Joseph Wang; Bassama a. Freiha -- Preconcentration and Differential Pulse Voltammetry of Butylated...

8/13/2019 Joseph Wang; Bassama a. Freiha -- Preconcentration and Differential Pulse Voltammetry of Butylated Hydroxyanis

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Anal yt i ca Chrmzca Act a, 154 (1983) 87-94Elsevler Science Publishers B.V., Amsterdam - Printed in The Netherlands

PRECONCENTRATION AND DIFFERENTIAL PULSE VOLTAMMETRYOF BUTYLATED HYDROXYANISOLE AT A CARBON PASTEELECTRODE

JOSEPH WANG* and BASSAM A. FREIHADepartment of Chemrstr y, New M exi co Stat e Uni versi t y, Las Cruces, NM 88003 (U.S.A.)Received 20th April 1983)

SUMMARYButylated hydroxyamsole BHA) and other tocopherols are shown to accumulate

very strongly onto carbon paste, with the surface species retaining their characteristicelectroactivlty. This accumulation serves as a preconcentration step which improves thevoltammetric measurement with respect to the sensitivity and selectivity. After 5-mmpreconcentration, a detection limit near 2 lo-@ M BHA is obtained. Enhanced selectivityLS achieved if the electrode is transferred into an electrolytic blank solution between thepreconcentration and measurement steps. The accumulated analyte can then be quantifiedm the presence of a 103-fold amount of a solution species with similar redox potential.The differential pulse stripping response is evaluated with respect to concentration depen-dence, reproducibility, preconcentration period, detection limit, and other variables. Thepreconcentratlon/medium exchange approach is exploited for selective detection of BHAin a flow mJection system. Applicability to various real samples is illustrated.

The determination of antioxidants and tocopherols is of special interest mthe food, pharmaceutical, or cosmetic industries. Various voltammetric pro-cedures have been introduced for this purpose utilizing mercury [l] platinum[ 21 and carbon [3, 41 working electrodes. These procedures are of onlymoderate sensitivity; the resulting detection limits (micromolar level) are notsufficient when traces of these important compounds have to be determined.In addition, rrreproducible voltammetric data may be caused by adsorptionof analyte(s) [3].

Recent studies have been initiated to exploit the accumulation of organiccompounds at electrode surfaces for enhancing the sensitivity and selectivityof their voltammetric measurement [5-S] . Various organic compounds suchas phenanthrenequinone [7], dopamine [9], chlorpromazine [5, lo] andadriamycin [6], have been quantified after their controlled adsorptiveaccumulation at solid electrodes. Alkaloids, surfactants and other com-pounds have been accumulated and measured at a static mercury drop elec-trode [ 81. In this report, the spontaneous controlled accumulation of BHAand other similar compounds at carbon paste electrodes is exploited to im-prove their voltammetric response. Although the extractive/adsorptiveaccumulation of organic molecules at carbon paste electrodes has been known0003-2670/83/ 3.00 o 1983 Elsevier Science Publishers B V


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88for more than 15 years [ll] ts analytical utility as a preconcentration stepprior to the voltammetric measurement has been reported only recently[5--81. Increased sensitivity is obtained as a result of the preconcentrationstep, and a high degree of selectivity is achieved by transferring the electrodeinto a blank solution between the preconcentration and measurement steps.The effects of various operational parameters on the response for butylatedhydroxyamsole are examined in this paper. The potential of this approach tobatch and flow systems is illustrated.EXPERIMENTAL

pparatusThe working electrode material was carbon paste. Two formulations of

the paste were employed, utilizing mineral oil and silicone grease (DowCorning) as pasting agents. I n both cases, the carbon paste was made bythoroughly mixing 2.5 g of graphite powder (Acheson Graphite, Grade 38)with 1.5 g of the pasting agent. The paste was packed mto one end of aglass tube (3-mm i.d., 5-mm o.d.); the other end of the paste was connectedto a copper wire. A fresh carbon paste surface was used each day. The cellwas a Bioanalytical Systems Model VC-2 voltammetric cell. In experimentsinvolving the medium-exchange procedure, a second (measurement) cell wasused. The cell was joined to the working electrode, reference electrode(Ag/AgCl, Model RE-1, Bioanalytical Systems) and platinum wire auxiliaryelectrode through holes in its teflon cover. A magnetic stirrer and a 1.2-cmlong stirring bar provided the convective transport during the preconcentra-tion. The flow injection system has been described earlier [12] . The injectionvalve was a Rheodyne Model 7010 with a 200~~1 sample loop. The electro-chemical detector was a Bioanalytlcal Systems Model TL-4 thin-layer cell.Differential pulse voltammograms were recorded with a Princeton AppliedResearch Model 174 polarographic system and a Houston Omniscribe strip-chart recorder. Instrument settings were as follows: 10 mV s-l scan rate,25 mV amplitude, l-s clock time.Reagents

Deionized water and absolute ethanol (U.S. Industrial Chemical Co.) wereused to prepare all solutions. The supporting electrolyte was 0.10 M phos-phate buffer (pH 7.4) prepared from a 1:4 mixture of KHzPOh and K,HPO,.Submillimolar stock solutions of butylated hydroxyanisole (BHA; reagentgrade; Sigma Chemical Co.) were prepared each day by dissolving the com-pounds in 2 ml of ethanol and making up to lOO-ml volume with deionizedwater. Various real samples were used as discussed below.Procedure

For the preconcentration step, the carbon paste electrode was immersed ina stirred lo-ml sample solution for a given time period. During this period,


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zero potential was applied at the electrode. The stirring was then stopped,and differential pulse measurement of the surface species was made. Inexperiments involving medium-exchange, the preconcentration proceeded atan open-circuit; the electrode was then removed from the sample solution,rinsed thoroughly with water for 15 s, and re-immersed m a second cell con-taining the supporting electrolyte solution of choice. The initial potentialwas then applied, and after the current decay, the voltammogram was re-corded. After the potential scan, the electrode was held at + 0.75 V for 2 minto clean it from the remaining adsorbable species. Specific details of theflow-injection procedure have been described [ 121.RESULTS AND DISCUSSION

The preconcentration of BHA (I ) is easily seen from differential pulsevoltammograms at a carbon paste electrode which has been immersed in aO

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stirred 3.7 X 10m7 M BHA solution for increasing periods of time (Fig. 1).The peak height increases with increasing preconcentration time, indicating

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e

a3 0.5 0 120 240POTENTIAL,V TIME s

Fig 1. Effect of preconcentration period on the differential pulse peak current from3.7 1O-7 M BHA in 0.1 M phosphate buffer. Preconcentration period. a) 0, b) 15,c) 45, d) 105, e) 300 s. Stirring rate, 550 rpm. Carbon mineral 011) paste electrode.


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an enhancement of BHA concentration at the carbon paste. For a 5-min pre-concentration period, a 7-fold enhancement of the peak current is observedover that attained by conventional solution-phase pulse voltammetry (com-pare curves a and e). As a result, submicromolar levels of BHA can be easilyquantified; in contrast, the same sample concentration approaches the detec-tion limit of conventional pulse voltammetry (curve a). Also shown m Fig. 1IS the resulting plot of peak current vs. preconcentration period. As thisperiod increases, the peak current rises rapidly at first and then it levels off.Such nonlinear dependence is expected for a preconcentration step of thisnature [7, 91. By holding the electrode at +0.75 V for 2 min the electrode iscleaned from the accumulated BHA as indicated from the subsequentdifferential pulse scan.

The strong accumulation is also indicated from experiments involving themedium-exchange procedure; when the electrode is transferred (between thepreconcentration and measurement steps) from the BHA solution to an elec-trolytic blank solution a welldefined BHA peak is obtained. The advantagesof the medium-exchange procedure are illustrated below. Peak currents re-corded followmg preconcentrations at 0.0 V and with open-circuit conditionsare similar. For convenience and speed, open-circuit conditions were used inconjunction with the medium-exchange procedure while a 0.0 V potentialwas used in the direct mode. Both formulations of the carbon paste (withthe mineral oil and silicone grease pastmg agents) showed similar degrees ofaccumulation. In contrast, no accumulation was observed when a glassy car-bon electrode was used. These data indicate that the accumulation processinvolves extraction into the paste, rather than adsorption at the electrodesurface; various hydrophobic compounds are known to extract mto carbonpaste electrodes [lo]. A similar degree of BHA accumulation was obtainedfrom a 10 ethanol/go phosphate buffer mixture. No accumulation wasobserved from a 50 ethanol/50 phosphate buffer mixture. The silicone-grease carbon-paste electrode was used in these experiments because of itsgreater stability in such mixtures. Another insight mto the nature of theaccumulation process was achieved by utilizing different compositions of thecarbon paste electrode. By increasing the weight percentage of silicone greasefrom 28 to 44 , a 50 increase in the peak current was observed (condi-tions: 3 min preconcentration from a 1 X 10m6 M BHA solution, followed byexchange to phosphate buffer, not shown). Similar increase in the signal wasobserved with chlorpromazine as the analyte [13]. The accumulation ofBHA is affected by the mass-transport conditions durmg the preconcentra-tion period. Peak currents of 9.6, 20.9, 21.5 and 23.0 nA were obtained atstrrring rates of 0,250,350 and 550 rpm, respectively (conditions: 3 X 10d6 MBHA m a 0.1 M phosphate buffer, carbon (mineral oil) paste electrode,differential pulse measurement after 2-min preconcentration and mediumexchange).

Figure 2 illustrates the response to successive standard additions of BHA(2.5 X 10m7 M increments). The well-defined peaks allow convenient measure-


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Fig. 2. Differential pulse voltammograms obtained after increasing the BHA concentrationin 2.5 X lo- M steps a-f). Preconcentration for 2 mm with 550 rpm stirring, 0.1 Mphosphate buffer. Carbon mineral oil) paste electrode. The zero current of the individualcurves is displaced for clarity.ment at the submicromolar concentration level. The plot of peak current vs.concentration (also shown in Fig. 2) is nonlinear. Such curvature is expectedfor a preconcentration step of this nature [6-g]. The quantitative signifi-cance lies in the fact that the peak current for a given time and stirring rate isreproducible and related to the bulk concentration. The reproducibility wasestimated by eight successive measurements on a 7.5 X 10 M BHA solution(conditions: 1 min preconcentration with 550 rpm stirring, carbon (mineraloil) paste electrode). The mean peak current was 4.99 nA with a range of4.80-5.26 nA. The relative standard deviation over the complete series was3.8 . The precision obtained compares favorably with that reported forother compounds accumulated at solid electrodes [6, 71. A detection limitnear 2 X 10-s M would be expected based on the signal-to-noise characteristics(S/N = 2) of Figs. 1 (curve e) and 2 (curve a). Measurements of a 5 X lOmaMBHA solution confirmed this detectability (not shown). Hence, at low analyteconcentration, the preconcentration provides a considerably improved re-sponse over that obtained by solution-phase pulse voltammetry.

Enhanced selectivity is another important advantage of the preconcentra-tion approach. This advantage is achieved by transferring the electrode fromthe complex sample to a blank electrolytic solution between the preconcen-tration and measurement steps. Thus, the surface-bound species is measured


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without interferences from solution phase species. Figure 3 illustrates thisimprovement in the response of BHA. Curves A(a) and A(b) are the differen-tial pulse r spons s for a solution containing 1 X 10m6M BHA and 1 X 10m3Mascorbic acid recorded with and without medium exchange, respectively.When the measurement is performed in the sample solution, the large ascorbicacid oxidation current masks completely the BHA peak. After exchanging tothe blank solution, the surface-bound BHA is quantified without the ascorbicacid interference. This improvement is emphasized by the significant change(25-fold) in the current scale. Thus, micromolar levels of BHA can be quanti-fied even when a 103-fold concentration of a species with similar redoxpotentials is present initially. Figure 3B illustrates the same advantage asapplied to real samples. The soft drink mix (Kool-Aid) contained a signifi-cant concentration of ascorbic acid (and other constituents) and microlevels of BHA. In the direct mode (curve b), large currents from macro solu-tion constituents swamp the peak current of the surface-bound species. After

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aJ,2 ox-POTENTJAL,VFig. 3. Differential pulse voltammograms: a) with and b) without medium exchangeafter the preconcentration step. Samples: A) 1 X low6 M BHA and 1 X 10e3 M ascorbicacid in 0.1 M phosphate buffer; B) Kool-Aid soft drink mix. Solution prepared by dis-solving 0.1 g of the mix in 0.2 m of ethanol and diluting to 10 ml. Exchange solution,0.1 M phosphate buffer. Preconcentration: A) 3 and B) 5 min; 550 rpm stirring. Carbonmineral oil) electrode.Fig. 4. Differential pulse response for a) One-A-Day multivitamin and b) Mazola cornoil samples. Preconcentration for 2 min at 550 rpm stirring; measurement after mediumexchange to a phosphate buffer solution. Carbon silicone grease) paste electrode. Themultivitamin tablet and 1.0 g of the corn oil were diluted in 100 ml of ethanol; 2 ml ofthe multivitamin solution or 1 ml of the oil solution was dissolved with phosphate bufferto 10 ml to prepare the sample. Exchange solution, 0.1 M phosphate buffer.


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medium exchange to a phosphate buffer solution (curve a), the BHA peak isdetected.Figure 4 demonstrates the feasibility of measuring other tocopherols, suchas vitamin E, in real samples using the preconcentration/medium-exchangeapproach. Curves (a) and (b) are the responses recorded in the phosphatebuffer solution after preconcentration from multivitamin and corn oilsamples, respectively. Because the medium-exchange procedure is utilized, theaccumulation of the tocopherols is obvious. No accumulation was observedwhen butylated hydroxytoluene (BHT) was tested. This can be used toimprove the selectivity toward BHA in the presence of BHT. For example,addition of 3 X 10 M BHT to a solution containing 3 X 10 M BHA didnot affect the response following preconcentration/medium exchange.The preconcentration approach can be exploited as a selective detectionmode for flow injection systems, based on a recently developed manifoldprocedure [ 121. Compared to conventional amperometric detection for flowinjection systems, this procedure provides remarkable selectivity toward theaccumulated analyte, as a result of combining its intimate contact with theelectrode surface with the medium-exchange procedure. For this purpose,the preconcentration step is started as the sample plug arrives in the detectorand terminated after it has passed through. Then, the stripping step is donewhile the carrier (blank) solution passes through the detector, i.e., mediumexchange is obtained. The preconcentration/medium-exchange flow-injectionmanifold was applied for selective and sensitive measurements of BHA(Fig. 5). Curves A (a-d) represent the response after successive injections of

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Fig. 5. Differential pulse voltammograms for injections of BHA solutions. A a-d): BHAsolutions of ascending concentrations, l-4 x 10 M. B a-f): Repeated injections of a4 x 10e6 M BHA solution; g, h) after addition of 1 x 10m4 M ascorbic acid to the BHAsolution. Carbon silicone grease) paste electrode. Carrier solution, 0.1 M phosphate buffer,Preconcentration, 1 min at 0.0 V; cleaning, 1 min at + 0.7 V. Flow rate, 0.4 ml min-.The zero current of the individual curves is displaced for clarity.


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BHA solutions of ascending concentrations (l-4 X 10m6M). Linear responseis obtained (after correcting for the blank response, dotted line). Least-squares treatment of these data yielded the equation I (nA) = (2.37 ?10.02)C(10e6 M) + 0.61 ?r 0.06 nA with S,, = 0.051 nA and r = 0 999 Curves B(a--h) illustrate the precision and selectivity of the flow-injection procedure.For the series of eight successive injections of a 4 X 10 M BHA solution, amean peak current of 8.55 nA was found, with a range of 8.1-9.2 nA anda relative standard deviation of 4.9 . Curves (a-f) of this series were re-corded with the BHA as single analyte in the sample; curves g and h wereobtained after addition of 1 X lo* M ascorbic acid to the same solution.These data demonstrate again that excess of solution phase species does notaffect the determination of BHA.REFERENCES

1 L. I. Smith, L. J . Spillane and I. M. Kolthoff, Anal. Chem., 64 (1942) 447.2 D. Cozzi, G. Rasp1 and L. Nucci, J . Electroanal. Chem., 12 (1966) 36.3 S. S. Atuma and J . Lindquist, Analyst, 98 (1973) 886.4 H. D. McBride and D. H. Evans, Anal. Chem., 45 (1973) 446.5 J . Wang and B. A. Freiha, Anal. Chim. Acta, 148 (1983) 79.6 E. N. Chaney and R. P. Baldwin, Anal. Chem., 54 (1982) 2556.7 H. Y. Cheng, L . Falat and R. L . Li, Anal. Chem., 54 (1982) 1384.8 R. Kalvoda, Anal. Chim. Acta, 138 (1982) 11.9 J . W. Sirla and R. P. Baldwin, Anal. L ett., 12 (1980) 577.10 T. B. J arbawi and W. R. Heineman, Anal. Chim. Acta, 135 (1982) 359.11 C. A. H. Chambers and J . L. Lee, J . Electroanal. Chem., 14 (1967) 309.12 J . Wang and B. A. Freiha, Anal. Chem., 55 (1983) 1285.13 B. A. Frelha, Ph.D. Thesis, New Mexico State University, Las Cruces, NM, 1984.

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