Journal of Automatic Chemistry, Vol. 15, No. 3 (May-June 1993), pp. 79-84

The role of atomic fluorescencespectrometry in the automaticenvironmental monitoring of trace elementanalysis

P. B. Stockwell and W. T. CornsP S Analytical Ltd, Arthur House, B4 Chaucer Business Park, Watery Lane,Kemsing, Sevenoaks, Kent TN15 6QY, UK

Considerable attention has been drawn to the environmental levelsof mercury, arsenic, selenium and antimony in the last decade.Legislative and environmental pressure has forced levels to belowered and this has created an additional burden for analyticalchemists. Not only does an analysis have to reach lower detectionlevels, but it also has to be seen to be correct. Atomic fluorescencedetection, especially when coupled to vapour generation techniques,offers both sensitivity and specificity.

Developments in the design ofspecified atomicfluorescence detectorsfor mercury, for the hydride-forming elements and also forcadmium, are described in this paper. Each of these systems iscapable of analysing samples in the part per trillion (ppt) rangereliably and economically. Several analytical applications aredescribed.

Introduction

There is considerable concern about pollution in theenvironment; the focus of much of this has been on suchheavy metals as mercury, arsenic, selenium, antimonyand cadmium. A brief r6sum of why some of theseelements are important and where they are used is givenbelow.

Mercury (Hg)

The toxicological effect of mercury compounds on bothplant and animal life has long been recognized, but it wasnot until the disaster at Minamata Bay in 1953 that thesubject received worldwide attention ]. Mercury occursnaturally in the environment in the form of mineraldeposits and also anthropogenically from industrial andagricultural wastes. Because of its high toxicity there hasbeen extensive research and development into techniqueswhich can be used to determine mercury in a variety ofsamples.

In addition to the toxic nature of mercury, it is oftenpresent in natural gas and petroleum products, each ofwhich can form the basis of feedstock for industrialreactions. Many of these take place in the presence ofaluminium rotors or condensers. Even low levels ofmercury can attack aluminium components, causingstress fractures [2]. These may result in a plant having toshut down for repairs, at a cost of millions of pounds.

Arsenic (As)In nature, arsenic seldom occurs in the free state, but,rather, is associated primarily with sulphide minerals ofthe general formula XmAsnSp where X silver, copper orlead. All the inorganic and organic compounds of arsenicin the environment are in the 3-, 3+ or 5+ oxidationstates.

Arsenic, and its various inorganic and organic com-pounds, figure in many industrial processes and appli-cations, for example the manufacture of certain glasses,semiconductor and photo-conductor materials, lead andcopper alloys, linoleum oil cloth, pigments, textileprinting, antifouling paints, tanning, taxidermy, adhe-sives for metals, cotton desiccation, wood preservatives,herbicides, fungicides, rodenticides and insecticides.Such varied uses, coupled with the mining and smeltingof arsenic-containing ores, coal combustion, waste incin-eration and agricultural burning, have resulted in aglobal atmospheric emission of 8 and 24 M kg/year fromnatural and anthropogenic sources, respectively. Formany years there has been environmental concernregarding the possible contamination by arsenic ofwater,soil and food.

Selenium (Se)Selenium is of great significance in environmentalanalysis, biochemistry and medical chemistry. On thegrounds of its ambivalent character (toxic, as well asessential), there is a particularly urgent need for an exactknowledge of the selenium content of biological andenvironmental materials. This is why there is a require-ment for methods permitting analytical access to sele-nium concentrations ranging between increased (toxic)via normal to low (deficiency) levels. The element itselfmakes this task very difficult because the differencebetween increased and normal concentration levels issmall, being only about one order of magnitude. Pro-cedures with a high power of detection are required,especially where selenium is to be determined in itsspecial binding forms, or when different species (Se [II],Se [0], Se [IV], Se [VI]) have to be detected, for examplein water analysis.

Antimony (Sb)Although antimony is non-essential for life, it is found inbiological specimens from people who have been exposedto industrial sources ofantimony or who are being treatedwith drugs containing the element. Antimony is used inindustry as a bearing metal, either by itself or alloyed

0142-0453/93 $10.00 ( 1993 Taylor & Francis Ltd.79

P. B. Stockwell and W. T. Corns The role of atomic fluorescence spectrometry in automatic environmental monitoring

with tin and lead, and antimonial lead is used in storagebatteries. Antimony compounds are used as flameretardants in plastics, polymers and rubber, and aspigments in paints. Other uses are in the manufacture ofenamels, glasses and glazes and as mordants for textiles.

The main medicinal use ofantimony compounds is in thetreatment of parasitic diseases. In this context it isreported that pentavalent (V) antimony is less toxic thantrivalent (III) antimony. Trivalent antimony is taken upby the red blood cells in humans, whereas pentavalentantimony remains in the plasma and it is more easilyexcreted than trivalent antimony.

The most likely route into the body from industrialexposure is by inhalation. This can arise from breathingthe dust or fumes from the grinding and high temperatureprocessing ofantimony and its compounds or from stibine(antimony hydride) which is particularly poisonous.

As legislative and environmental pressures increase,detection levels are required to be lower and lower. Tomaximize the analytical precision it is also important thatthe measurements, and therefore the sample pretreat-ment, are optimized to ensure reliable measurements.This often leads to single element analytical systems.Analysts are faced with the dilemma of achieving thedetection levels promoted by instrument manufacturers.When the analyst is faced with the analysis of soil,biological samples and the like, the limit of detectionrequired stretches analytical techniques. Fluorescencetechniques often offer two orders of magnitide sensitivityimprovement over absorption techniques and allowanalysts to achieve the detection limited required andtraceability ofthe results. Furthermore, in atomic absorp-tion, non-specific absorption from molecular species mayoccur, resulting in positive bias. This phenomenon is notusually observed using fluorescence techniques [3].

Cadmium (Cd)Cadmium is toxic to humans, it accumulates in thekidneys and too high a level results in renal failure.Unlike copper and nickel, it is not phytotoxic and it isreadily taken up into plants, especially leafvegetables. Soif it is disposed to land it can result in human toxicity,whereas excess copper, nickel or zinc in a soil willprobably kill the plants well before the humans canconsume it.

Cadmium metal is very volatile compared with manyother toxic metals and this is another major area of risk.Therefore if cadmium alloys are heated, there is a veryserious risk due to the resulting cadmium vapour. The EClimit for drinking water is 5 tg/1. In the UK, themaximum permitted level is 3 mg/kg Ds for soils and alsofor sludge to land.

Cigarettes contain elevated levels of cadmium and raisedCd levels are detectable in the kidneys of smokers.

Measurement

The elements discussed above are of environmentalconcern and therefore it is necessary to measure themaccurately and reliably. Unfortunately, they are oftenpresent in small quantities; and they are difficult toanalyse by conventional techniques.

Of the trace elemental analysis techniques, atomicfluorescence offers improved selectivity compared withoptical emission spectrometry, because the resolution isdetermined by the line width rather than by the detectionsystem. Fluorescence techniques offer good advantages interms of linearity and detection levels. By its inherentnature the signal obtained can be increased by theintensity of the lamp or excitation source. The limitationsare scatter and background levels of impurities. Atomicabsorption, on the other hand, although it has beenextensively used, suffers from the fact that it is non-linearand measurements at lower levels are extremely difficult.

Theoretical aspects of atomic fluorescence

A typical atomic fluorescence arrangement, shown infigure 1, consists of an intense excitation source focusedonto an atomic population in a flame. Fluorescenceradiation, which is emitted in all directions, then passes toa detector in the same plane, usually positioned at right-angles to the incident light.

The source may be either an atomic line source or acontinuum, and serves to excite atoms by the absorptionofradiation at specific wavelengths. The atomics are thendeactivated partly by collisional quenching with flamegas molecules and partly by emission of fluorescentradiation in all directions.

The wavelength ofthe fluorescence radiations is generallythe same or longer than the incident radiation. Thewavelengths of the emitted radiation is characteristic ofthe absorbing atoms and the intensity ofthe emission maybe used as a measure of their concentration.

At low concentrations, this intensity is governed by thefollowing relationship:

I-= KOIoC

Hydrogen Diffusion Flame

Lensl Bclcd Discharge Hollow

]- Cathode Lamp

Inlcrfcrcncc

/ Filler

Figure 1. Typical atomic fluorescence optical arrangement.

8O

P. B. Stockwell and W. T. Corns The role of atomic fluorescence spectrometry in automatic environmental monitoring

where

If the intensity of fluorescent radiation.C the concentration of metal ion in solution.

K a constant.

Io the intensity of the source at the absorption linewavelength.

ID the quantum efficiency for the fluorescent pro-cess, which may be defined as the ratio of thenumber of atoms which fluoresce from theexcited state to the number of the atoms whichundergo excitation to the same excited state fromthe ground state in unit time.

There are five basic types of fluorescence which occurin flame measurement: (a) resonance fluorescence;(b) direct-line fluorescence; (c) stepwise-line fluorescence;(d) thermally assisted direct-line fluorescence; (e) ther-mally assisted anti-Stokes fluorescence. These have beendescribed in detail by Thompson and Reynolds [4].

Maximizing the benefits of atomic fluorescence

It is puzzling, considering its inherent advantages, thatatomic fluorescence has not been more successful.Primarily this has been due to the matrix interferenceeffects which occur when real samples are analysed.Coupling a fluorescence measurement technique to avapour generation technique has the potential to over-come all of these problems with an additional bonus. Thepretreatment to generate the vapour will remove agreat majority of the interferents and the bonus is theincreased transfer ofefficiency ofthe element ofinterest tothe measurement cell. Stockwell [5] has illustrated theimprovements made by a vapour generation technique tothe sensitivity of an ICP-OES system, i.e. by a factor of500 times.

Each ofthe elements discussed here can be determined byvapour generation technique and each has accessiblefluorescence lines, good available light source and

therefore provide good prospects of measurement. Thevapour generation techniques required involve:(a) generation of mercury itself as a vapour;(b) generation of the hydrides of As, Se, Sb, Te; and(c) generation of the tetraethyl cadmium species. P SAnalytical, in association with Yorkshire Water Labora-tory Services and the University of Plymouth, haspioneered the development of these coupled techniquesand several instruments are now readily available forroutine use.

Determination of mercury by atomic fluorescence

Mercury is an ideal element for determination byfluorescence: it is atomic at room temperature and alsoabsorbs and fluoresces at the same wavelength. Intensemercury sources are readily available and there is littleproblem in producing an atomic cell when the techniqueis coupled to vapour generation. Thompson and Godden[6] proposed the use offluorescence for the determinationof mercury in 1975. Godden and Stockwell [7] havedescribed the development of a specific fluorescencespectrometer for mercury analysis, and this work resultedin a commercial instrument: the Merlin FluorescenceDetector.

The determination of mercury at low levels requiresconsiderable care and attention to detail. Appropriatemethodologies are described in Yorkshire Water’s Meth-ods of Analysis [8]. Basically the method requires thereaction of the samples.in an acid medium with tin (II)chloride. This procedure is automated using a vapourgenerator, in which the sample, blanks and the tin (II)chloride are introduced using peristaltic pumps into agas/liquid separator. Figure 2 shows a schematic rep-resentation of an automated instrument for mercurydetermination. The mercury is sparged from the gas/liquid separator by a flow of argon gas and the streamcontaining the mercury is fed into the Merlin Fluores-cence Detector using a specific interface for analysis. Theinstrument measures total inorganic mercury and sam-ples which contain organic mercury must be digested

NaBH4 Peristaltic Pump Dryer Gas Out Dryer Gas In

Blank Ii

Autosampler aWmitlhiinngg 21iieon tCaamr: PI’ iHMYe : FDItrc:enccFigure 2. Schematic representation of an automated system for mercury determination.

81

P. B. Stockwell and W. T. Corns The role of atomic fluorescence spectrometry in automatic environmental monitoring

prior to analysis. The preferred bromate/bromide diges-tion method ensures that complete breakdown oforgano-mercury compounds is achieved within an hour [8].Reagents and standards are prepared according toYorkshire Water’s Methods. The analytical instrument isset up according to the manufacturer’s recommendationsand allowed to stabilize. A quick check with colouredsolutions will ensure that the equipment is functioningcorrectly. The reductant, reagent blank and samples arethen presented to the vapour generator. The equipment isthen calibrated over the range ofinterest. A typical signalresponse is shown in figure 3. Where the blank andsample matrices are completely matched, the resultantsignal represents the true level of mercury in the .sample.This signal represents the output typically obtained from0"010 ppb ofmercury in water. The calibration graphs arecomputed by the method of least squares and showexcellent correlations which confirm the linearity andsensitivity of the method and Merlin’s applicability tomercury analysis. The Merlin system provides a rapid,sensitive and fully automatic system for mercury determi-nations at ultra-trace levels. It can be applied to a widerange ofsamples, and systems are being successfully usedin laboratory and process control applications. Theaccuracy of the technique is shown in table 1. Itsperformance exceeds all other commercial systems, eventhose relying on some method ofpreconcentration on goldtraps etc.

Determination of arsenic, selenium and antimony

Mercury is a relatively simple element to determine byfluorescence; however, the technique can be extended toother elements. Recent research by the authors with theUniversity of Plymouth has looked at the scienceinvolved, as well as instrument optimization and simplifi-cation. The instrumentation concepts have focused onfour main areas: (a) the light source; (b) the atom cell;(c) the optical design; and (d) read-out and data-processing.

For the determination of arsenic and other hydrideforming elements, boosted discharge hollow cathode

Reading Sid 2 Run (10 ppt Hg) Peak Area: 2209.3%secBaseLine: 1.2% Peak Height: 44.1%

Time 272secs

Range 1000 Filter 32 Signal> 0.100% Standby

Figure 3. Typical signal response for 0.01 ppb mercury using acontinuous flow vapour-generator coupled to atomic fluorescencespectrometry.

lamps have been found suitable for the excitation sourceof novel detectors. A simple hydrogen diffusion flameprovides a viable atom cell.

The optical design is more complex and requiresattention to the selection ofoptical filters and reduction ofscatter. Changing a lamp allows a range ofdetectors to beavailable commercially with little additional modificationto the design. Data-processing and control facilities areprovided using similar software as used with the MerlinDetector. Instrumentation for arsenic atomic fluores-cence consists of a boosted discharge hollow cathodelamp as an excitation source, a hydrogen diffusion flameas an atom cell, a collection of lenses to collect and focususeful radiation, an interference filter to achieve wave-length isolation and a solar blind photomultiplier tube.Illumination from the excitation source is at right anglesto the detection axis to suppress detection of radiationfrom the excitation source. A multi-reflectance filterspecifically developed to maximize the optical trans-mission at the wavelength of interest has proved to be aversatile and sensitive option which allows measurementof arsenic, selenium, antimony and tellurium at signifi-cantly lower levels than can be achieved elsewhere [9].Figure 4 shows the detectors for both mercury and thehydride forming elements. Both use similar data collec-tion and output electronics.

The analytical operation of the arsenic fluorescencedetection system is similar to that for mercury, but uses adifferent chemistry. Sodium tetrahydroborate is usedinstead of tin (II) chloride and its reaction withhydrochloric acid produces a steady stream of hydrogen.

Table 1. Certified reference materialsfor mercury.

Certifiedreferencematerial Certified value Value obtained

IAEA/W-4Simulated water

Seronorm urineBatch 108

BCR 143Sewage sludgeAmended soil

BCR 144Sewage sludgeAmended Soil

NIST SRM 16416Fresh water

NIST RM 8407River sediment

NIST RM 8408River sediment

Coniston Oil

BESTMarinesediment

NIST SRM 2670Urine

2.50 + 0-13 tg/1 2"45 + 0"04 tg/1

51"0 + 5"0 tg/1 59-0 + 6-0 tg/1

3"92 + 0"23 tg/g 3"82 + 0"13 tg/g

1"39 + 0"17 tg/g 1"49 + 0"22 tg/g

1"52 + 0-04 tg/ 1-47 + 0"04 tg/ml ml

50 + 2 tg/g 50 + tg/g

107 +2 tg/g 106 + 2 tg/g

100 rag/1 101 mg/10"092 tg/g 0"094 +

0"002 tg/g

0"105 0"103 +0"008 g/ml 0"005 g/ml

82

P. B. Stockwell and W. T. Corns The role of atomic fluorescence spectrometry in automatic environmental monitoring

Figure 4. Detectors for mercury and hydrideforming elements.

The hydrogen converts the arsenic (Ill) to the hydrideform and provides the fuel for the atom cell (i.e. hydrogenflame). Arsine, produced by the reaction of the tetrahy-droborate with the acid, is transferred by the argonstream into the hydrogen diffusion flame. The signalshape is similar to that produced for mercury, except thatthe plateau is achieved somewhat quicker and thememory effects are also minimized. Figure 5 shows atypical peak shape for 0"020 ppb arsenic solution.

The calibration graph is linear between 0-100 ppb ofarsenic. Typically, detection levels in the region of 0.010ppb are achievable using the vapour generator coupled tothe specific fluorescence detection system describedabove. The accuracy of the technique is shown in table 2,where the results for certified water reference materialsare given. Selenium, antimony and tellurium are alsodetermined in a similar fashion.

Determination of cadmium as tetraethyl compound

With the success of vapour generation approaches formercury and the hydride forming elements, the extensionto other elements, especially those with good atomizationsources such as cadmium and lead is obvious. Severalgroups [10, 11] have investigated the conversion ofcadmium and lead to the tetraethyl compounds. Goodall

and co-workers [12] coupled the PSA vapour generationtechnology to a specific atomic fluorescence detector forcadmium determination. The technology is similar tothat used for the Excalibur detector, except that nohydrogen is produced by the chemical reaction to formthe tetraethyl compound. An additional flow of hydro-gen/argon is required to create the atomization cell. In

Reading Std Run (0.05 ppb As) Peak Area: 1440.7%secBaseLine: 1.2% Peak Height: 24.3%

Time- 136secs

Range 1000 Filter 32 Signal> 0.100% Standby

Figure 5. Typical signal response for 0"02 ppb arsenic usingcontinuous flow hydride generation coupled to atomic fluorescencespectrometry (Excalibur, PSA).

83

P. B. Stockwell and W. T. Corns The role of atomic fluorescence spectrometry in automatic environmental monitoring

Table 2. Certified reference materials: arsenic.

Certified reference Certified value Value obtainedmaterial (g/1) (zg/l)

CASS2 1"01 0"07 1-08 + 0" 18(Near-shore seawater)

SLEW1 0-765 + 0"093 0"697 + 0"020(Estuarine water)

addition, a cadmium discharge lamp is used as theatomization source, rather than a boosted hollow cathodelamp- and the power supply module is simpler inconcept. The optics and data collection systems arebasically those designed for the Excalibur. A prototypeinstrument has been constructed along the above linesand initial results draw significant promise. Detectionlevels in the low ppt levels are possible and theperformance for certified reference materials are shown intable 3.

Conclusion

Atomic fluorescence is already proving to be a veryreliable and sensitive means of determining mercury,arsenic, selenium and antimony. Recent research hasclearly demonstrated that it is also useful for themeasurement of cadmium and lead. Commercially avail-able detectors are likely to be available by the end of 1994.

Table 3. Certified reference materials: cadmium.

Certified referencematerial Certified value Value obtained

NIST 1643c 12"2 + 0-1 g/1 12"6 + 0-5 g/1(Water)

BCR 144 (Sewage 3"41 + 0"25 zg/g 3-34 + 0"15 g/gsludge)

BCR 145 (Sewage 18.0 + 1"2 zg/g 18"24 0"7 g/gsludge)

Acknowledgements

The advancements in the analytical applications ofatomic fluorescence could not have been achieved sosuccessfully and in a such a short time scale withoutassistance from a number of sources. Their assistance isgratefully acknowledged here. Professor L. Ebdon, Dr P.Goodall and Dr S. Hill from the University of Plymouthare thanked, as is Dr Clive Thompson ofYorkshire WaterLab Services, Sheffield. The methods for mercury inwater were developed at Yorkshire Water Lab Services,Sheffield Laboratory, Charlotte Rd, Sheffield $2 4EQ,UK.

References

1. SMITH, W. E. and SMITH, A. E. (Eds), Minimata. Holt,Rinehardt and Winston, New York, 1975.

2. ENGLISH, J. J., A mercury induced aluminium alloy pipingfailure in an ethylene manufacturing plant. Paper presentedat the First Ethylene Producers Conference, 4 April 1989.American Institute of Chemical Engineers.

3. WEST, C. D., Analytical Chemistry, 46 (1974), 797.4. THOMPSON, K. C. and REYNOLDS, R. J., Atomic Absorptio.n,

Fluorescence and Flame Emission Spectroscopy, A PracticalApproach. Charles Griffin & Co., 1978.

5. STOCIWELL, P. B., Laboratory Practice, 39 (1990), 29.6. THOMPSON, K. C. and GODDEN, R. G., Analyst, 100 (1975),

544.7. GODDEN, R. G. and STOCIWELL, P. B., Journal ofAnalytical

Atomic Spectrometry, 4 (1989), 301.8. Yorkshire Water, Methods ofAnalysis, 5th edition. Yorkshire

Water, Leeds, 1989 (Section 9).9. CORNS, W. T., STOCIWELL, P. B., EBDON, L. C. and HILL,

S. J., Journal of Analytical Atomic Spectrometry, 8 (1993), 71.10. D’ULIvO, A. and CHEN, Y., Journal of Analytical Atomic

Spectrometry, 4 (1989), 319.11. STURaEON, R. E., WILLIE, S. N. and BERMAN, S. S.,

Analytical Chemistry, 61 (1989), 1867.12. GOODAI.I., P., HILL, S.J., EBDON, L. C., STOCKWELL, P. B.

and THOMPSON, K. C.,Journal ofAnalytical Atomic Spectrometry(in press).

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