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ASPND7 27(2) 35–68 (2006)ISSN 0195-5373

AtomicSpectroscopy

March/April 2006 Volume 27, No. 2

In This Issue:On-line Flow Injection Flame AAS Determination of Cobalt in Soil and Sediment Samples With 5,7-Dichloroquinoline-8-ol-Embedded Polymeric MaterialsR.S. Praveen, Sobhi Daniel, and T. Prasada Rao ............................................ 35

Characterization of Tire Bead Wire Coating With Special Emphasis on Tin Estimation Using an Atomic Absorption Spectrometer With a Flow Injection Analysis System (AAS-FIAS)N. Mandal, P. Sajith, S. Dasgupta, S. Bandyopadhyay, R. Mukhopadhyay, and A.S. Deuri .................................................................. 44

Concentrations of As, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, Pb, and Zn in Uruguayan Rice Determined by Atomic Absorption SpectrometryMario Rivero-Huguet, Raquel Huertas, Lorena Francini, Liliana Vila, and Elena Da ................................................................................ 48

Flame AAS Determination of Total Lead in Soil Sediments Near Highways in Maracaibo City, Venezuela Víctor A. Granadillo, Milkelly del V. Nava, Denny R. Fernández, Aracelis del C. Vásquez, Blanca Semprún, Maigualida Hernández, and Marinela Colina ......................................................................................... 56

Chelating Resin Micro-column Separation/ Preconcentration and Electrothermal Vaporization ICP-OES Determination of Trace Bismuth in Environmental and Biological Samples Yi-Wei Wu, Jian-Kun Duan, Zu-Cheng Jiang, and Bin Hu ............................. 62

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AtomicSpectroscopy

Vol. 27(2) March/April 2006

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35Atomic SpectroscopyVol. 27(2), March/April 2006

On-line Flow Injection Flame AAS Determination of Cobalt in Soil and Sediment Samples

With 5,7-Dichloroquinoline-8-ol-Embedded Polymeric Materials

R.S. Praveen, Sobhi Daniel, and *T. Prasada RaoInorganic Materials Group

Regional Research Laboratory (CSIR)Trivandrum – 695 019, India

INTRODUCTION

Cobalt is an essential micro–nutrient for a range of metabolicprocesses in man, animals, andplants (1,2). For the trace leveldetermination of cobalt and othertransition metals, environmentalsamples pose definite challenges tothe analytical chemist since typicalconcentration levels are two ordersof magnitude lower in geologicalsamples than the detection limits offlame atomic absorption spectrome-try (FAAS) analysis (3). Therefore,in the determination of trace andultratrace amounts of cobalt in geo-logical samples, a preconcentrationprocedure is often required. Con-

Many chelating sorbents havebeen reported in the literature forselectively sorbing cobalt fromaqueous solutions, but only a lim-ited number of them can be appliedto FI-on line preconcentration (seeReference 8 and further referencestherein). This is perhaps due to theprime requirement that a sorbent inFI-AAS must have faster sorptionand desorption. Hence, one has tobe careful in choosing a sorbentmaterial in on-line solid phaseextraction (SPE) (9).

In this study, we developed asimple, sensitive, and selective flowinjection on-line preconcentrationand separation method for the rou-tine determination of trace amountsof cobalt in geological samplesbased on 5,7-dichloroquinoline-8-ol-embedded polymer materials.

*Corresponding author.E-mail: [email protected]: (91) 471-2515317Fax: (91) 471-2491712 / 2490186

ventional off-line procedures forpreconcentration and separation,although effective, are usually time-consuming, tedious, require largequantities of sample and reagents,and are vulnerable to contamina-tion and analyte losses (4).

Flow injection (FI) on-line pre-concentration and separation witha microcolumn has proved to be asuitable method and has beenwidely used in trace element analy-sis due to its simple automatedoperation and high reproducibility(5). FI-online preconcentration cou-pled with FAAS provides enhancedsensitivity, efficient removal of thematrix, low cost of the FAAS instru-ment, substantially higher samplethroughput and, more importantly,employs a contamination-freeclosed system (5–7).

ABSTRACT

Dichloroquinoline-8-ol-embed-ded styrene-ethylene-glycoldimethacrylate polymer materialswere prepared via the bulk, pre-cipitation, and suspension poly-merization methods using similarcompositions. The polymeriza-tion was carried out by thermalmeans in the presence of 2,2’-azobisisobutyronitrile as initiatorand 2-methoxy ethanol as poro-gen. The above synthesized poly-mer materials were characterizedphysically and morphologicallyby using the FTIR, TGA, CHN,and SEM techniques. These poly-meric materials were packed intohomemade micro columns andused for on-line solid phaseextraction (SPE) preconcentra-

tion of trace amounts of cobalt.The preconcentrated cobaltspecies were eluted with 0.01mol L–1 of nitric acid and injecteddirectly into the nebulizer of aflame atomic absorption spec-trometer (FAAS) for quantification.

The enrichment factors forbulk, precipitation, and suspen-sion polymerization materials,were 12.5, 12.5, and 6.0, respec-tively, with a 1-min preconcen-tration time and a samplethroughput of 30 h–1. The detec-tion limits corresponding to 3times the standard deviation ofthe blank were found to be 10,10, and 80 µg L–1 with a preci-sion of 2.1, 2.5, and 2.6% (RSD)for five successive determina-tions of 80 µg L–1 of cobalt. Furthermore, the retention

capacities determined by thebatch method were 11.46, 11.42,and 8.52 mg of Co(II) per g ofbulk, precipitation, and suspen-sion polymerization materials,respectively. Thus, enrichmentfactor, detection limit, and reten-tion capacities are in the order:precipitation ~ bulk > suspen-sion. No significant interferencewas observed from neutral elec-trolytes and 100-fold amounts ofFe(III), Ni(II), Mn(II), Cu(II), andZn(II). In addition to testing theaccuracy of the developed proce-dure for the determination ofcobalt in certified referencematerials of soil and marine sedi-ments, the method was appliedsuccessfully to the determinationof cobalt in real soil andsediment samples.

36

EXPERIMENTAL

Instrumentation

A PerkinElmer® AAnalyst™ 100flame atomic absorption spectrome-ter (PerkinElmer Life and AnalyticalSciences, Shelton, CT, USA) wasused, equipped with deuteriumbackground correction and aPerkinElmer Lumina™ cobalt hol-low cathode lamp. This lamp wasoperated at 10 mA using a 0.7-nmslit width. The wavelength usedwas 240.7 nm. The system wasequipped with a 10-cm air-acety-lene burner head and a stainlesssteel nebulizer using flow rates of4.0 L min-1 (air) and 1.0 L min-1

(acetylene). The burner height wasadjusted to about 30 mm from basefor optimum sensitivity. The nebu-lizer uptake rate was adjusted toprovide optimum response for con-ventional sample aspiration.

A PerkinElmer FIAS™-400 flowinjection system connected to theFAAS was used for on-line precon-centration of cobalt. The automaticoperation of the injection valve andthe two multichannel peristalticpumps were programmed usingthe spectrometer software(PerkinElmer AA WinLab™ v. 3.0).Tygon® peristaltic tubing was usedto pump the sample at 3.0 mLmin–1 and PTFE tubing of 0.3 mmi.d. was used for all connections inorder to minimize dead volume.Homemade conically shaped micro-columns were used, packed with20 mg of materials prepared by thebulk, precipitation, and suspensionpolymerization methods. Time-resolved absorbance signals ofcobalt were displayed on the com-puter monitor along with the peakheight and integrated absorbancevalues.

FT-IR spectra were recorded in thewave number range 4000–400 cm–1

using a MAGNA FTIR-560 spectrom-eter (Nicolet, USA). The CHNanalysis was carried out using aPerkinElmer elemental analyzer.

Thermo-gravimetric analysisstudies were carried out using aModel TGA–50H (Shimadzu, Japan).The surface morphology of the 5,7-dichloroquinoline-8-ol-embeddedpolymeric materials was carried outusing a JEOL Model JSM 5600LVscanning electron microscope(SEM). A Model Innova 4330 refrig-erated incubation shaker (USA) wasused for adsorption kinetic andadsorption isotherm studies. AModel LI-120 digital pH meter(ELICO, India) was used for pHmeasurements.

Reagents, Standard Solutions,and Samples

The polymeric materials 5,7-dichloroquinoline-8-ol (DCQ),styrene, ethylene glycoldimethacrylate (EGDMA), 2,2’-azo-bisisobutyronitrile (AIBN), and2-methoxyethanol were obtainedfrom Aldrich, Milwaukee, WI, USA.A cobalt stock solution (1000 µgmL–1) (Aldrich) was prepared bydissolving 0.4039 g of CoCl2.6H2Oin 100 mL of deionized water. Asolution of 5% ammonium acetate(pH 7.0) was prepared to maintainthe pH of the sample solution(7.0±2.0) during on-line flow injec-tion solid phase extraction-precon-centration experiments.

Certified reference materials(CRMs) of soil and marine sedimentwere purchased from the Interna-tional Atomic Energy Agency (IAEA,Soil 7) and the National ResearchCouncil, Canada (MESS-3), respec-tively. One sample each of housesoil, river sediment, and marinesediment sample was collected inTrivandrum and after mineraliza-tion, each sample was subjected tothe determination of cobalt.

Polymerization Studies

Bulk PolymerizationThe 5,7-dichloroquinoline-8-ol

(2 mmoles)-embedded polymermaterials were prepared with10 mmoles each of styrene as

monomer and EGDMA as cross-link-ing monomer. The polymerizationmixtures were prepared in 2-methoxyethanol (10 mL) as poro-gen and 2,2’-azobisisobutyronitrile(AIBN) (0.05 g) as initiator, cooledto 0oC, purged with N2 for 10 min,and then sealed in an R.B. flask.The bulk polymerization was car-ried out by keeping it in an oil bathat 80oC while stirring for 2–3 h.The polymer materials thusobtained were ground and sievedto get particle sizes ranging from45–212 µm. These particles werewashed thoroughly with water and1.0 mol L–1 HCl to remove theunembedded chelating agents andreactants.

Precipitation PolymerizationSynthesis of microspheres by

precipitation polymerization wasperformed by using the same com-position as used for the polymermaterials for bulk polymerization,with the exception of using a 40-mLvolume of porogen.

Suspension PolymerizationSynthesis of the beads by suspen-

sion polymerization was performedas follows:

Step 1: Polyvinyl alcohol (PVA,molecular weight ~400, 0.1 g) wasdissolved in hot water (about 90oC,30 mL), then cooled to room tem-perature. The solution was purgedwith N2, placed in a water bath(60oC), and then stirred at 400 rpmfor 30 min.

Step 2: The same compositionas for the above bulk polymeriza-tion was used (porogen, DCQ,styrene, EGDMA, and AIBN) andsubjected to N2 purging. Then thissolution was added dropwise to theabove PVA solution and stirred for2 h.

The polymerizing solution wasput in an oil bath at 80oC for 24 h.The beads obtained were collected,washed with hot water, and dried.

37

Vol. 27(2), March/April 2006

and suspension polymerizationmethods, are listed in Table I. Thistable also shows the calculated /theoretical percentages of C, H, andN. The agreement between calcu-lated and experimentally found val-ues is quite good. Thus, the closesimilarity of % N in the polymermaterials between the experimen-tal and calculated values confirmsthat DCQ is indeed embedded inthe styrene-EGDMA polymericmatrix.

SEM Studies

The morphology of the polymersprepared by the precipitation, bulk,and suspension polymerizationmethods was assessed by SEM(micrographs are shown in Figure 1).In all cases, the morphology was aspredicted. For bulk polymerization,the morphology of the crushedpolymer is shown. In the other twomethods, microspheres and beadswere produced by precipitationand suspension polymerization. Asseen from the micrographs, there isa systematic morphological changein the material’s shape and size ofthe polymers synthesized by thebulk, precipitation, and suspensionmethods. Bulk polymerizationshows agglomerated polymericmaterials in irregular shapes rang-ing from 1–20 µm. In the case ofthe precipitation method, the poly-meric material shapes into regular

Procedure for On-line Precon-centration

The FI manifold (0.3 mm i.d.)used in this study for on-line pre-concentration and elution isdescribed by Sperling et al. (10).The pH of the sample solution wasadjusted to ~7.0 and pumpedthrough homemade micro-columnspacked with 20 mg of polymericmaterials prepared by the bulk, pre-cipitation, and suspension methods.The preconcentration time wasapproximately 1 min and thesorbed cobalt species were elutedwith 0.01 mol L–1 of nitric acid at aflow rate of 4 mL min–1. Calibrationcurves were drawn by pumping10–160 µg L–1 of cobalt standardsolutions through the micro-columns.

Analysis of Soil and SedimentCRMs, and Real Samples

A 0.5-g soil or sediment samplewas weighed into a platinum cru-cible, 5 mL of HF and 1 mL of conc.H2SO4 were added, and the mixturewas heated to 150oC on a hot plate.The process was repeated threetimes. The residue was cooled andfused with 2 g of KHSO4 at 800oCin an electric bunsen burner for30 min. The melt was cooled anddissolved in 50 mL of deonizedwater and made up to 100-mL vol-ume. These sample solutions weresubjected to preconcentration andanalysis by the procedure describedabove. The cobalt content of thesoil and sediment samples wasestablished by reference to a cali-bration graph prepared in accor-dance with the above procedure.

RESULTS AND DISCUSSION

Characterization Studies

Characterization of 5,7-dichloro-quinoline-8-ol-embedded polymermaterials, synthesized by the pre-cipitation, bulk, and suspensionmethods, was carried out asdescribed below.

IR Spectra

The IR spectral studies ofstyrene-EGDMA and DCQ-embed-ded styrene-EGDMA, prepared bythe different polymerization meth-ods, indicate that the spectra con-tain a strong peak at 765 cm–1

corresponding to the C–Cl bond.This shows the presence of DCQ inall of the materials except for thestyrene-EGDMA polymeric material.This observation also indicates that5,7-dichloroquinoline-8-ol is indeedembedded in the polymer matrix inall of the materials prepared by theprecipitation, bulk, and suspensionmethods.

TGA Studies

The TGA studies of styrene-EGDMA and DCQ-embeddedstyrene-EGDMA polymers, preparedby the precipitation, bulk, and sus-pension methods, reveal an addi-tional decomposition patternstarting at ~180oC in case of thelatter polymer materials. Thesestudies confirm the IR evidencethat DCQ is indeed embedded inthe polymer materials since themelting point of DCQ is 180oC.

Elemental Analysis

The elemental analysis data ofstyrene-EGDMA and DCQ-embed-ded styrene-EGDMA polymers, pre-pared by the precipitation, bulk,

TABLE IElemental Analysis Studies

Polymerization Calculated (%) Experimentally Found (%)Method C H N C H N

A. PrecipitationStyrene-EGDMA 71.52 7.28 - 71.50 6.99 -DCQ-Styrene-EGDMA 68.90 6.67 0.80 68.70 6.70 0.84

B. BulkStyrene-EGDMA 71.52 7.28 - 71.20 6.79 -DCQ-Styrene-EGDMA 68.90 6.67 0.80 68.29 6.82 0.80

C. SuspensionStyrene-EGDMA 71.52 7.28 - 71.40 6.85 -

DCQ-Styrene-EGDMA 68.90 6.67 0.80 67.98 6.68 0.88

38

spherical particles of uniform sizes(300–400 nm). The roundness ofthe particles is not very perfect. Inthe suspension method, the mater-ial crystallizes into nanosized per-fectly uniform spherical beads(~200 nm). Also, the materials orga-nize into very regular shapes incase of the precipitation and sus-pension polymerization methodsonly.

Evaluation Of Main Experimen-tal Variables

Effect of pHA series of solutions containing

0 and 80 µg L–1 of cobalt solutionwere taken, the pH of the solutionswas adjusted to between 2 and 10,and pumped through micro-columns packed with 20 mg ofpolymeric materials prepared bythe precipitation, bulk, and suspen-sion methods. The SPE of cobaltwas found to be constant and maxi-mum in the pH range of 6–10 (seeFigure 2). The pH of the samplesolution was adjusted to ~7.0 insubsequent studies after the addi-tion of 1.0 mol L–1 of ammoniumacetate solution. The analytical sig-

nal for on-line FIA-FAAS determina-tion of cobalt lies in the order: pre-cipitation ~ bulk > suspension.

Choice of EluentVarious miscible solvents (acidi-

fied to pH ~2.0) such as methanol,dimethyl formamide, dimethylsulphoxide, acetonitrile, and 0.01mol L–1 of HCl and HNO3 weretested for the elution of previouslysorbed cobalt onto 5,7-dichloro-quinoline-8-ol-embedded polymericmaterials prepared by the precipita-tion, bulk, and suspension meth-ods. It was found that nitric acidalone in the range of 0.01–0.1 mol L–1

quantitatively elutes cobalt for thethree materials. Again, the analyti-cal signals obtained for FIA-FAASdetermination of cobalt follow theorder: precipitation ~ bulk > sus-pension for all eluents.

Optimization of FI Analysis FlowConditions

High sample loading flow ratesare important for efficient precon-centration and high samplethroughput. In general, FI sampleflow rates are limited by the backpressure produced by the column

Fig. 1. SEM photographs of 5,7-dichloroquinoline-8-ol (DCQ)-embed-ded polymer materials synthesized bythe(a) bulk, (b) precipitation, and (c)suspension methods.

Fig. 2. Effect of pH on FI-FAAS signals of of 80 µgL–1 of cobalt pumped throughmicro-columns packed with DCQ-embedded polymer materials synthesized by thebulk, precipitation, and suspension methods.

39


and/or sorption efficiency. In thisstudy, no degradation of sorptionefficiency was observed up to aloading flow rate of 12.0, 3.0, and6.0 mL min–1 of sample in case ofthe precipitation, bulk, and suspen-sion polymerization materials,respectively. The optimum sampleflow rates, which do not result inback pressure, were found to be12.0, 3.0, and 6.0 mL min–1 formicro-columns packed with 20 mgof materials prepared by the precip-itation, bulk and suspension meth-ods, respectively. These flow rateswere used for subsequent studies.

An elution flow rate of 4.0 mL min–1

provided optimum sensitivity andelution peaks with minimum tail-ing. No provision was made tocompensate for the lower flow ratedelivered by the FI system; how-ever, the transfer capillary to thenebulizer (PTFE, 0.3 mm i.d.)restricted the uptake rate to valuesclose to the flow rates provided bythe FI system. Operating the nebu-lizer at this flow rate does notlower the sensitivity relative to adecrease in the flow rate due to thepotential improvement in nebulizerefficiency under starved conditions(10,11). The lower sample flow ratein the FI mode in comparison tothe conventional free uptake of thenebulizer is also beneficial fordroplet diameter distribution,which results in smaller dropletsand is therefore less prone to vapor-ization interferences (11).

Performance of On-Line Precon-centration System

The characteristic data listed inTable II show the performance ofthe on-line FI preconcentration ofcobalt with micro-columns packedwith 20 mg of 5,7- dichlorquino-line-8-ol-embedded polymer materi-als obtained by the precipitation,bulk, and suspension methods. Theefficiency of SPE was investigatedby analyzing the previouslycollected column eluent from stan-dard solutions of cobalt using the

same preconcentration technique.From the results obtained byrepeated preconcentration, a reten-tion efficiency of >99% was calcu-lated for the precipitation and bulkmaterials. The SPE elution sequencefor the three methods is highlyreproducible and provides an over-all precision of 2.1, 2.5, and 2.6%(RSD), respectively, for five succes-sive/determinations of 80 µg L–1 ofcobalt passed through micro-columns packed with 20 mg ofpolymer materials. A linear relation-ship was observed betweenpreconcentration time and anenrichment factor up to a 2-minloading time. Using precipitation-,bulk-, and suspension-based columnmaterials and a 1-min loading timeprovided a 12.5-, 12.5-, 6.0-foldenhancement in sensitivity, whilea 2-min loading time provided a 25-,25-, 12-fold enhancement in sensitiv-ity, respectively, in comparison toconventional FAAS. The detectionlimits corresponding to 3 times thestandard deviation of the blankwere found to be 10, 10, and 80 µg L–1

of cobalt with precipitation-, bulk-,and suspension-based column mate-rials, respectively. The linear equa-tion with regression was as follows:

Aprecipitation (Ap) = 0.0005+0.25 x C

Abulk (Ab) = 0.0005+0.30 x C

Asuspension (As) =0.0005+0.07 x C

The correlation coefficients are0.998, 0.999, and 0.996, where Ap,Ab, and As are absorbances and Cis the concentration of cobalt inµg mL–1. All statistical calculationsare based on the average of tripli-cate measurements. Furthermore,a 1-min loading time allows a sam-ple frequency of 30 h–1. Higher sen-sitivities can be obtained bymodifying the method, i.e., usinglonger preconcentration times atthe expense of lower samplethroughput. Under the optimal con-ditions described above, the role ofDCQ-embedded polymer materialsobtained by the precipitation, bulk,and suspension methods in on-linepreconcentration of cobalt wasinvestigated by adsorption kineticsand adsorption isotherm studies.

Adsorption Kinetics

The time required for loadingcobalt onto DCQ-embedded poly-meric materials prepared by theprecipitation (Figure 3a), bulk (Fig-ure 3b), and suspension (Figure 3c)methods was determined by agitat-ing 20 and 60 mg (curves A and Bin Figure 3) of cobalt in a refriger-ated incubation shaker at 30oC for0.5, 1, 2, 4, 8, 10, 20, 40, and 60min. The amount of cobalt loadedonto DCQ-embedded polymermaterials was determined by FAAS.The equilibrium time in whichDCQ-embedded polymer materialsattain 50% saturation with cobalt

TABLE IIAnalytical Performance Data of On-line SPE-FIA-FAAS Determination

of Cobalt Using Precipitation, Bulk, and Suspension Materials

Parameter Precipitation Bulk Suspension

1 Linear Range (µg L–1) 10–160 10–160 80–1602 Sensitivity Enhancementa 12.5 12.5 63 Concentration Efficiencya 12.5 12.5 64 Precision (RSDb) at 80 µg L–1 (%) 2.1 2.5 2.65 Sample Consumption (mL) 12 3 66 Loading Time (min) 1 1 17 Sample Frequency 30/h 30/h 30/h

8 Detection Limit (3σ) µg L–1 10 10 80

a Compared to conventional nebulization; b n = 5.

40

(i.e., when the amount of metal ionsorbed on the SPE is half of its max-imum sorption capacity) is calledloading half-time (t1/2). From curvesA and B in Figure 3 it is clear thatthe loading half-time is <30 s. Thekinetics of cobalt sorption on DCQ-embedded polymer materials fol-lows the first order rate expressiongiven by Lagergren (12):

log (qe – q) = log qe – Kads t/2.303

where q and qe are the amountsof cobalt adsorbed (mg per g) ofDCQ-embedded polymer materials)at time t (min), and equilibriumtime (60 min), respectively, andKads is the rate constant of adsorp-tion. Linear plots of log (qe – q)versus t (Figure 4) show the applic-ability of the equation for the threeDCQ-embedded polymer materials.The correlation coefficients andKads calculated from the slopes ofFigure 4, are shown in Table III.

Adsorption Isotherms

The adsorption of cobalt wasstudied as a function of DCQ-embedded polymer materialsobtained with the precipitation,bulk, and suspension methods byequilibrating for 60 min in a refrig-erated incubation shaker. Theamount of cobalt loaded onto theDCQ-embedded polymer materialswas determined by FAAS. The Lang-muir treatment (13) is based on theassumption that (a) maximumadsorption corresponds tosaturated monolayer of adsorbatemolecules on the adsorbed surface,(b) the energy of adsorption is con-stant, and (c) there is no transmi-gration of adsorbate in the plane ofthe surface. The equation is given isas follows:

Ce / qe = 1 / (q0b) + Ce / q0

where Ce is the equilibrium con-centration (mg L–1), qe is theamount adsorbed at equilibrium,and qo and b are Langmuirconstants related to adsorptioncapacity and energy of adsorption,respectively.

from the linear regression fits of theLangmuir plots are shown in TableV. It can be seen that the bindingcapacities (qo) are in the order: pre-cipitation > bulk >suspension. Theessential characteristics of the Lang-

The linear plot of Ce/qe versus Ce

shows that adsorption obeys theLangmuir adsorption model (Figure5). The correlation coefficient (R)for the linear fit of the Langmuirplots, qo and b values determined

Fig. 3. Effect of agitation time on theadsorption of cobalt (a) bulk, (b) precipitation, and (c) suspension. Cobalt concentration (A) 20 mg L–1 and (B) 60 mg L–1, pH 7.0, adsorbent dose =250 mg per 100 mL.

Fig. 4. Lagergren plots for the adsorp-tion of cobalt: (a) bulk, (b) precipita-tion, and (c) suspension. Cobalt concentration (A) 20 mg L–1,(B) 60 mg L–1, pH 7.0, adsorbent dose =250 mg per 100 mL.

41


muir isotherm can be expressed interms of a dimensionless constant,equilibrium parameter, RL, whichis defined by RL = 1/(1+bCo),where b is a Langmuir constant andCo is the initial concentration ofcobalt (13). The RL values observedbetween 0 and 1 indicate favorableadsorption of cobalt onto the DCQ-embedded polymer materials (seeTable V).

The Freundlich equation wasalso applied to the adsorption. Thisequation is basically empirical butoften useful as a means of datadescription. It generally agreesquite well with the Langmuir equa-tion and the experimental data overa moderate range of adsorbate con-centrations. The Freundlich

rials. A plot of log (x / m) versus logCe (see Figure 6) is linear and theconstants Kf and n obtained areshown in Table VI. The values of 1 < n < 10 show a favorable adsorp-tion of cobalt onto the DCQ-embed-ded polymer materials (14). Thecorrelation coefficients for the

isotherm is represented by the fol-lowing equation (14):

log (x/m) = log Kf + (1/n) log Ce

where Ce is the equilibrium con-centration (mg L–1) and x m–1 is theamount adsorbed per unit mass ofthe DCQ-embedded polymer mate-

TABLE IIICorrelation Coefficient and Kads Values of DCQ-Embedded Polymer

Particles Obtained by Different Polymerization Methods

Precipitation Bulk Suspension

Cobalt Correl. Coeff. Kads Correl. Coeff. Kads Correl. Coeff. Kads(µgL-1 ) (R) (L min–1) (R) (L min–1) (R) (L min–1)

20 0.993 1.01 0.980 1.04 0.991 1.10

60 0.997 0.50 0.990 1.21 0.997 0.62

TABLE IVR, q0, and b Values Determined

From Langumir Plots for the Adsorption of Cobalt

Precipi- Bulk Suspen-tation sion

R 0.999 0.988 0.990qo 20.30 16.11 2.75

b 0.04 0.16 0.26

Fig. 5. Langmuir plots of bulk, precipitation, and suspensionpolymers for the adsorption of cobalt. Cobalt concentration:20–100 mg L–1, pH 7.0, equilibration time: 60 min, adsorbent dose = 250 mg per 100 mL.

TABLE VEquilibrium Parameter, RL

Initial Co RL ValueConc. Precipi- Bulk Suspen-

(mg L–1) tation sion

20 0.58 0.23 0.1340 0.41 0.13 0.0960 0.32 0.09 0.0680 0.26 0.07 0.05

120 0.19 0.05 0.03

Fig. 6. Freundlich plots of bulk, precipitation, and suspensionpolymers for the adsorption cobalt. Cobalt concentration:20–100 mg L–1, pH 7.0, equilibration time: 60 min, adsorbent dose = 250 mg per 100 mL.

TABLE VIKf and n Values Deduced From

Freundlich Plots for the Absorption of Cobalt

Precipi- Bulk Suspen-tation sion

Kf 5.66 5.31 1.90

N 5.18 3.96 2.70

42

dissolution procedure described inthe Experimental section. The solu-tion was subjected to preconcentra-tion and analysis by adopting theon-line preconcentration proceduredescribed in the Experimental sec-tion after adjusting the pH to7.0±2.0. The cobalt content estab-lished by the present procedureagrees well with the certified values(see Table VII), verifying the accu-racy of the developed procedurefor the analysis of soil and sedimentsamples.

Analysis of Real Soil andSediment Samples

Soil, river, and marine sedimentsamples were mineralized and thecobalt content was determined asdescribed in the Experimental sec-tion. As can be seen from theresults listed in Table VIII, the SPEon-line preconcentration methoddeveloped in the present studyshows that the low-cost FIA-FAASinstrument used for the determina-tion of trace amounts of cobalt insoil and sediment samples providesgood analytical data.

CONCLUSION

This study shows that dichloro-quinoline-8-ol-embedded polymericmaterials, synthesized by the pre-cipitation and bulk polymerizationmethods, results in quantitativesorption of trace amounts of cobalt,

Freundlich plots were found to be0.997, 0.996, and 0.999 for theDCQ-embedded polymer materialsprepared by the precipitation, bulk,and suspension methods, respec-tively, indicating a better fit for theexperimental data in comparison tothe Langmuir plot.

Effect of Neutral Electrolytesand Coexisting Ions

The tolerance of various neutralelectrolytes and coexisting ionsusually present in soil or sedimentsamples in the determination of80 µg L–1 of cobalt was systemati-cally studied using the FIA-FAASprocedure described in a previoussection. A deviation from the stan-dard absorbance value greater than3% was taken as interference. Thus,it can be stated that NaCl (5%), KCl(0.5%), CaCl2 (0.1%), MgCl2 (0.1%),and 2 mg L–1 of Fe(III), Ni(II),Mn(II), Cu(II), and Zn(II) do notinterfere in the FIA-FAAS determina-tion of cobalt. This observation sug-gest that the developed procedurecan be used for the selective deter-mination of cobalt in the presenceof matrix elements which arelikely to coexist in geological andenvironmental samples.

Analysis of Certified ReferenceMaterials

Accuracy of the developed pre-concentration procedure wastested by analyzing certified refer-ence materials of soil (IAEA–Soil 7)and marine sediment (MESS-3).These soil and sediment sampleswere mineralized by following the

unlike with the suspension method.However, as described elsewhere(14,15), the polymeric materialsobtained by bulk polymerizationlead to large and irregularly sizedparticles which have to be groundand sieved. The grinding and siev-ing operations performed eithermanually or mechanically are slowor produce irregular particles withrather limited control over particlesize and shape. Hence, the bulkpolymerization particles (thoughadequate for developing new on-line column materials and for usein mechanistic studies) are not suit-able for large-scale sample through-put. On the other hand,precipitation polymerization leadsto microspheres which can bedirectly used as on-line columnmaterials. On-line solid phaseextraction preconcentration withpolymer materials prepared by pre-cipitation polymerization followedby FIA-FAAS analysis allows thedetermination of cobalt as low as10 µg L–1. The enrichment factorsfor bulk, precipitation, and suspen-sion polymerization materials, were12.5, 12.5, and 6.0, respectively.The detection limits correspondingto 3 times the standard deviation ofthe blank were found to be 10, 10,and 80 µg L–1 with a precision of2.1, 2.5, and 2.6% (RSD) for fivesuccessive determinations of80 µg L–1 of cobalt. Furthermore,the retention capacities determined

TABLE VIIAnalysis Of Certified Reference

Materials (CRMs)

Cobalt Found (µg per g )

CRMs Present CertifiedMethod Value

IAEA Soil-7 8.8±1.1 8.9±1.0

MESS-3 14.3±2.0 14.4±2.0

TABLE VIIIAnalysis of Soil and Sediment Samples

Sample Cobalt Added Cobalt Found Recovery(µg of Co per g) (µg of Co per g) (%)

House Soil, – 9.20±1.2 –Trivandrum, India 5.0 14.15±1.4 99.0

10.0 19.20±2.0 100.0

Karamana – 18.50±2.0 –River Sediment, 15.0 33.50±3.0 100.0Trivandrum, India 30.0 48.40±5.0 99.7

Arabian Sea – 17.60±1.8 –Sediment, 15.0 32.60 100.0Trivandrum, India 30.0 47.40±5.0 99.3

43


by the batch method were 11.46,11.42, and 8.52 mg of Co(II) per gof bulk, precipitation, and suspen-sion polymerization materials,respectively. Thus, enrichment fac-tor, detection limit, and retentioncapacities are in the order: precipi-tation ~ bulk > suspension.

The developed on-line FIA-FAASprocedure is simple and rapid (sam-ple throughput = 30 h–1) and allowsthe reliable analysis of soil and sedi-ment samples for cobalt determina-tion. Furthermore, the developedpreconcentrative separation proce-dure when coupled on-line to theelectrothermal AAS, ICP-AES, orICP-MS techniques can offer betterchoices for the determination ofcobalt in a variety of real samplessuch as seawater or biological sam-ples.

ACKNOWLEDGMENT

The authors R.S Praveen andS. Daniel are thankful to CSIR,New Delhi, India, for awardingjunior/senior fellowships, respec-tively.

Received March 17, 2005.

REFERENCES

1. E.J. Underwood, Trace Elements InHuman And Animal Nutrition, 4thEdn., Academic Press, New York(1977).

2. D.Purves, Trace Element Concentra-tion Of The Environment, ElsevierScience Publishers, Netherlands(1985).

3. R.R.Barefoot, Trends Anal. Chem. 18,702 (1999).

4. Peng Liu, Zhixing Su, Xiongzhi Wu,and Qiaosheng Pu, J. Anal. At.Spectrom.17, 125 (2002).

5. Z. Fang, Flow Injection Separationand Preconcentration, VCH, Wein-heim, Germany (1993).

6. B. Welz, Microchem. J. 45, 163(1992).

7. Z. Fang, Flow Injection AtomicAbsorption Spectrometry, Wiley,Chichester, U.K. (1995).

8. A.M. Starvin,V.M. Biju,andT.Prasadarao, At. Spectrosc.25(6), 238 (2004).

9. Shaoming Zhang, Quiosheng Pu,PengLiu, Qiaoyu Sun, and Zhixing Su,Anal.Chim. Acta 452, 223 (2002).

10. M. Sperling, S. Xu, and B. Welz,Anal. Chem. 64, 3101 (1992).

11. S. Karthikeyan, T. Prasadarao, andC.S.P. Iyer, Talanta 49, 523 (1999).

12. D.B. Singh, D. Prasad, D.C.Rupainwar, and V.N. Singh, Water,Air, Soil Pollut. 42, 376 (1989).

13. I.J. Langmuir, J. Am. Chem. Soc. 40,1361 (1918).

14. A.G. Mayes, In B. Sellergren (Ed.),Molecularly Imprinted Polymers,Elsevier, Amsterdam, The Nether-lands, pg. 305 (2001).

15. S. Daniel, P. Prabhakararao, and T.Prasadarao, Anal. Chem. Acta. 536,197 (2005).


*Corresponding author.E-mail: [email protected]./Fax: +91 - 2952–220669

Characterization of Tire Bead Wire Coating With Special Emphasis on Tin Estimation Using an Atomic Absorption Spectrometer

With a Flow Injection Analysis System (AAS-FIAS)a

N. Mandal, P. Sajith, S. Dasgupta, *S. Bandyopadhyay, and R. MukhopadhyayHari Shankar Singhania Elastomer and Tyre Research Institute (HASETRI)

P.O.-Tyre Factory, Kankroli, Rajasthan, India 313342and

A.S. Deuri R&D Centre, J.K. Tyre, Jaykaygram, P.O.-Tyre Factory, Kankroli, Rajasthan, India 313342

ABSTRACT

Bead wires used by the tireindustry are made up of steeland coated with a low level ofcopper and tin. The coating oversteel is required for good adhe-sion between the rubber and thebead wire. Conventional flameatomic absorption spectrometer(FAAS) instrumentation cannotdetermine low levels (1–3%) oftin present in bead wire.

In this paper, an attempt hasbeen made to estimate the tincontent and also to characterizequantitatively the coating pre-sent in bead wire using a flameatomic absorption spectrometer(FAAS) in combination with aflow injection analysis system(FIAS). Repeatability and repro-ducibility of the measurementswere found to be acceptable forthis type of analysis and theresults are reported here.

INTRODUCTION

Bead wires of 0.965 mm and1.60 mm are widely used in the tireindustry. The function of bead wireis to hold the tire to the rim of thevehicle. Bead wire is made up ofsteel and is normally coated withcopper and tin (bronze coating).

The first in-depth study on theeffect of coating properties wasdone by Maeselle and Debruyne(1). In their study, the effect of cop-per concentration used for brasscoating is discussed. They reportedthat a variation of plating conditionsleads to different brass grain size,plating weight, mechanical defor-mation of the electro-deposit, ther-mal treatment, and compoundcomposition. The optimum coppercontent of brass coating at constantplating weight was reported as 60%,67–70%, or 75% copper, dependingon the rubber compound used inadhesion tests.

The bonding mechanism of rub-ber to steel was extensively studiedby W.J. van Ooij (2) and N.L Hewitt(3). They reported that the adhe-sion is explained on the basis of anauto-catalytic effect of a thin film ofcuprous sulfide formed on the brasssurface by the combined action ofsulfur and accelerators. The filmtransfers sulfur atoms to rubbermolecules, which results in highrubber surface polarity. This sur-

face bonds to brass by physicalinteraction.

The bead wire used by the tireindustry normally contains 97–99 %copper (Cu) and 1–3% tin (Sn). Thecoating weight is generally found inthe range of 0.15–0.65 g/kg. How-ever, the tin content cannot bedetermined by FAAS since thedetection limits for AA measurementin aqueous solution is approximately10 mg/L (ppm) Sn even when usinga high sensitivity nebulizer (4,5).Thus, if a 10-g sample is used foranalysis, only µg/L (ppb) Sn will beavailable in solution.

There are different ways bywhich tin in µg/L (ppb) levels insolution can be estimated. Forexample, using an atomic absorp-tion spectrometer with a flow injec-tion analysis system (AAS-FIAS),inductively coupled plasma (ICP),or a graphite furnace atomicabsorption spectrometer (GFAAS).Both ICP and GFAAS are used fortin estimation with low detectionlimits. These instruments are suit-able for industries that requiremetal estimation for both hydride-forming and non-hydride-formingmetals. However, these instrumentsare very costly. However, AAS-FIASis economical and useful for thedetermination of metals that canform hydrides, such as Sn, Sb, As,Bi, Hg, Se, and Te.

Copper can be determined byconventional FAAS. In this study,special emphasis is given to tindetermination by AAS in combina-tion with FIAS.

EXPERIMENTAL

Instrumentation

A PerkinElmer® Model 3300atomic absorption spectrometer,equipped with a FIAS™-400 flowinjection analysis system, was usedin this study (PerkinElmer Life andAnalytical Sciences, Shelton, CT,USA). The instrumental parametersare given in Table I.

a This article was previously publishedin Tire Technology International 2005reprinted herewith with permissionof the authors.

45


The pump speed programmingfor the FIAS–400 is given in Table II.The reagents used are listed inTable III.

Estimation Principle of AAS-FIAS

The AAS-FIAS technique is basedon the hydride generation principle(6). The hydride technique involvesthe reaction of an acidified aqueoussample with a reducing agent suchas sodium borohydride. The sodiumborohydride/acid reaction gener-ates hydrides as shown in the fol-lowing equation:

NaBH4 + 3H2O + HCl →H3BO3 + NaCl + 8H*

Em++H* → EHn +H2 (excess)

where E = the analysis of interestand m may or may not equal to n.

Calculation

The concentrations of copperand tin are given in Equations A andB (7,8), while Equation C calculatesthe plating weight.

Equation A:Tin concentration (%) =

Sn concentration (mg/L) × 100

Cu conc. (mg/L) + Sn conc (mg/L)

Equation B:Copper concentration (%) =

Cu concentration (mg/L) × 100

Cu conc. (mg/L) + Sn conc. (mg/L)

Equation C:Plating weight (g/kg) =

(mg/L of Cu + mg/L of Sn ) × made up to volume in L

Sample weight in grams

Preparation of Sample Solution

To estimate the tin content (%)in the coating of bead wire, thecoating must first be extracted toenable the analysis by FIAS or anyother technique (7,8). For this pur-pose, the bead wire was cut intoapproximately one-inch lengths andthen cleaned with acetone andtoluene to remove organic foreignmatter from the surface. The wire

was kept at 105oC for 30 min andthen cooled in a desiccator. About10 g of the sample was strippedwith 10 mL conc. HNO3. The solu-tion was transferred to a 100-mLstandard flask with double-distilledwater. Concentrated HNO3 wasused to make iron passive as wellto remove the coating. The samplesolution was diluted to a 50–100dilution factor (DF) and made up to100-mL volume with a carrier solu-tion, so that the matrix effect is nul-lified and at the same time thesensitivity is improved.

Preparation of Standard Solution

The standard solution wasdiluted to a measurable range withthe carrier solution, using a dilutionfactor of 10. Therefore, thematrixes of the sample as well asthe standard are the same.

Preparation of Carrier Solutionand Reducing Agent

Preparation of the carrier andreducing solutions is very impor-tant. If these are not prepared prop-erly, the signal may not be good oreven be absent. Exactly 0.4% NaBH4

in 0.05% NaOH was used as thereducing agent. Sodium hydroxide

TABLE IAAS-FIAS Instrumental Parameters

Parameters Copper Tin

Wavelength 324.8 nm 286.3 nmSlit Width 0.7 nm 0.7 nmTechnique Background-corrected Atomic absorption

atomic absorption using impact bead

Data Processing Time average Peak height with 37-point smoothening

Cell Temperature – 900oCRead Time 5 sec 15 sec

Flame Type C2H2 : Air –

TABLE IIPump Speed Programming for FIAS

Step Time (s) Pump 1 Speed Pump 2 Speed Valve

Prefill 15 100 120 Fill1 10 100 120 Fill

2 15 0 120 Inject

TABLE IIIReagents Used

Reagent Source

Suprapur® Hydrochloric Acid Merck Ltd., Mumbai, IndiaNitric Acid Merck Ltd., Mumbai, IndiaBoric Acid Merck Ltd., Mumbai, IndiaSodium Borohydride Sd Fine Chemicals, Mumbai, India

Sodium Hydroxide Merck Ltd., Mumbai, India

46

was used for enhancing the stabilityof sodium borohydride. Saturatedboric acid with 1% (v/v) HCl wasused as the carrier solution. Highpurity chemicals were usedthroughout. All solutions werefiltered to remove any suspendedparticle that might cause blockagein the FIAS. Gas bubbles formed inthe reducing solution are disturbingas they may affect repeatability.Therefore, a freshly preparedreducing solution was used, whichwas stirred continuously with amagnetic stirrer. In case of tinestimation, the sensitivity can beimproved by using argon carriergas with 1% oxygen.

Carrier Gas Flow

Argon gas was used as the car-rier gas, which carries hydridesfrom the mixing chamber to thequartz tube. The argon flow ratehighly influences sensitivity. If theflow is too high, the metal atom orhydride cloud is dispersed toorapidly. If the flow is too low, theconcentration of hydrides in thequartz tube is poor. To obtain thehighest sensitivity, optimization of the flow rate is required. A 100±10 mL /min flow rate wasselected for our experiments.

Setting up of FIAS Instrumentfor Sn Determination

A tin hollow cathode lamp wasused as the radiation source,installed into the spectrometer andaligned (6). The quartz cell was alsoaligned to get maximum energy tothe spectrometer. The carrier andreductant fluid flow was adjustedso that the ratio of the carrier-to-reductant flow was about 2:1, witha carrier flow between 9 mL/minand 11 mL/min. The carrier gasflow was optimized. If a peakappeared too quickly, the carriergas flow was slightly increased. Ifthe peak maximum appeared toolate, the carrier gas flow wasslightly decreased. To condition thequartz cell, the cell was placed in abath of concentrated hydrofluoricacid (40%) for 20 minutes. The cellwas then thoroughly rinsed underrunning water, further rinsed withdouble-distilled water, and subse-quently dried (9). After system opti-mization, the blank and standardwere run and the signal was mea-sured according to its peak height.Figure 1 shows how peak heightvaried with Sn concentration. Thecalibration graph obtained after theanalysis is shown in Figure 2. Thesample was subsequently analyzed.


Repeatability of the method wascarried out by analyzing five speci-mens of the same sample. To vali-date the technique, reproducibilitywas checked by analyzing two dif-ferent samples after a one-weekinterval. All standards and sampleswere prepared fresh for each analy-sis. The short-term repeatabilityresults of 25 replicates of tin andcopper measurements are shownin Figures 3 and 4, respectively.

The repeatability of bead wirecharacterization is shown in TableIV. The results of the reproducibil-ity study is shown in Table V.

Fig. 1. Variation of peak height with concentration.

Fig. 2. Variation of absorbance with concentration.

47


TABLE IV. Repeatability of Tin Estimation

Replicates Sample Tin Copper Plating Weight(g) (%) (%) Weight (g/kg)

1 10.1353 1.0 99.0 0.492 10.0321 1.0 99.0 0.483 10.1531 1.1 98.9 0.484 10.0262 1.2 98.8 0.48

5 10.2074 1.1 98.9 0.49

Average 1.1 98.9 0.48

Standard deviation 0.07 0.07 0.003

Fig. 4. Short-term repeatability of copper measurements.

Fig. 3. Short-term repeatability of tin measurements.

CONCLUSION

In this study, tire bead wire coat-ing including the estimation of tincontent was quantitatively charac-terized using an atomic absorptionspectrometer in combination witha flow injection analysis system(AAS-FIAS). The repeatability andreproducibility values of both cop-per and tin estimation were foundto be acceptable and the methoddescribed appropriate for regularanalytical laboratory use.

ACKNOWLEDGMENT

The authors would like to thankthe Management of HASETRI andJK Tyre for their support to publishthis paper.

Received February 20, 2006.

REFERENCES

1. A. Maeseele and E. Debruyne, Rub-ber Chemistry and Technology 42,613 (1969).

2. W.J. van Ooij, Rubber Chemistry andTechnology 51, 52 (1978).

3. N.L Hewitt, Rubber World, pg. 26(September 1994).

4. PerkinElmer Manual, "AnalyticalMethods for Atomic AbsorptionSpectroscopy," PerkinElmer Lifeand Analytical Instruments, Shel-ton, CT, USA (1994).

5. Vogel’s " Textbook of QuantitativeChemical Analysis," 5th edition,ELBS, Longman, pg. 805 (1989).

6. PerkinElmer Manual, "Flow InjectionMercury/Hydride Analysis – Rec-ommended Analytical Conditionsand General Information,"PerkinElmer Life and AnalyticalInstruments, Shelton, CT, USA(1994).

7. J. Tabilo and B.D. Nelson, Tyre Tech-nology International, AnnualReview, pg. 72 (1997).

8. ASTM D2969, Standard Test Methodfor Steel Tire Cords, pg. 769(2002).

9. Sunil Jai Kumar and N. N. Meeravali,At. Spectrosc. 17, 27 (1996).

TABLE V. Reproducibility

Sample ID Parameters Day 1 Day 2

Sample 1 Sn (%) 1.8 2.0Cu (%) 98.2 98.0Plating Weight (g/kg) 0.48 0.48

Sample 2 Sn (%) 1.2 1.5Cu (%) 98.8 98.5Plating Weight (g/kg) 0.48 0.48


*Corresponding author. E-mail: [email protected]: +59826013724 Ext. 368Fax: +59826018554

Concentrations of As, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg,Mn, Mo, Na, Ni, Pb, and Zn in Uruguayan RiceDetermined by Atomic Absorption Spectrometry

*Mario Rivero-Huguet, Raquel Huertas, Lorena Francini, Liliana Vila, and Elena DarréDepartment of Atomic Spectrometry

Laboratorio Tecnológico del Uruguay, LATU Avda. Italia 6201 CP11500, Montevideo, Uruguay

ABSTRACT

The United Nations GeneralAssembly declared the year 2004the International Year of Rice andthe concept "Rice is Life". Thelargest nutritional problemsoccurring globally are protein-energy malnutrition, and Ca, Fe,I, Zn, and vitamin A deficiency.

In this report, 49 rice samples(Oryza sativa L.) were digestedby dry ashing in order to deter-mine As, Cd, Cr, and Pb by ETA-AAS; while Ca, Co, Cu, Fe, K, Mg,Mo, Mn, Na, Ni, and Zn weredetermined by FAAS; and Hg byCV-AAS using microwave-assisteddecomposition. The followingconcentration ranges wereobtained for Ca (9.1–15 mg/100 g),Cd (2.30–4.12 µg/kg), Co (41–60µg/kg), Cu (1.33–180 mg/kg), Fe(4.41–7.15 mg/kg), K (167–217mg/100 g), Mg (45–121 mg/100 g),

INTRODUCTION

On December 16, 2002, theUnited Nations General Assembly(UNGA) declared the year 2004 theInternational Year of Rice (IYR) andthe concept "Rice is Life". In declar-ing IYR, UNGA recognized that riceis the primary food source for morethan half of the world’s population,and that enhancing the sustainabil-ity and productivity of rice-basedproduction systems will require thecommitment of many parts of soci-ety as well as government and inter-governmental action (1).

Rice (Oryza sativa L.) is consideredthe main staple food for several

countries (Myanmar, Lao People’sDemocratic Republic, Vietnam,Cambodia, Bangladesh, Indonesia,Thailand, Philippines, Nepal, P.R.China) and is a major source ofnutrients. In developing countries,rice accounts for 715 kcal/capita/day;27% of dietary energy supply, 20%of dietary protein, and 3% of dietaryfat. However, while rice providesa substantial amount of dietaryenergy, it has an incomplete aminoacid profile and contains limitedamounts of essential micronutrients.Today, more than 2,000 millionpeople suffer from micronutrientmalnutrition (Ca, Fe, I, Zn, and vita-min A deficiency). Malnutritionreduces the productivity of adults,reduces children’s mental and phys-ical development, and leads to pre-mature death, particularly for

women and children. If food sys-tems do not provide sufficientquantities and enough diversity offood on a continuous basis, malnu-trition will ensue particularly forthe poor, and their health and wel-fare will deteriorate. Nutritionalconsiderations, therefore, wereessential to the International Yearof Rice and the concept that Rice isLife (2–5).

Currently, studies are alsofocused on the improvement of thenutritional quality by geneticmanipulation and the fortificationof rice.

Since rice is a staple food con-sumed worldwide, its nutritional vs.toxic composition is of specialinterest.

Mo (0.52–0.97 mg/kg), Mn (5.45–25.4 mg/kg), Na (0.95–2.50 mg/100g), Ni (0.53–0.72 mg/kg), and Zn (5.86–12.6 mg/kg).

Mean recoveries of elementsfrom fortified rice were: 87±12%for As, 95.3±8.9% for Ca,106.2±7.7% for Cd, 103.3±6.5%for Co, 89.4±8.1% for Cr,99.3±4.6% for Cu, 103±10% forFe, 96.3±9.3% for Hg, 95.4±12%for K, 98.3±8.0% for Mg,93.4±7.8% for Mo, 95.3±9.9% forMn, 89±12% for Na, 90.3±9.7%for Ni, 91.2±5.5% for Pb and92.0±9.4% for Zn. The concen-trations of the minerals andmicroelements studied fall withinthe typical range of rice grownaround the world. Potassium wasthe most abundant mineral, fol-lowed by Mg and Ca; amongmicroelements, the concentra-tions of Cu, Fe, Mo, Mn, Na, andZn in rice were outstanding.

It was also found that themilling process highly affects theK, Mg, Mn, Na, and Zn concentra-tions, while it has little influenceon Ca, Co, Cu, and Fe. On theother hand, there is a loss of Ca,Fe, and Mn during the parboilingprocess.

Recent studies have shownthe potential to exploit thegenetic variation of rice seedswith regard to the concentrationof some minerals (Ca, Fe, Zn,etc.) without affecting yield oradding new traits.

All rice samples tested showedlower levels of As, Cd, Hg, and Pbin comparison to the maximumlimit permitted by governmentorganizations. Thus, theconsumption of Uruguayan ricepresents no health threat regard-ing the concentration of toxicelements such as As, Cd, Cr, Hg,and Pb.

49


food contamination. Intake of rela-tively low doses of these elementsover a long period can lead to mal-function of organs and chronic toxi-city. Determination of base-linelevels of Cd, Pb, Hg, and As in agri-cultural and horticultural crops isnecessary to evaluate their toxico-logical significance and to establishaction levels (8). These potentiallytoxic metals can be present in theenvironment from both natural andanthropogenic sources and enterthe human body via the two mainroutes of ingestion and inhalation.The latter pathway is of minorimportance except for industrialworkers and people living close toemission sources. For the generalpopulation, ingestion is the majorroute of intake with food being themain contributor (9,10).

Cadmium

In the case of Cd, food serves asthe major source for the generalpopulation. Concentrations of Cdin foods depend on soil contamina-tion and the rate of uptake byplants from the soil. It is wellknown that among the cerealcrops, Cd uptake from soil by riceis high and, therefore, rice is amajor source of cadmium toxicityin rice-consuming countries. Approx-imately 40–60% of Cd intake inJapan comes from rice alone. Thus,rice serves as an index for environ-mental monitoring of Cd in rice-eat-ing countries. However, the dietaryintake of Cd from rice depends onthe consumption level which variesfor different occupational groups (11).

Chromium

Chromium is an essentialelement for the carbohydrate, cho-lesterol, and protein metabolism,while Cr toxicity depends on themetal’s chemical form. Cr(VI) com-pounds show a toxic, mutagenic,and even carcinogenic character.Cr(III), which is the mostfrequently found form in foodsand beverages, has low toxicity.Although humans can absorb Cr

by inhalation or dermal contact, Crintake through diet is the primaryroute of entry into humans (12).

Arsenic

Arsenic toxicity also depends onthe chemical form of the element,with As(III) being the most potent,followed by As(V), then monomethyl-arsenate, and finally dimethylarsen-ate. When ingested, inorganicarsenic may cause acute or chronictoxicity. Epidemiological evidencesupports the conclusion that As isa human carcinogen associatedmainly with lung, skin, and bladdercancer. These facts show the needfor determining As concentrationsin food (13).

Lead

Lead is a non-essential elementthat is toxic to biota at elevatedconcentrations. In nature, Pb com-monly occurs in its monovalent(Pb+1), divalent (Pb+2), and tetrava-lent (Pb+4) states, with divalent Pbbeing the most common. Lead canalso form organometallic compounds,such as tetraethyllead, which wasthe largest single source of Pb cont-amination into the atmosphere inUruguay prior to January 2004,when it was used as an antiknockadditive in gasoline. Studies haveshown the carcinogenicity of Pb inthe gastrointestinal tract, respira-tory system, and kidneys (13).

Mercury

Mercury occurs in differentforms such as mercury 0 (liquid andvapor), inorganic mercury (I) and(II), alkylmercury, and phenylmer-cury. The antimicrobial actions ofHg have also been known for sometime. Organomercurials, particu-larly the alkyl- and arylmercurycompounds, are more active as bac-tericides or fungicides than theinorganic salts. The poisonousproperties of Hg have been knownthroughout recorded history andare reported in ancient oriental andRoman literature (13).

Uruguay has emerged as amedium-size rice producer and isLatin America’s major riceexporter, now listed as one of theworld’s top ten. Although it is agreat producer of rice, dietaryintake surveys in Uruguay list a con-sumption of 10 kg per capita/year,lower than the consumption inAsia (81 kg/year), but higher thanthat in Europe (3 kg/year), and farbelow Brazil which is the majorconsumer in the region with a con-sumption per capita of 39 kg/year(6,7).

Genetic factors, soil and weatherconditions, and the use of fertilizersaffect the final level of mineral andcontaminant components in rice.The human food chain is linkedthrough vegetable, fruit, and tuberconsumption to the nature of thesoil which supplies, for example,the mineral ions. These are in turnderived from the parent rock andfrom decaying plant and animalresidues.

On the other hand, many indus-trialized processes give rise to envi-ronmental problems with increasedlevels of contamination from ele-ments such as As, Cr, Hg, and Pbwhich can have profoundly delete-rious effects on health. The jointFAO/WHO (Food and AgricultureOrganization/World Health Organi-zation) requires detailed informa-tion on the concentration levels ofelements in agricultural crops toassess the toxicological and nutri-tional significance of human andanimal intake of these elements.The normal "background" concen-trations of toxic elements, such asPb, Cd, Cr, and As, must be knownto develop limitations on the intakeof these elements from foods. Infor-mation on background levels pro-vides guidance in evaluating theeffect of soil additives, such asphosphatic fertilizers and sewagesludge (containing Pb, Cd, Cr, andAs), as well as the effect ofcommercial food handling and pro-cessing steps which can result in

50

Although the toxic effects of allof these elements have been knownfor a long time, studies are still on-going to gain a better understand-ing of their effects within thebiosystems.

Atomic Absorption Techniquesfor Metals Determination inRice

For the determination of metalions in rice, atomic absorptionspectrometry (AAS) is thetechnique most widely usedbecause of its versatility, precision,and accuracy. AAS determinationsare usually made by flame atomicabsorption spectrometry (FAAS)when metal concentrations arehigh enough (Cu, Zn, Fe, Ca, Mg, K,Na, Mn, Mo, Ni, and Co). For ele-ments with low limits of detection(As, Cd, Cr, and Pb), the more sen-sitive electrothermal atomizationatomic absorption spectrometry(ETA-AAS) technique is used, whilecold vapor AAS (CV-AAS) is used forHg determinations..

Sample Digestion

Prior to analysis, rice samplesneed to be brought into solution.The two most widely usedtechniques are based on dry ashingor wet digestion. Both techniqueshave advantages as well as limita-tions. The choice of techniqueshould be based on the needs ofthe specific user. Dry ashing ischeap and provides good detectionlimits, but is time-consuming (stricttemperature control should beobserved to avoid metal losses) andsensitive to contamination.

Wet digestion is quick, requireslittle attendance, and is normallynot as sensitive to contamination asdry ashing, but is labor-intensiveand usually results in rather dilutesolutions.

Since the introduction ofmicrowave energy for wet diges-tion in 1975, microwave–assisteddecomposition has been widelyemployed and numerous applica-tions have been described both foropen-vessel and closed-vessel diges-tions. Closed-vessel digestion hasseveral advantages over open-vesseldigestion: smaller quantities ofreagent (no evaporation), less cont-amination, and a higher reactionrate. The most commonly used aciddigestion of biological materials isnitric acid, but various differentacid mixtures have also been used(14,15).

The aim of this study was theanalysis for the first time in

Uruguay of four different forms ofUruguayan rice (brown, parboiledbrown, milled, and parboiledmilled) for the determination of As,Cd, Cr, and Pb by ETA-AAS in sam-ples digested by dry ashing. How-ever, microwave-assisted decom-position of the samples was usedfor the determination of Ca, Co, Cu,Fe, K, Mg, Mo, Mn, Na, Ni, and Znby FAAS, and Hg by CV-AAS.

EXPERIMENTAL

Instrumentation

A Perkin-Elmer® Model 5000atomic absorption spectrometerwas used, equipped with an HGA®-500 graphite furnace, MHS-10hydride generation system, anddeuterium background correction(PerkinElmer Life and AnalyticalSciences, Shelton, CT, USA). Thegraphite furnace and flame settingsare listed in Tables I and II, respec-tively.

TABLE IInstrumental Conditions and Furnace Program

for the Determination of As, Cd, Cr, and Pb

As Cd Cr Pb

Wavelength (nm) 193.7 228.8 357.9 283.3Spectral Bandwidth (nm) 0.7 0.7 0.7 0.7Drying (°C) 110 110 110 110Ashing (°C) 900 250 1200 450Atomization (°C) 2700 2100 2700 2300Cleaning (°C) 2700 2700 2700 2700Sample Injection (µL) 50 10 50 50Background Correction D2 Lamp D2 Lamp D2 Lamp D2 LampMeasurement Mode Peak Area/ Peak Area/ Peak Area/ Peak Area/

Height Height Height Height

TABLE II. Instrumental Parameters for Flame AAS Determination

Caa Co Cu Fe Kb Mga Mo Mn Nab Ni Zn

λ (nm) 422.7 240.7 324.8 248.3 769.9 285.2 313.3 279.5 589.0 231.1 213.9

H Slit (nm) 0.7 0.7 0.7 0.2 1.4 0.7 0.7 0.7 0.7 0.2 0.7

Flame Type AAOF AAOF AAOF AAOF AAOF AAOF NOARF AAOF AAOF AAOF AAOF

Range (mg/L) 0.2–5.0 0.02–1.0 0.003–1.0 0.1–5.0 2.0–30 0.2–4.0 0.2–5.0 0.01–1.0 1.0–8.0 0.1–2.0 0.004–1.0

BG D2 Lamp D2 Lamp D2 Lamp D2 Lamp

AAOF=Air-acetylene oxidizing flame, NOARF=Nitrous oxide-acetylene reducing flame, BG= Background correction.a 0.1% La2O3 was used to control the depression of the signal caused by some elements.b 0.1% CsCl was used as ionization buffer.

51


Rice Samples

Forty-nine commercial samplesof long grain rice (Oryza Sativa L.),cultivated in Uruguay, wereselected for this study. The sampleswere in the form of brown, milled,parboiled milled, and parboiledbrown rice.

Prior to analysis, approximately500 g of sample was ground in ananalytical mill (Analytical Mill A10,Kinematica GAC, Luzern, Switzer-land) and passed through a 1-mmsieve.

Reagents and StandardSolutions

All chemicals used in the samplepreparation and analysis were ofanalytical grade or better.

Standard stock solutions of1000 µg/mL were prepared for As,Ca, Cd, Co, Cr, Cu, Fe, K, Hg, Mg,Mo, Mn, Na, Ni, Pb, and Zn (FlukaChemie GmbH, Germany, andAccuStandard Inc., New Haven, CT,USA), certified by the manufacturerto ± 1% (w/v) and traceable to NIST(National Institute of Standards andTechnology, Gaithersburg, MD,USA).

All solutions were prepared withultrapure water with a specificresistivity of 18 MΩ.cm obtainedby filtering distilled water through aMilli-Q™ Plus purifier system imme-diately before use (Millipore Corpo-ration, Bedford, MA, USA).

Procedure of Sample Decomposition

Dry ashing of the rice samplesfor the determination of As, Cd, Cr,and Pb by ETA-AAS was performedin a multiplace mineralizationblock. The sample (~5 g) wasweighed into a beaker resistant tohigh temperatures and, while con-tinuously shaking the suspension,5 mL ashing aid solution [10% w/vMg(NO3)2 in ethanol] was added.Next, the following temperatureprogram was used: heating at

125°C until nearly dry, then heatingfrom 125–450°C for 2 hours, andholding at 450°C for 12–14 hours.If white ashes are not obtained afterthis temperature cycle, 1 mL HNO3

should be added dropwise (brownfumes should be seen) and then1 mL 30% H2O2 added; afterwards,repeat the heating cycle until whiteashes are observed.

Once white ashes occur fromthe digestion cycle, 10 mL HNO3

(10%) was added and the solutionobtained was homogenized in anultrasonic bath and thencentrifuged.

Duplicate blanks were preparedby adding 5 mL ashing aid solutionfor the digestion procedure.

Microwave oven digestion wasperformed in order to determineCa, Co, Cu, Fe, K, Hg, Mg, Mo, Mn,Na, Ni, and Zn by FAAS. Samples of0.50 g were digested in Tetrafluo-rmethaxil (TFM) Teflon™ vesselswith 5 mL HNO3 using a MilestoneMLS 1200 MEGA microwave oven(Milestone, Italy) and following theheating program listed in Table III.

For the determination of mer-cury by CV-AAS, analyte additionswere performed before digestionand the microwave oven digestionas described above was followed.

Calibration and AccuracyStudies

Direct calibration against acidi-fied standard solutions was carriedout for Ca, Co, Cu, Fe, K, Mg, Mo,Mn, Na, Ni, and Zn and by the stan-dard additions technique for As, Cd,Cr, Hg, and Pb.

To ensure a high level of analyti-cal reliability, recovery studies wereincluded for each batch to estimateanalytical accuracy. The rice sam-ples were spiked with two differentconcentrations of a certified solu-tion (Trace Metal Standard I and III,J.T. Baker Inc., Philipsburg, PA,USA) and then submitted to thedigestion procedure. Each batchalso contained a certified referencematerial (NIST CRM 1568a RiceFlour).

The limits of detection (LOD)obtained in this study are listed inTable III. These values were calcu-lated on the basis of the concentra-tion giving a reading equivalent totwice the standard deviation of theblanks.

The recommendations listed inthe EURACHEM/CITAC guide(EuroAnalytical Chemistry/Co-Oper-ation on International Traceabilityin Analytical Chemistry) were fol-lowed to identify and estimate theindividual contributions to the totaluncertainty of the methodemployed (16).


The metal ion composition ofbrown, milled, parboiled milled,and parboiled brown rice is listedin Table IV and was calculated onthe wet basis to allow a comparisonwith the literature data of the prod-uct when purchased.

The concentrations of all the ele-ments studied (As, Ca, Cd, Co, Cr,Cu, Fe, Hg, K, Mg, Mo, Mn, Na, Ni,Pb, and Zn) in this survey fallwithin the typical range (as can beseen in Table IV) of rice fromaround the world (7,11,17–20).

TABLE IIIHeating Programa Used for theDetermination of Ca, Co, Cu, Fe, Hg, K, Mg, Mo, Mn, Na, Ni,

and Zn

Step Power Time(W) (min)

1 250 12 0 53 250 54 400 5

5 650 5

a Caution: Digestion vessels must becooled for an appropriate time beforeopening in order to avoid burns fromhot and corrosive vapors and to elimi-nate sample loss.

52

tural practices employed in theproduction of rice in Uruguay.

It has been demonstrated thatunder fasting conditions the gas-trointestinal absorption of toxicmetals can be significantlyincreased (13). For instance, Pbabsorption in the human adult is inthe order of about 10–15% (up to50% for children) and under fastingcircumstances it increases up to45%. Studies have also shown thatcertain dietary factors such as milkfasting result in low calcium andvitamin D intake, causing iron defi-ciency and may enhance leadabsorption from the gut (13). Sincerice is considered a main staplefood and a major source of nutrientfor the poor who lack access todiverse foods, strict control shouldbe enforced on the maximum levelof toxic metals allowed in thisimportant crop.

rice grain arsenic. P.N. Williams etal. (24) have recently reported highlevels of As (up to 0.46 mg/kg) inrice from the United States. Thesehigh levels are attributed to the useof As herbicides in cotton fields inthe early to mid-20th century andcurrently some of these areas areused for rice production.

Although the Cr content in riceis not yet regulated in Uruguay, it isimportant to know the Cr levels infoodstuff because of the element’stoxicity. The presence of this metalcan be attributed to the use of stain-less steel equipment or contamina-tion during processing.

The results of our study verifythat the levels of As, Cd, Cr, Hg,and Pb concentrations found incommercially available rice arebelow the legislated limits. Theselow levels confirm the good agricul-

Toxic Elements

All rice samples tested showedlower cadmium and lead concentra-tions than the maximum limit per-mitted by the Codex Alimentarius(0.1 mg/kg and 0.2 mg/kg, respec-tively) (21,22). Uruguayan legisla-tion (23) regulates the levels of Asand Hg in foodstuffs (2.0 mg/kg and0.05 mg/kg, respectively). As canbe observed in Table IV, As and Hgconcentrations found in this workare far below the maximum con-centrations allowed by this regula-tion. These low levels confirm thatthe rice growing areas as well asthe groundwater employed for irri-gation are situated in non-contami-nated sites. It is well known that insome countries, i.e., Bangladesh,irrigation of the paddy fields witharsenic-contaminated groundwaterhas led to arsenic buildup in paddysoil, with subsequent elevations in

TABLE IVMetal Content in Different Forms of Rice Produced in Uruguay

Units Brown Milled Parboiled Parboiled LOD Typical CRM RecoveryBrown Brown Range (%) (%)

As µg/kg < LOD < LOD < LOD < LOD 50 0.01–0.61 87±12

Ca mg/100 g 9.1±1.4 15±1.8 10±1.3 10±1.2 0.010 9–50 99±10 95.3±8.9

Cd µg/kg 3.43±0.68 4.12±0.72 2.30±0.69 3.75±0.66 1.0 <1–230 106.2±7.7

Co µg/kg 57±11 41±15 59.6±8.4 50±10 20 10–100 103.3±6.5

Cr µg/kg < LOD < LOD < LOD < LOD 5.0 <1–100 89.4±8.1

Cu mg/kg 1.80±0.16 1.52±0.16 1.45±0.15 1.33±0.16 0.010 1–6 95.2±8.4 99.3±4.6

Fe mg/kg 7.14±0.64 7.15±0.66 5.90±0.71 4.41±0.77 0.050 2–50 89±11 103±10

Hg µg/kg < LOD < LOD < LOD < LOD 1.0 <0.1–29 96.3±9.3

K mg/100 g 217.3±4.9 167±5.6 210.6±3.7 167.2±5.8 0.10 60–280 99.4±8.4 95.4±5.7

Mg mg/100 g 121±11 46.4±5.5 116±13 45.4±5.9 0.050 20–150 91.3±7.7 98.3±8.0

Mo mg/kg 0.82±0.21 0.97±0.19 0.52±0.16 0.10 0.18–3.0 93.4±7.8

Mn mg/kg 25.4±2.8 6.55±0.85 6.28±0.88 5.45±0.79 0.010 2–36 91±11 95.3±9.9

Na mg/100 g 2.23±0.21 1.20±0.11 0.95±0.11 2.50±0.21 0.40 1.7–34 96.7±7.8 89±12

Ni mg/kg 0.62±0.15 0.72±0.13 0.53±0.10 0.613±0.097 0.020 < 0.2–1.2 90.3±9.7

Pb µg/kg <LOD < LOD < LOD < LOD 5.0 2–70 91.2±5.5

Zn mg/kg 12.6±1.0 6.10±0.73 11.6±1.2 5.86±0.75 0.010 6–28 102.8±4.4 92.0±9.4

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Minerals and Microelements

In addition to determining toxicelements in rice, the level of miner-als and microelements was alsostudied. As listed in Table IV, potas-sium was the most abundant min-eral (217, 211, 167, and 167 mg/kgfor brown, parboiled brown, milledand parboiled milled rice) found inrice, followed by Mg and Ca.Among microelements, the pres-ence of Cu, Fe, Mo, Mn, Na, andZn in rice was outstanding (formilled rice, the values found were1.5 mg/kg Cu, 7.25 mg/kg Fe,0.97 mg/kg Mo, 6.55 mg/kg Mn,1.2 mg/100 g Na, and 6.1 mg/kg Zn).

The concentration of most ele-ments presented in Table IV weresimilar in brown and parboiledbrown rice, except for Mn whichwas higher in brown rice (25.4 and6.28 mg/kg, respectively).

It is generally accepted that asgreater amounts of rice bran areremoved from the grain duringmilling and polishing, morevitamins and minerals are lost. Forall the samples analyzed in thiswork, milled rice shows a signifi-cantly lower content of K, Mg, Mn,Na, and Zn than brown rice. Butthe milling process seems to havelittle influence on the Ca, Co, Cu,Fe, and Ni levels. Similar resultswere reported in rice cultivated inBrazil (7).

The distribution pattern ofmetal ions in the rice grain is notyet clearly known. Some authors(4,7,26–27) point out that micro-elements (Cu, Fe, Mn, and Zn) arelikely to be uniformly distributed inthe grain and macroelements (P, K,Ca, and Mg) seem to be presentchiefly in the external layers of thegrain (aleurone, pericarp). Researchhas shown that element concentra-tions are highly affected by themilling process (25), while othershave found a strong interactionbetween, for instance, iron contentand milling, which suggests that

much of the iron concentration isin the outer layers (aleurone andpericarp) of rice (25).

The effects of rice grain process-ing with regard to the mineral lev-els in the edible product (i.e.,milled and parboiled rice grain) arestill being evaluated. On the otherhand, the parboiling process seemsto have little effect on the mineralcontent in rice (except for Ca, Fe,and Mn) as can be seen in Table IV.The high loss of these minerals wasnot expected. It is assumed that theminerals spread to the external lay-ers of the grain when soaked andsteamed and are subsequentlyremoved by milling. R.J.B. Heine-mannn et al. (7) also observed sig-nificant losses of Ca and Mn in theparboiled milled rice cultivated inthe south of Brazil.

It has been reported thatparboiled rice is of superior nutri-tional value in comparison to milledrice, mainly due to the retention ofminerals and water-soluble vitamins(26). The higher retention ofmicronutrients in parboiled rice hasbeen assigned to their solubilizationand migration to the center of thegrain and their setting during thestarch gelatinization process (26).

Nevertheless, the significance ofthe nutritional benefits of parboiledrice is still arguable, mostly due tothe lack of uniform commercialprocesses applied in different coun-tries. It is believed that the reten-tion pattern of some minerals is theresult of the interaction of differentfactors such as mineral location inthe grain and their solubility duringsoaking, different ratios of migra-tion, as well as variations in thehydrothermal process and millingresistance of the parboiled grain.Further studies need to be carriedout to achieve a more completeunderstanding of mineral retention.

Staples are generally not consid-ered an important source of miner-als in the diet. However, because

rice is an important staple food, anyincrease in mineral concentrationmight well have a significant effecton human nutrition and health.

Genetic Alteration of Rice

Currently, several research insti-tutions are working toward improv-ing the nutrient content throughgreater utilization of rice geneticresources. Since 1995, researchersat the International Research RiceInstitute (IRRI), Manila, Philippines,have been evaluating the geneticvariability of Fe and Zn concentra-tion in rice. Screening tests run toidentify germplasm with high ironand zinc found a statistically signifi-cant correlation with phytate as theiron and zinc concentrationsincreased, so did phyate. But thenutritional implications with regardto the bioavailability of theincreased Fe and Zn content coun-terbalanced with increased phytateare still under investigation (5,27).

Scientific programs are alsofocused on increasing the iron con-tent of the grain and the element’sbioavailability once ingested. Atwo-fold increase was achieved bytransferring the coding sequence offerritin from a bean (Phaseolus vul-garis) into rice endosperm (28). Atthe same time, to increase thebioavailability of iron, β-carotenewas inserted, and the phytate genefrom Aspergillus fumigatus wasinserted to break down phytic acidwhich binds iron (5).

Yet, the controversy and ques-tions remain concerning theadvances in biotechnology. Specifi-cally, there is uncertainty regardingthe degree to which micronutrientlevels can be increased, as well asthe effect of this increase on yield,disease resistance, and palatability.The opportunities created throughgenetic alteration of the ricegenome also create new impera-tives for biosafety.

54

Enrichment and Fortificationof Rice

As we mentioned previously, thecomplete milling and polishing thatconverts brown rice to white ricereduces the amount of vitamins andminerals in the final product. Stud-ies have shown that riceenrichment and fortification tech-niques restore the levels of vitaminsand minerals in milled rice at firstremoved from the grain during themilling process (7). Enriched rice iscoated in a protective materialwhich helps to prevent nutrientloss due to washing. The success ofvarious fortification strategies, par-ticularly those involving fortifica-tion with iron, is mixed. Thefortification of foods with ironremains technically complex.Those iron compounds with thegreatest bioavailability (ferrous sul-phate and ferrous fumarate) signifi-cantly alter the palatability of food,whereas large declines in theuptake of iron are seen when amore palatable iron compound(elemental iron or ferricorthophophate) is used (28).

Fortified rice holds great poten-tial in reducing micronutrient defi-ciencies. Its success, however, willdepend on making the enrichedrice available, affordable, and palat-able for the consumer (29).

CONCLUSION

Since rice is a staple food con-sumed worldwide, its nutritional vs.toxic composition is of specialinterest. Hence, the knowledge ofthe metal ion concentrations in riceis crucial to determine the amountof toxic elements and nutrientsconsumed with rice. It also helps tobuild up nutrition tables widelyused to calculate food nutrientintake.

The report of the 16 elements(As, Ca, Cd, Co, Cr, Cu, Fe, Hg, K,Mg, Mo, Mn, Na, Ni, Pb and Zn) inUruguayan rice here presented hasno precedent in the country. SinceUruguay is one of the world’s topten rice producers, the awarenessof toxic elements (As, Cd, Cr, Hg,and Pb) and mineral and microele-ments is vital for the trade market.This type of survey is very impor-tant since the determination of ele-ments in rice may help prestigiousrice producers from counterfeitrice and permit source confirma-tion for government certification.

The methodology employed forthe determination of As, Cd, Cr,and Pb by ETA-AAS, and Ca, Co, Cu,Fe, K, Mg, Mo, Mn, Na, Ni, and Znby FAAS, and Hg by CVAAS wasappropriate and the resultsobtained were in general asexpected.

The data generated show thatthe concentrations of the toxic ele-ments As, Cd, Pb, and Hg in therice samples tested are significantlylower than the maximum tolerancelimits established by internationalorganizations. These low levels con-firm the good production practicesemployed in Uruguay and showsthat the consumption of this ricepresents no health threat.

In addition, the concentrationsof all the elements determined inthis survey fall within the rangetypical of rice grown around theworld.

Received October 7, 2005.

REFERENCES

1. FAO, 2004,http://www.fao.org/rice2004/

2. R.M. Welch, G.F. Combs, Jr., and J.M.Duxbury, Issues Sci. Tech. 14, 50(1997).

3. FAOSTAT, 2001, FAO statistical data-bases. Retrieved 2 May 2001 fromthe World Wide Web:http://apps.fao.org.

4. R.M. Welch, Plant and Soil 247, 83(2002).

5. G. Kennedy and B. Burlingame, FoodChemistry 4, 589 (2003).

6. International Rice Research Institute(IRRI), 2004 Rice supply/utilizationbalances, by country andgeographical region, selectedyears,http://www.irri.org/science/rices-tat/

7. R.J.B. Heinemann, P.L. Fagundes,E.A. Pinto, M.V.C. Penteado, andU.M. Lanfer-Marquez, J. Food Com-position and Analysis 4, 287(2005).

8. D. Wiersma, B.J. van Goor, and N.van der Veen, J. Agricultural andFood Chem. 34, 1067 (1986).

9. J. Sherlock, Experimentia 40, 153(1984).

10. J. Sherlock, EnvironmentalGeochemistry and Health 9, 43(1987).

11. R. Srikanth, D. Ramana, and V. Rao,Food Additives and Contaminants5, 695 (1995).

12. M.E. Rivero-Huguet, At. Spectrosc.4, 177 (2004).

13. E. Merian, 1991, Metals and TheirCompounds in the Environment,Occurrence, Analysis and Biologi-cal Relevance (VCH Verlagsgesell-schaft mbH, Weinheim, VCHPublishers, Inc., New York).

14. A. Morales-Rubio, A. Salvador, M.de la Guardia, and R. Ros, At.Spectrosc. 11(1), 8 (1993).

15. L. Jorhem and J. Engman, J. AOACInternational 5, 1189 (2000).

55


16. S.L.R. Ellison, M. Rösslein, and A.Williams (Eds.), EURACHEM/CITACGuide, Quantifying Uncertainty inAnalytical Measurement, 2nd Edi-tion, EURACHEM, Berlin, Germany,2000, Internet version:http://www.eurachem.bam.de/guides/quam2.pdf.

17. K.A. Wolnik, F.L. Fricke, S.G. Capar,M.W. Meyer, R.D. Satzger, E. Bon-nin, and C.M. Gaston, J. Agricul-tural and Food Chemistry 33, 807(1985).

18. X. Ji and J. Ren, At. Spectrom. 3,224 (1995).

19. M.E. Soares, M.L. Bastos, F.Carvalho, and M. Ferreira, At. Spec-trosc. 13, 149 (1995).

20. USDA, U.S. Department of Agricul-ture, Agricultural Research Service,USDA Nutrient Database for Stan-dard Reference (2004).

21. CODEX ALIMENTARIUS, 2001,Codex Maximum Level for Cd inCereals, Pulses and LegumesCAC/GL 39-2001.

22. CODEX ALIMENTARIUS, 2001,Codex Maximum Level for Pb inFoods, Codex Stan 230-2001, Rev.12003.

23. Ministerio de Salud Pública de laRepública Oriental del Uruguay2001, Reglamento BromatológicoNacional, Decreto N° 315/994,Rev. 2001.

24. P.N. Williams, A.H. Price, A. Raab,S.A. Hossain, J. Feldmann, and A.A.Meharg, Env. Science and Technol-ogy 39, 5531 (2005).

25. G.B. Gregorio, Journal of Nutrition3, 500 (2002).

26. G.B. Gregorio, D. Senadhira, T.Htut, and R.D. Graham, Food Nutri-tion Bulletin 21, 382 (2000).

28. G. Kennedy, B. Burlingame, and N.Nguyen, International Rice Com-mission Newsletter, 202-206(2002).

29. L. Fresco, J. Food Composition andAnalysis 4, 249 (2005).


*Corresponding author.E-mails: [email protected],[email protected]: + 58-261-759 81 54Fax: + 58 261-759 81 25

Flame AAS Determination of Total Lead in SoilSediments Near Highways in Maracaibo City, Venezuela

*Víctor A. Granadilloa, Milkelly del V. Navaa, Denny R. Fernándeza, Aracelis del C. Vásqueza,

Blanca Semprúna, Maigualida Hernándeza, and Marinela Colinab

a Laboratorio de Instrumentación Analítica, Departamento de Química, Facultad Experimental de Ciencias, Universidad del Zulia, Maracaibo 4011, Venezuela

b Laboratorio de Química Ambiental, Departamento de Química, Facultad Experimental de Ciencias, Universidad del Zulia, Maracaibo 4011, Venezuela

INTRODUCTION

During the last decades, anthro-pogenic activities have increasedthe dispersion and accumulation ofhighly toxic compounds into theenvironment (1–4). Institutions andgovernmental agencies that investi-gate, legislate, and control environ-mental problems around the worldhave demonstrated that this type ofcontamination affects humanhealth, particularly with regard todegenerative illnesses (3,5). Differ-ent international studies are avail-able which have indexed the casesinvolving metals such as mercury,arsenic, cadmium, lead, aluminum,chromium, vanadium, and nickel(3,5,6).

Greater than 90% of the lead(Pb) contamination in Venezuelancities is due to the combustion ofgasoline with high concentrationsof lead antiknock (tetraethyl lead)used in automobiles [>300 mg/L Pb(7)]. The high automobile density(>10,000 cars/day) in MaracaiboCity, Venezuela, is responsible forthe high levels of environ- mentalPb (up to 3000 mg/Kg Pb) (8). Forthis reason, the populations in ourcities are exposed to the toxicactions of Pb by ingestion of pol-luted foods or by inhalation of leadcompounds.

The economical growth of theZulia State in particular is sustainedby industries such as petrol compa-nies, petrochemistry, polymers, andmetal mechanics, which use a great

variety of metals and chemical com-pounds for obtaining theirproducts. Most of these compoundscan be toxic when they exceed thesafe threshold values for the envi-ronment and the human organism(7–9). Over the last 50 years, Mara-caibo City has experienced drasticurban growth, changing from asmall city of 250,000 inhabitants toa metropolis of more than2,000,000 (8). The subsequenturban activities resulted in anincrease in automobile traffic and,consequently, a high consumptionof leaded gasoline. Nowadays, highlevels of lead are found in the urbansediment of the main avenues andstreets of this city (≥ 3000 mg/KgPb) (7,8).

In early 1999, the Venezuelangovernment implemented the saleof unleaded gasoline in order toreduce the high Pb contaminationof the country’s main urban areas.However, due to the fall of purchas-ing power of the Venezuelanpopulation, use of cheaper leadedgasoline persists (8).

It is important to point out thatPb is one of the most importanttoxic metals studied around theworld due to its dispersion throughthe oceans, the high concentrationsfound in the urban environment,and the pathological effects onhuman beings (10). Lead in combi-nation with other heavy metalssuch as mercury, cadmium, and alu-minum forms a group of highlytoxic elements that do not performany metabolic functions in tissues,organs, and fluids of the organism(10). Whole blood lead concentra-tions above 400 mg/L are danger-ous, damaging organs and vitalsystems, even causing death (11).

ABSTRACT

An FAAS-based method previ-ously analytically developed forthe total determination of Pb wasemployed to establish the Pb lev-els in 76 urban sediment samplescollected near four highways inMaracaibo City, Venezuela. Theproposed method is simple, usescontrolled heating at 60–70°C,atmospheric pressure, anddiluted acid media during treat-ment of the solid samples. Thesefacts avoided the loss of Pb byvolatility, which generally isresponsible for experimentalerrors in the estimation of thereal concentration of this analyte.

Accuracy of the FAAS methodwas evaluated by analyzing threecertified materials, obtaining amean relative error of 1.08%between the certified and experi-mental values. The average preci-sion expressed as RSD was1.93%. The detection limit (3σ)and characteristic concentrationwere 0.25 and 0.43 mg/L Pb,respectively.

The Pb mean concentrations(± 1SD, mg/kg) of the differentsampling points were: A-5: 3,385± 2,503; A-L: 3,554 ± 1,712; C-1:2,252 ± 2,224; and C-2: 2,457 ±1,666. These concentrationswere approximately 300% higherthan the values found in a studycarried out between 1984 and1985. High concentrations of Pbfound in Maracaibo City arerelated to the high automobiletraffic, which consumes exorbi-tant volumes of leaded gasoline.

The developed analyticalmethod for the FAAS-determina-tion of Pb in urban sediment sam-ples was accurate, precise, andfree of interferences, constitutingan analytical tool for the evalua-tion of this toxic element in envi-ronmental samples.

57


The clinical risk for children hasincreased due to Pb intoxicationand produces irreversible healthdamage (11).

Previous works (12–16) reportedthe concentrations of depositedtotal lead in heavy traffic andschool areas as well as thegeographical distribution of Pb inthe urban areas of Maracaibo City.In these investigations, relativelyhigh levels of deposited total leadwere found in the urban areas. The largest concentrations wereobtained for the central (1517±396 mg/kg Pb) and northern (1954±555 mg/kg Pb)regions (16). In general, the con-centrations of Pb in the urban sedi-ment samples were between 220and 1954 mg/kg (16).

Analytical methods for Pb deter-mination have been developed dueto the environmental relevance ofthis metal (7,8,17–19). Flameatomic absorption spectrometry(FAAS) is one of the techniquesmore widely used for this type ofanalysis because it providesadequate detection limits (i.e., inmg/L), sensitivity, and selectivity(8,17–19). In order to provide reli-able and reproducible results, FAASrequires appropriate pretreatmentprocedures of the solid environ-mental samples due to the volatilityof Pb using acid digestion withatmospheric pressure and controlledtemperatures (60–70°C) (7,8).

In this work, we determined thePb levels of 76 urban sediment sam-ples collected near four highwaysof Maracaibo City using an FAAS-based method previously analyti-cally developed (15–17). Thenovelty of the proposed method isbased on the control of Pb loss dur-ing digestion of the solid samples.This analytical aspect can beignored by some chemists but it isgenerally responsible for importantexperimental errors in the estima-tion of the real concentration ofthis analyte.

EXPERIMENTAL

Instrumentation

The determination of lead wascarried out using a PerkinElmer®Model 460 flame atomic absorptionspectrometer (PerkinElmer Life andAnalytical Sciences, Shelton, CT,USA). The operating parameters ofthe instrument are shown in Table I.

Reagents and StandardSolutions

All chemical reagents used wereof analytical grade. The concen-trated solution of lead (~1000 mg/L)was prepared using a concentratedTitrisol® solution of Pb (Merck,Darmstadt, Germany). The standardsolutions for the calibration curveswere performed daily by serialdilution of the concentrated solu-tion of the analyte in 0.01 M nitricacid (Merck). For each sample,the lead concentrations were deter-mined using 7-point calibrationcurves, and aqueous standard solu-tions of 0.0, 5.0, 10.0, 15.0, 25.0,35.0, and 50.0 mg/L Pb.

The evaluation of the accuracyof the method was carried outthrough the analysis of standardreference materials: Buffalo RiverSediment NIST SRM 2704 (NationalInstitute of Standards and Technol-ogy, Gaithersburg, MD, USA), andtwo certified reference materials:Pond Sediment NIES No. 2 andVehicle Exhaust Particulates NIESNo. 8 (National Institute for Envi-

ronmental Studies, Ibaraki, Japan).In addition, the accuracy of theemployed method was evaluatedthrough recovery studies of thespiked samples.

Sampling of Urban VialSediment

The urban sediment samples(USS, n = 76) were collected nearfour heavily traveled highways inMaracaibo City: "Av. 5 de Julio"(A-5), "Av. La Limpia" (A-L),"Circunvalación 1" (C-1), and"Circunvalación 2" (C-2). The sam-ples were obtained in March andMay 2001 from areas adjacent tothe main roads during the regionaldry season with high temperaturesand winds of appreciable magni-tude. Samples ranging from500–1000 g were obtained in tripli-cate taken from the surface of theterrestrial layer and placed intopolypropylene bags with hermeticseals. Along each avenue, differentsamples were collected; each sam-pling point was located about 200or 300 m from the next, dependingon the road longitude. The numberof samples collected was 21, 22, 14,and 19 for A-5, A-L, C-1, and C-2,respectively.

Treatment of the Samples

Each sample was sifted to elimi-nate strange bodies (i.e., stones,glass pieces and/or wood, witheredleaves from trees, cigarette butts,etc.) and then size-grain homoge-nized. Once sifted, the sampleswere dried in a convection oven forgraveness at 60±2°C. Temperaturecontrol was necessary to avoid lossdue to volatilization of the metal.The volatile character is a well-known physicochemical propertyof this analyte. This drying step wasalso applied to the three certifiedtest sample materials to evaluatethe accuracy of the analyticalmethod.

The dry solid samples were pre-viously acid-digested in triplicateemploying atmospheric pressure

TABLE IOperating Parameters for

Flame AA Determination ofTotal Pb in Urban Sediment

Samples

Parameter Value

Wavelength 283.3 nmSpectral bandwidth 0.7 nmType lamp Hollow cathodeLamp current 8 mA

Gases in the flame Air-acetylene

58

and controlled heating by convec-tion at temperature intervalsbetween 60 and 70°C (7,8). A2.0000-g dry soil sediment sample(or a predetermined weight of thecertified material) and 4 mL of con-centrated nitric acid free of Pbwere placed in a 250-mL beakerwith a clock glass and digested for10 min at 65 ± 5°C. The sampleswere then diluted with 25 mL ofdeionized water and digested for50 min; thus, the total digestiontime for each sample was 1 hour.The resulting solution was trans-ferred to a 25-mL volumetric flaskand brought to volume with deion-ized water. The representativeblank solutions were preparedusing the same digestion conditions.


Analytical Figures of Merit

The linearity of the calibrationcurve was obtained up to 50 mg/LPb. For each sample, the Pb con-centration was determined using7-point calibration curves withaqueous standard solutions of 0.0,5.0, 10.0, 15.0, 25.0, 35.0, and50.0 mg/L Pb, as shown in Figure1(A). Absorbance (A) was found tobe linearly related to the concentra-tion (c) using the equation: A =0.0093c + 0.0025 (the standarddeviations of intercept and slopewere 0.00002 and 0.00008, respec-tively; r = 0.9999, p <0.001).These graphs allowed the analyticaldetermination of Pb in urbansediment samples.

The within- and between-runprecision of the method proposedwas evaluated by determining Pb inthree real samples of 20-fold dilutedsediment (Table II). Three aliquotsof each real sample were analyzed(five runs each) using the instru-mental and operating parametersprovided in Table I. The obtainedresults show an RSD average of<5% (~1.93%), resulting in meanRSD values of 2.20 and 1.66%,respectively. These results demon-

strate that the analytical methoddeveloped for Pb was adequatelyreproducible and appropriate forthis type of analysis.

The accuracy of the proposedspectrometric method for the deter-mination of Pb was verified by ana-lyzing three certified materials:Buffalo River Sediment NIST SRM2704 (NIST, Gaithersburg, MD,USA), Pond Sediment NIES No. 8(NIES, Ibaraki, Japan), and VehicleExhaust Particulates NIES No. 8(NIES; Ibaraki, Japan) (Table III).The results indicate that the mea-sured concentrations obtainedusing the FAAS-based method forPb did not differ significantly withrespect to certified values. It veri-fies the appropriate accuracy in theanalytical determination of themetal considered in this investiga-tion. This parameter was also evalu-ated by means of recovery studiesresulting in an average Pb recoveryof 100.8 ± 3.5% (Table IV), demon-strating the appropriate reliabilityof the proposed method.

Due to the lack of a backgroundcorrector in the spectrometer usedfor the determination of Pb in thedigested sediment samples, a spec-tral interference study could not becarried out. It is important to notethat due to the digestion processcarried out with the samples, anacid matrix comparable to the sim-ple matrix of the aqueous solutionswas obtained. In consequence, thedigested certified samples, havinga matrix similar to the real samples,showed a statistical correspondencebetween the certified and experi-mental metal concentrations.Therefore, it can be assumed witha high grade of reliability that spec-tral interferences did not exist.

The study of non-spectral inter-ferences was carried out by com-paring the slopes of the workingcurves with those obtained by themethod of standard addition. Theslopes obtained for the calibrationand standard addition curves were

0.0093 (r = 0.9999, p < 0.001), and0.0092 (r = 0.9996, p < 0.05),respectively [Figure 1(B)]. Themean relative error between slopeswas 1.08%, which indicates anabsolute parallelism. These resultsimplied the absence of non-spectralinterferences in the FAAS analysesfor the developed Pb method andpermitted the use of either the calibra-tion graphs or the standard additionsmethods for metal quantification.

The detection limit and the char-acteristic concentration were 0.25mg Pb/L and 0.43 mg Pb/L, respec-tively. The experimental value forthe characteristic concentrationwas near to the theoretical valuereported for the instrument: 0.55mg Pb/L (20). This similarity wasexpected because the characteristicconcentration is independent of thematrix under study, if the analysesusing FAAS are free from interfer-ences. In consequence, the experi-mental values indicated theappropriate sensitivity of thedescribed methodology; therefore,these findings allowed the determi-nation of lead in the environmentalsamples listed.

Levels of Pb in Urban SedimentSamples of Maracaibo City

The concentration of Pb (mean± SD, mg/Kg) in the different sam-ples were: A-5: 3,385 ± 2,503; A-L:3,554 ± 1,712; C-1: 2,252 ± 2,224;and C-2: 2,457 ± 1,666. These highconcentrations found at the studiedavenues have increased when com-pared to the values found in a previ-ous study carried out between 1984and 1985 (i.e., A-5: 1822 ± 476;A-L: 792 ± 421; C-1: 764 ± 253; andC-2: 758 ± 292) (16,17). These highPb concentrations in MaracaiboCity are associated with the hightraffic, which consumes high vol-umes of leaded gasoline. Table Vshows the average Pb concentra-tions in mg/kg at the differentavenues in the two sampling peri-ods in 1985 and 2001. The percent-age increase for Pb concentrationsnear these highways is also indicated.

59


The percentage increase of totallead was high (i.e., between 186and 449%), which is reliable proofof the elevated levels of environ-mental contamination of MaracaiboCity during the last 16 years. Theresidential and commercial build-ings located in the studied areas actas physical barriers, avoiding thefree displacement of lead by meansof wind and thus favoring their sed-imentation (17). It is important tonote that in 1985, the concentra-tion of deposited total lead in the A-5 area was statistically different (p<0.05) with respect to the others;but by 2001, the Pb values werefound to be the same as for the fourareas tested.

CONCLUSION

High concentrations of Pb foundin Maracaibo City, Venezuela, aredue to the high automobile trafficwhere exorbitant volumes ofleaded gasoline are used (about>300 mg/L Pb). The percentageincrease in total lead concentrationreveals the high grade of lead cont-amination of Maracaibo City. Theseconcentrations is a potential healthdanger for the population of thisVenezuelan city.

The developed analyticalmethod for the FAAS determinationof Pb is based on using controlledheating at 60–70°C, atmosphericpressure, and diluted acid media forthe treatment of the solid samples.The method is accurate, precise,and free from interferences. Thisanalytical approach employed inthe treatment of the solid samplesreported in this work would be ofbenefit to other laboratories world-wide.

As a final commentary, theauthors would like to mention thatin August of 2005, the Venezuelangovernment implemented the saleof only the unleaded gasoline. Thisstep will provide new and futurevariants of investigations regardingthis toxic element and allow to

Fig. 1. Calibration curve (A) and study of non-spectral interferences (B) for theFAAS determination of Pb in soil sediments near highways in Maracaibo City(Zulia State, Venezuela), showing adequate parallelism between calibration (l)and standard addition (m) curves.

TABLE IIWithin-run and Between-run Precision Studies for the Determination

of Pb by FAAS in Urban Sediment Samples of Maracaibo City

Sample Mean Conc.a Within-run Between-run(mg/L) SD RSD (%) SD RSD (%)

A-5 3.12 0.02 0.64 0.03 0.96A-L 3.85 0.15 3.90 0.10 2.60C-1 4.44 0.09 2.03 0.07 1.58C-2 5.38 0.12 2.23 0.08 1.49

a In 20-fold diluted sample, prepared in triplicate, and five replicate absorbancereadings.

60

study and evaluate whether or notPb will remain permanently in theurban atmosphere of MaracaiboCity.

ACKNOWLEDGMENTS

This research was partially sup-ported by previous grants from theConsejo de Desarrollo Científico yHumanístico (CONDES) from theUniversidad del Zulia (L.U.Z.), theFondo Nacional de Ciencia y Tec-nología (FONACIT, Venezuela), andLaboratorio de InstrumentaciónAnalítica (L.I.A.-L.U.Z.).

Received March 17, 2005.

REFERENCES

1. K.C. Yu, C.Y. Chang, L.J. Tsai, andS.T. Ho, Water Sci. Technol. 42,193 (2000).

2. M. Shinya, T. Tsuchinaga, M. Kitano,Y. Yamada, and M. Ishikawa,Water Sci. Technol. 42, 201(2000).

3. E.R. Ladich, F.G. Mullicck, and J.A.Centeno, Metal Ions in Biology andMedicine 6, Eds. J.A. Centeno, Ph.Collery, G. Vernet, R.B. Finkelman,H. Gibb, and J.C. Etienne, pp. 3-5,John Libbey Eurotext, Paris,France (2000).

4. K Nath, Presented at the CongresoMundial sobre Contaminación delAire en Países en Vías de Desar-rollo, San José, Costa Rica (1996).

5. D. Jost and E. Schmid, Presented atthe Fourth International Confer-ence on Mercury as a Global Pollu-tant, Hamburg, Germany (1996).

6. L. Piccinini, P. Borella, A. Bargellini,U. Morandi, A. Stefani, V. Arigliano,and P. Davalli, Metal Ions in Biol-ogy and Medicine 5. Eds. Ph.Collery, P. Brätter, V. Negretti deBrätter, L. Khassanova, and J.C.Etienne, pp. 619-623, John LibbeyEurotext, Paris, France (1998).

7. M. Nava, D. Fernández, C. García,and V.A. Granadillo, Presented atthe 52a Convención Anual de laAsoVAC, Barquisimeto, Venezuela(2002).

8. M. del V. Nava, D.R. Fernández, A.del C. Vásquez, M. Colina, and V.A.Granadillo, Presented at the 8thRio Symposium on Atomic Spec-trometry,Rio de Janeiro, Brazil(2004).

9. M.E. Romero, M.B. Bernard, and S.Hde Bautista, Presented at theFourth International Conferenceon Mercury as a Global Pollutant,Hamburg, Germany (1996).

10. K. Groen, R. Nijdam, and A.J.A.MSips, Trace Elements in Man andAnimals - TEMA 9, pp. 206-207,Ottawa, Canada (1997).

11. Z. Kilic, O. Acar, M. Ulasan, and M.Ilim, Food Chem. 76, 107 (2002).

TABLE IIIAccuracy Studies for the Determination of Pb by FAAS Employing

Standard Reference Materials

Mean Concentration ± SD (µg/g)Material Certified Measured Relative Error

Value Value (%)

NIST SRM 2704 a 161 ± 17 160.2 ± 5.4 0.50n = 7

NIES No. 2 b 105 ± 6 103.5 ± 2.8 1.43n = 7

NIES No. 8 c 219 ± 9 221.9 ± 6.2 1.32n = 5

a Buffalo River Sediment (NIST, Gaithersburg, MD, USA).b Pond Sediment (NIES, Ibaraki, Japan).c Vehicle Exhaust Particulates (NIES, Ibaraki, Japan).

TABLE IVRecovery of Pb Added to

Sediment Samples

Concentration (mg/L Pb) RecoveryAdded Expected Found (%)

5 7.15 7.09 99.25 8.70 8.60 98.95 10.08 10.23 101.510 12.15 13.00 107.010 13.70 13.35 97.410 15.08 14.96 99.215 17.15 18.00 105.015 18.70 18.05 96.5

15 20.08 20.64 102.8

Average recovery ± SD = 100.8±3.5TABLE V

Increase of Total Sedimented Lead Between 1985 and 2001 Near Four Highways in Maracaibo City

Avenue Mean Concentration ± SD (mg/kg–1 Pb) Increment 1985 2001 (%)

A-5 1822 ± 476a 3385 ± 2503 186A-L 792 ± 421 3554 ± 1713 449C-1 763 ± 255 2252 ± 2224 295

C-2 758 ± 292 2457 ± 1666 324

A-5: Av. 5 de Julio, A-L: Av. La Limpia, C-1: Circunvalación 1; C-2: Circunvalación 2.a p < 0.05.

61


12. H. González and R.A. Romero, Pre-sented at the II Encuentro Nacionalde Química Analítica, San Felipe,Venezuela (1984).

13. A. Fernández and R.A Romero, Pre-sented at the II Encuentro Nacionalde Química Analítica, San Felipe,Venezuela (1984).

14. H. González, A. Fernández, and R.A.Romero, Acta Cient. Ven. 35, 424(1984).

15. H.A. González, Characterization,evaluation and distribution of theconcentrations of deposited totallead in pre-scholar areas of the cityof Maracaibo (years 1984 and1985). Universidad del Zulia, Mara-caibo, Venezuela (1986).

16. H. González, A. Fernández, and R.Romero, Ciencias 6, 63 (1988).

17. V.A. Granadillo, Levels of total sedi-mented lead in four automotorhigh-circulation ways of Maracaibocity determined spectrometrically,Universidad del Zulia, Maracaibo,Venezuela (2003).

18. S.S. Kartal and L. Elci. Anal. Chim.Acta 413, 33 (2000).

19. J.L. Gómez-Ariza, I. Giráldez, D.Sánchez-Rodas, and E. Morales,Anal. Chim. Acta 399, 295 (1999).

20. Analytical Methods for AtomicAbsorption Spectrophotometry,PerkinElmer, Norwalk, CT, USA(1982), now changed to:PerkinElmer Life and AnalyticalSciences, Shelton, CT, USA.


Chelating Resin Micro-column Separation/Preconcentration and Electrothermal Vaporization

ICP-OES Determination of Trace Bismuth inEnvironmental and Biological Samples

Yi-Wei Wu, Jian-Kun Duan, Zu-Cheng Jiang, and *Bin HuDepartment of Chemistry, Wuhan University, Wuhan 430072, P.R. China

ABSTRACT

A new method for the deter-mination of trace Bi in environ-mental and biological samples bychelating resin YPA4 microcol-umn separation/preconcentra-tion in conjunction withelectrothermal vaporization-inductively coupled plasma opti-cal emission spectrometry(ETV-ICP-OES) has been devel-oped. The resin loaded with Biwas prepared by slurrying andthen directly introduced into thegraphite furnace without anypretreatment. The factors affect-ing the adsorption and vaporiza-tion behavior of Bi wereinvestigated in detail. It wasfound that chelating resin YPA4

is not only an effective solidphase extractant for the quickadsorption of Bi in a wide acidityrange of 0.1~3.5 mol L–1 HCl, butalso a chemical modifier to vapor-ize Bi quantitatively from the fur-nace at the low vaporizationtemperature of 1000oC. With theuse of YPA4 as a chemical modi-fier, the sensitivity was increasedby a factor of 6. Under optimumconditions, the adsorption capac-ity of YPA4 for Bi was 63.5 mgg–1; the detection limit (3σ) ofthe instrument for Bi was 300 pg,with the relative standard devia-tion (RSD) of 3.5% (n=8, C=2 µgmL–1). The proposed method wasapplied to the ETV-ICP-OES deter-mination of trace Bi in environ-mental and biological referencematerials, and the determinedvalues were in good agreementwith certified values.

INTRODUCTION

Due to its chemical and physicalproperties, bismuth and its com-pounds are widely used insemiconductors, cosmetics, phar-maceuticals, and metallurgical addi-tives, as well as in the preparationand recycling of uranium nuclearfuels (1). A number of toxic healtheffects in humans and animals,such as osteoarthropathy, hepatitis,and neuropathology (2), have beenattributed to Bi compounds. Sincethe use of Bi in different areas ofindustry has increased, Bi has nowexcessively spread into the envi-ronment. As a result, the chance ofexposure of organisms to bismuthhas risen significantly. These casesunderline the necessity for meth-ods to determine Bi concentrationsin environmental, biological, andother systems (3–15).

However, concentrations of Biin real samples are usually at theµg g–1 – ng g–1 levels, and could notbe determined directly by conven-tional techniques owing to insuffi-cient sensitivity and matrixinterference. Hence, an efficientseparation and preconcentrationprocedure is usually necessaryprior to determination.

The most widely usedtechniques for the separation andpreconcentration of trace bismuthinclude coprecipitation (14),hydride generation (6,8,10), liquid-liquid extraction (3), and ion-exchange and sorption(9–11,16–20). Among these tech-

*Corresponding author.E-mail: [email protected]

niques, the methods using ion-exchange resin or sorbent extrac-tion, such as modified silica (17),ion exchange resin (9), chelatingresin (18–20), metal oxide (21), andactivated carbon (22) have provedto be effective (9–11,16–22). Inaddition, the use of chelating resinfor separation and preconcentra-tion is especially simple and lesstime-consuming than other meth-ods (19).

Using Muromac A-1 chelatingresin, Sung et al. (19) simultaneouslydetermined the concentration ofbismuth, cadmium, and lead inurine by ETAAS. Yamini et al. (9)successfully determined ultra-traceBi in water samples by ETAAS usingC18 loaded with Cyanex 301 as thesolid phase extractant. However, allof these methods involved elutionwith nitric acid as the eluant whichis a laborious process and prone tocontamination. Therefore, techniquesfor solid phase extraction, whichcould directly analyze theadsorbent loaded with the analyteswithout any complicated elutionprocess, are much more attractive.

Eletrothermal vaporization(ETV), as an alternative samplingdevice for inductively coupledplasma optical emission spectrome-try (ICP-OES) or inductively cou-pled plasma mass spectrometry(ICP-MS), possesses several benefitsover conventional nebulization(e.g., small sample size requirements,the capability for solid and slurryanalysis, ashing-matrix separation,etc.) (23). However, ETV-ICP-OESor ETV-ICP-MS does have limitationswhich include severe memoryeffects, incomplete vaporization,and transport loss. The use of

63


chemical modifiers has beenproven to be very effective toprompt the vaporization of analyte,suppress transport loss, eliminatememory effects, and therefore toimprove the analytical performanceof the method.

In recent years, various chemicalmodifiers, such as halogenatingreagents [especially polytetrafluo-roethylene (PTFE)] (24), DDTC(diethyldithiocarbamate) (25), andPd (12) have been reported for ETVplasma spectrometry determinationof refractory elements. Chang et al.(12) employed the salt of platinumgroup Rh and Pt/Pd as chemicalmodifiers to determine trace Bi insteel-making flue dust and in seareference water samples by ETV-ICP-MS, respectively. Coedo et al.(26) developed a method to deter-mine As, Sn, Sb, Se, Te, Bi, Cd, V,Ti, and Mo in steelmaking fluedusts. Slurry samples were analyzeddirectly by ETV-ICP-MS, and anadded quantity of Rh acting as themodifier resulted in an increase forthe same analytes in matrix-slurrysolutions. However, a very highvaporization temperature (2900oC)was used which is unfavorable forthe ETV device, especially, thegraphite tube.

The aim of this work was toexplore the possibility of usingYPA4 resin (aminoisopropylmercap-tan type with a polythioether back-bone, which a total content of Sand N in the resin of 24.89% and7.82%, respectively) as both anadsorbent and a chemical modifierfor ETV-ICP-OES determination ofBi, and to develop a new methodthat possesses good selectivity andexcellent sensitivity for the determi-nation of trace Bi. YPA4 resinloaded with the analyte wasdirectly introduced into the ETVdevice without any pretreatment,and thus the complicated elutionprocess was avoided. The factorsaffecting the adsorption and vapor-ization behavior of Bi were investi-

gated in detail. Finally, theproposed method was applied tothe determination of trace Bi inenvironmental and biological refer-ence materials to validate the accu-racy of the method.

EXPERIMENTAL

Instrumentation

The graphite furnace sampleintroduction device (Beijing SecondOptics, Beijing, P.R. China) andICP-OES instrument (Beijing Broad-cast Instrument Factory, Beijing,P.R. China) used in this work wereidentical with that reported previ-ously (27). The ICP spectrometricsystem was equipped with a 2-kWplasma generator and a conven-tional silica plasma torch. A WF-1Btype heating device with a match-ing graphite furnace was used foranalyte vaporization. Radiationfrom the plasma was focused as a1:1 image on the entrance slit of aWDG 500-1A type monochromator(Beijing Second Optics, Beijing,P.R. China) with a reciprocal lineardispersion of 1.6 nm mm–1. Thetransient emission signals from theplasma were detected with a R456type photomultiplier tube (Hama-matsu, Japan), fitted with a labora-tory-built direct current amplifier,and recorded by a U-135C recorder(Shimadzu, Japan). The instrumen-tal operating conditions and wave-lengths are listed in Table I.

Standard Solutions andReagents

The bismuth stock solutions(1.000 mg mL-1) were prepared bydissolving high-purity Bi (The FirstReagent Factory, Shanghai, P.R.China) in 1 mol L–1 nitric acid. Theworking standard solutions of bis-muth were prepared by diluting thestock solution with 0.5 mol L–1 HCl.All other reagents used were of ana-lytical reagent grade. Doubly dis-tilled water was used throughout.

Preparation of YPA4

The chelating resin YPA4, as indi-cated above, is an aminoisopropyl-mercaptan type with a polythio-ether backbone, having a total Sand N content in the resin of24.89% and 7.82%, respectively(produced at and purchased fromthe Department of Polymer Chem-istry, Wuhan University). The syn-thesis, IR spectroscopy characteri-zations, and elemental analysis havebeen described previously (28–29).The chelating resin YPA4 with a140 mesh size was immersed inacetone and 1 mol L–1 HCl for 24 h,respectively, then filtered andwashed with doubly distilled water,and dried prior to storage for usefor the adsorption of bismuth.

Separation and Preconcentra-tion Procedure

HCl was selected as the adsorp-tion medium. A 0.5~10.0 µg amountof sample solution containing Biwas transferred to a 1-mL centrifugetube, the acidity adjusted to thedesired value with 4 mol L–1 HCl,and the final volume diluted to1.0 mL. Then, 10 mg of the YPA4

chelating resin was added, and themixture was stirred mechanicallyfor 5 min to facilitate adsorption of

TABLE IETV-ICP-OES Instrumental

Operating Conditions

Wavelength Bi: 306.7 nmIncident Power 1.0 kWCarrier Gas (Ar) 0.6 L min–1

Coolant Gas (Ar) 16 L min–1

Plasma Gas (Ar) 0.8 L min–1

Observation Height 12 mmEntrance Slit Width 25 mmExit Slit Width 25 mmDrying Temp. 100oC,

ramp 10 s, hold 20sVaporization Temp. 1000oC, 4 s Clearing Temp. 2500oC, 2s

Sample Volume 10 mL

64

Bi onto the chelating resin. Aftercentrifugation, the supernatant wasremoved. The resin loaded with Biwas evaporated carefully to neardryness and then prepared to a1.0-mL slurry by adding 0.1% (m/v)agar.

Recommended Procedure forETV-ICP-OES

After selecting the analyticalwavelength and using a pneumaticnebulization system, the sampleintroduction system was discon-nected from the plasma torch, andreplaced by the graphite furnacedevice. After plasma stabilization,10 µL of the prepared sample waspipetted into the graphite furnacewith a micro-syringe, and the sam-ple inlet hole sealed with a graphitecone. Under the optimized condi-tions, the analytes loaded ontoYPA4 were quickly vaporized fromthe graphite furnace andintroduced into the ICP by Ar car-rier gas. The emission signal wasrecorded and peak height was mea-sured for quantification.


Effect of Acidity on Adsorptionof Bi

HCl was selected as the adsorp-tion medium and the adsorptionpercentage of Bi on YPA4 was eval-uated at varying acidities. Figure 1shows the effects of acidity onadsorption percentage of Bi.

As can be seen, the metal ion Biwas adsorbed quantitatively by thechelating resin YPA4 at an HCl acid-ity ranging from 0.1 mol L–1 to 3.5mol L–1. The adsorption percent-ages were over 90%, which showedthat the resin possesses the possi-bility of extracting Bi over a wideacidity range. For this study, 0.5mol L–1 HCl was selected as theacidity for separation and precon-centration of Bi.

Effect of Equilibrium Time

It is well known that a rapidadsorption is of great importancefor an ideal adsorption material.The adsorption kinetics of the resinwas studied in 0.5 mol L–1 HCl. Theresults of the dependencies of theadsorption time of Bi on the chelat-ing resin YPA4 in HCl show that Biwas adsorbed quantitatively within2–5 min, which can be considereda fast adsorption kinetics process.The 20–25% content of S in theresin and a strong affinity of Bi tothe S donor atom may easilyexplain the rapid adsorption behav-iors of Bi on the resin in HClmedium.

Static Adsorption Capacities(Qs)

The adsorption capacity of mate-rials is an important factor to evalu-ate the adsorption performance.Using 0.5 mol L–1 HCl and a 5-minstirring time, a 20-mg portion of theresin was shaken with 25 mL aque-ous solution containing 5–2000 µgof Bi. After the distribution equilib-rium was reached, the concentra-tion of Bi in solution was determined.The results showed that the staticadsorption capacity of the resin forBi was 63.5 mg g–1, which indicates

that the resin possesses a highadsorption capacity for Bi.

Interference Effects

The effects of common co-exist-ing ions in environmental and bio-logical samples on Bi(III) wereexamined. Using 0.5 mol L–1 HCland a 5-min stirring time, differentconcentrations of the metal ionswere added to the solution contain-ing 5 µg Bi. The tolerances of thecoexisting ions are given in Table II.It can be seen that the YPA4 chelat-ing resin showed excellent selectiv-ity toward Bi and thus is an idealmaterial to separate and precon-centrate Bi in complicated samples.

TABLE IITolerance Limits for CoexistingIons in the Adsorption of the

Studied Elements

Coexisting Tolerance Limit Ions of Ions

K+, Na+ 50,000 mg L–1

Mg2+ 40,000 mg L–1

Ca2+, Sn2+ 15,000 mg L–1

Cu2+ 10,000 mg L–1

Pb2+ 5,000 mg L–1

SO42– 160,000 mg L–1

Fig. 1. Effect of acidity (HCl) on adsorption percentage of Bi (%). Bi: 6.0 µg mL–1; Sample volume: 10 mL.

65


tion is based on the assumptionthat the complexes formedbetween Bi and the function groupof YPA4 are stable and easilyvolatile, and that the analyte wasvaporized and transported from thegraphite furnace into ICP in theform of a complex. Thus, the trans-port efficiency of Bi from ETV tothe plasma was greatly increased(3). Without YPA4, a higher vapor-ization temperature is required forBi. A possible reason may be thatalthough BiCl3 possesses goodvolatility (boiling point at 441oC)and excellent chemical stability inacid media, BiCl3 can be hydrolyzedat the drying stage and transformedinto BiOCl, and finally into Bi (boil-ing point at 1564oC) or Bi2O3 (boil-ing point at 1887oC). Thus, a highervaporization temperature isrequired to vaporize Bi from thegraphite furnace.

Optimization of ETV Conditions

Pyrolysis Temperature and TimeIn general, the selection of an

appropriate pyrolysis temperatureis very important for removing theorganic matrix of the resin and pre-venting analyte loss during pyroly-

Vaporization Behaviors of Bi inETV-ICP-OES

As described above, the chelat-ing resin YPA4 showed goodadsorption selectivity, high adsorp-tion capacity, excellent stability,and fast kinetic characteristics forBi. However, our preliminaryexperimental results showed that itis very difficult to desorb Bi fromthe resin even by using strong com-plexing agents such as thiourea.Additionally, the elution process istedious and time-consuming. Thus,in this work, the resin loaded withBi was directly introduced into theelectrothermal vaporizer in theform of a slurry without desorption.

Figure 2 shows the typical signalprofile of Bi without the addition ofYPA4 and also after adsorption byYPA4. As can be seen, the presenceof YPA4 greatly changed the vapor-ization behaviors of Bi. An intenseand sharp emission signal profilewas obtained for Bi after adsorptionby YPA4, and Bi was quantitativelyvaporized at the low temperature of1000oC (Figure 2-D). In addition, noobvious memory effect wasobserved at a temperature of2500oC (Figure 2-E), indicating thatBi had been vaporized completely.On the other hand, using the sameconditions but without YPA4, aweaker emission signal for Bi wasobtained and peak height was justone sixth of that obtained by Biloaded on YPA4 (Figure 2-A).Although no memory effect existedat 1000oC (Figure 2-B), a severememory effect was found at2500oC (Figure 2-C). These resultsindicated that (a) the chelatingresin YPA4 is an effective chemicalmodifier for ETV-ICP-OES determi-nation of Bi at a low temperature of1000oC and (b) when compared tothe results obtained without use ofYPA4, the presence of YPA4 notonly remarkably enhanced the sen-sitivity of Bi, but also greatly low-ered the vaporization temperatureof Bi (from the conventional1600oC to 1000oC). The explana-

sis. During the pyrolysis stage, thetemperature was ramped from100oC to the desired final tempera-ture; thereafter, the furnace waskept at that temperature for 20 s toensure that the charred resin wassufficiently vaporized from the fur-nace. It was found that the analyteloss during pyrolysis occurredwhen the pyrolysis temperaturewas ≥500oC. Thus, a pyrolysis tem-perature of 300oC was selected.However, when the resin wasdirectly vaporized after drying at100oC for 20 s without adoptingthe pyrolysis stages, the emissionsignal intensity of Bi was almost thesame as that obtained by adoptingthe pyrolysis stage. To avoid theloss of Bi caused by the resin pyrol-ysis, the resin loaded with Bi wasdirectly vaporized from thegraphite furnace after maintainingthe temperature of 100oC for 20 swithout the pyrolysis stage.

Under the selected drying tem-perature of 100oC , the effect ofdrying time on Bi signal intensitywas investigated. The resultsshowed that there were no obviousdifferences in signal intensity of Biwhen the drying time was changing

Fig. 2. Signal profiles for Bi.A: Only 20 ng Bi at 1000oC; B: Residual signals of empty firing at 1000oC after A;C: Residual signal of empty firing at 2500oC after A; D: 20 ng Bi with YPA4 at 1000oC;E: Residual signals of empty firing at 2500oC after D; Conditions: Drying 100oC, ramp 10 s, hold 20 s.

66

from 5 s to 40 s. Therefore, a dryingtime of 20 s was chosen.

Effect of Vaporization Tempera-ture

Using a drying temperature of100oC and a drying time of 20 s, theinfluence of vaporization tempera-ture on signal intensity of Biadsorbed by YPA4 was studied andthe results are shown in Figure 3.As can be seen, the emission signalof Bi was observable over the tem-perature range of 500oC and wasenhanced with an increase in tem-perature up to 1000oC. The maxi-mal signal was obtained at about1000oC and remained unchangedin the temperature range from1000 to 1600oC. However, theanalytical signal of Bi slightlydecreased with an increase in tem-perature from 1600 to 1800oC. Avaporization temperature of 1000oCwas therefore selected for Bi.

Figure 3 also shows the effectof vaporization temperature on thesignal intensity of Bi in 0.5 mol L–1

HCl medium (without YPA4). Ascan be seen, the emission signal ofBi was observed at the temperatureof 600oC, the emission signalenhanced with an increase in tem-perature, and reached a plateau atabout 1600oC. The results indicatedthat Bi was not vaporized from theelectrothermal vaporizer as thecompound of BiCl3. The reasonmay be that, on the one hand, thevolatilization of HCl took place atthe drying stage, or, on the otherhand, BiCl3 was hydrolyzed at thisstage. The simple hydrolysisprocess may be presented as fol-lows (30):

BiCl3 Ô BiOCl Ô Bi24O31Cl18

The compound Bi24O31Cl18 mayfinally transform into Bi2O3 or Biduring the ETV heating cycle, andBi was vaporized as the form ofBi2O3 or Bi. Bi2O3 and Bi have a rela-tively high boiling point of 1887oCand 1564oC, respectively, and ahigher vaporization temperature is

required for its complete vaporiza-tion.

Using the same conditions,dependence of the blank signal ofthe resin YPA4 on the vaporizationtemperature was also examinedand the results are also shown inFigure 3. No obvious blank signalof the resin YPA4 was found overthe whole temperature rangetested.

Effect of Vaporization Time Under the selected vaporization

temperature of 1000oC, the effectof vaporization time on signal inten-sity of Bi was studied. The resultsshowed that there were no obviousdifferences in signal intensity of Biwhen the vaporization time waschanged from 3 s to 10 s. However,when the vaporization time wasless than 3 s, vaporization of Bi wasnot complete. Thus, a vaporizationtime of 4 s was required.

Effect of Amount of YPA4

In this work, YPA4 was used asan adsorbent for Bi. As describedabove, under the optimized condi-tions, the adsorption capacity ofthe resin for Bi is 63.5 mg g–1. Onthe other hand, YPA4 was alsotested as a chemical modifier for

ETV-ICP-OES determination of Bi.Thus, the effect of the amount ofYPA4 in the slurry on the emissionsignal intensity of Bi in ETV-ICP-OESwas examined. For this purpose,the following experiment wasdesigned: The slurry of 6% YPA4

loaded with the analyte was pre-pared by mixing the YPA4-loadedanalyte with 0.1% (m/v) agar as thestabilizer. The slurry series (1%,0.5%, 0.2%) were obtained by dilut-ing 2% YPA4-loaded analyte slurrywith 0.1% agar solution. By thistreatment, the analyte in the resinslurry was diluted 2-, 4-, and 10-fold, respectively.

Figure 4 shows the resultsobtained for the above experiment.It can be seen that the signal inten-sity of the analyte increases linearlywith an increaseof YPA4 from 0.2%to 1% (corresponding to anincrease in analyte concentrationlinearly). It can also be seen thatthe amount of YPA4 ranging from0.2% to 1% has almost no obviouseffect on the signal intensity of theanalyte. However, when the con-tent of YPA4 was increased to 2%,the signal intensity of Bi decreasedrapidly. For this reason, a 1% YPA4

was chosen for this work.

Fig. 3. Effect of evaporation temperature on signal intensity.Conditions: Bi 20 ng adsorbed by YPA4; Drying 100oC, ramp 10 s, hold 20 s; Vaporization time 4 s.

67


Detection Limit and PrecisionAccording to the definition of

IUPAC, the detection limit (3σ)(calculated as three times the stan-dard deviation of the backgroundnoise signal intensity) of the instru-ment for Bi was 300 pg and the rel-ative standard deviation (RSD) was3.5% (n=8, C=2 µg mL–1).

Sample AnalysisIn order to establish the validity

of the procedure, the methoddescribed above was applied to thedetermination of Bi in biologicaland environmental standard refer-ence materials (GBW07605 TeaLeaves and GBW07406 Soil,obtained from the Institute ofGeophysical & Geochemical Explo-ration, CAGS, P.R. China).

A 0.8510-g of GBW07605 TeaLeaves sample was weighed into aquartz beaker, dissolved in 15 mLof HNO3–HClO4 (4:2, v/v); thenusing mild heating conditions(approximately 400oC), the samplesolution was vaporized to near dry-ness, and finally dissolved in 10 mLof 0.5 mol L–1 HCl.

Then, 10 mg YPA4 was added tothe above digested tea leaves sam-ple solution and then preparedaccording to the proceduredescribed above. The averageresults of five replicate determina-tions and the certified results aregiven in Table III. As can be seen,the analytical results obtained werein good agreement with the certi-fied values.

A 0.020-g sample of GBW07406Soil was weighed into a PTFE (poly-tetrafluoroethylene) beaker, dis-solved in 10 mL of HNO3–HClO4–HF(4:2:1, v/v/v); then using mild heat-ing conditions, the sample solutionwas vaporized to near dryness, andfinally dissolved in 10 mL of 0.5mol L–1 HCl.

A 10-mg amount of YPA4 wasadded to the digested soil samplesolution, and then prepared accord-ing to the procedure described

above. The average values of fivereplicate determinations and thereference values are also given inTable III. Good agreement betweenthe determined values and the ref-erence values was obtained.

CONCLUSION

In this study, the chelating resinYPA4 containing the S and N donoratoms was used as both an adsor-bent and a chemical modifier forETV-ICP-OES determination of Bi.The double effects of YPA4 resinprovide a new effective strategy forthe separation/preconcentrationand determination of trace/ultra-trace elements. In the presence ofYPA4, Bi was quantitatively vapor-ized at the low vaporization tem-perature of 1000oC. Compared withthe conventional vaporization tem-perature of 1600oC, a decrease of600oC in vaporization temperatureis beneficial in prolonging the life-time of the evaporator. In addition,

the sensitivity was increased by afactor of 5–6 with YPA4 as thechemical modifier. Since no elutionand no pretreatment (slurrying) ofthe eluant are required, the wholeanalytical operation is simplified.Therefore, it is possible to concen-trate analytes from a small samplevolume with a higher enrichmentfactor. The proposed method isapplicable to the determination oftrace/ultratrace Bi in complicatedmatrixes, such as in biological, envi-ronmental, and geological samples.

ACKNOWLEDGMENTS

This project was financially sup-ported by the National Nature Sci-ence Foundation of China and theWuhan Municipal Science &Tech-nology Committee.

Received July 4, 2005.

Fig. 4. Effect of concentration of YPA4 resin on signal intensity.Conditions: Bi 20 ng adsorbed by YPA4; Drying 100oC, ramp 10 s, hold 20 s; Vaporization 1000oC, 4 s.

TABLE IIIAnalytical Results of Bi in Tea Leaves and Soil Reference Materialsa

Element GBW07605 Tea Leaves GBW07406 SoilDetermined Certified Determined Certified

Bi 0.055±0.001 µg g–1 0.063±0.008 µg g–1 51.8 ±3.9 µg g–1 49±7 µg g–1

a Average ± s.d. (n=5).

68

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