Vol. Therole of discrete sample injection in trace mercury ...JournalofAutomaticChemistry,Vol. 18, No. 4 (July-August 1996), pp. 153-162 Therole ofdiscrete sampleinjection in trace

Journal of Automatic Chemistry, Vol. 18, No. 4 (July-August 1996), pp. 153-162

The role of discrete sample injection intrace mercury analysis by atomicfluorescence spectrometry

P. B. Stockwell, W. T. Corns and N. BrahmaP S Analytical Ltd, Arthur House, Unit 3, Crayqelds Industrial Estate,Main Road, Orpington, Kent BR5 3HP, UK

Coupling specific atomic fluorescence spectrometers to vapourgeneration techniques is a highly sensitive approach to thedetermination of trace levels of mercury. In many sample types thelevels of the mercury content are extremely high and the matrixmay have a deleterious e2ffect on the measurement. This paperdiscusses the application of discrete sample injection techniques tobroaden the range of analytes tested and the levels analysed. Thelimitation of linear dynamic range for fluorescence is theself-absorption e/ffect. Reducing the eective sample size to below100 # litres allows a linear calibration up to 10 parts per million(ppm). This sample limitation, coupled to the software’s abilityto reset the sampling valve should the signal level exceed themaximum setting, ensures that levels of up to lOOppm can bepresented to the analyser. An additional advantage of the discretesample injection applies to complex analytical samples, for exampleconcentrated sulphuric acid. The eective dilution provided by thismeans overcomes any matrix interferences and quickly providescorrect data. With proper care, the analytical range of the systemdescribed can extend over seven orders of magnitude from less than1 part per trillion (ppt) through to 10 ppm.

Introduction

Over the past decade there has been considerable concernabout the levels of heavy metals in the environment,especially mercury, arsenic, selenium and antimony. Sincethe authors became interested in this field, levels ofmercury have received by far the most attention.Currently the legislation sets out the limits for mercuryas the total content, whatever the form in which themercury may be present. However, there is a pressing casefor analysing the various species of mercury present;methylmercury, for example, is more than 1000 timesmore toxic than mercury in its inorganic forms. RecentlyJones et al. [1-1 have suggested a simple gas chromato-graphical separation system, linked to a specific atomicfluorescence detector, to determine such species in a rangeof materials including soils, sludges and effluent.

In 1988, the reorganization of the UK water industry intoprivatized companies and a policing facility provided bythe National Rivers Authority laboratories drew attentionto the monitoring ofmercury in drinking water. The levelsrequired, and the sampling frequency which were dictatedby the legislation, meant that a radically new direction forinstrumentation was needed. The batch methods usingatomic absorption detection available at this timeprovided neither the detection limits nor the throughputnecessary.

Thompson and Godden I-2] described an atomic fluor-escence method for the measurement of mercury; Goddenand Stockwell [3], using an available molecular fluor-escence detector with subtle modifications, designed asimple but effective commercial variation of this with theadditional potential for complete automation. In 1989,P S Analytical introduced the world’s first fully automatedmercury analyser based on these developments. Since thenmore than 20 commercial competitors have been intro-duced around the world. With each of these makingvarious claims as to detection capabilities, it would seemto the analytical community that the determination ofmercury at low levels is just a trivial matter. This is veryfar from the truth because at the levels required, oftenbetween 10-10 and 10-12 g litre, it is very dicult to getrepresentative samples and reproducible results. With careto the sampling and methodology, levels below ppt canbe measured.

In the UK’s water industry the atomic fluorescencemeasurement coupled to vapour generation techniqueshas become well established. The use of a hygroscopicmembrane drier tube to continuously remove moisturedeveloped by the vapour generator has been particularlyuseful in laboratory applications [4]. In addition, therange of analytes and concentration levels analysed hasbeen increased using discrete sample injection techniques[5].

Figure shows the layout of the fully automatedinstrument which can be used to analyse liquid samples.It comprises a random access autosampler, a vapourgenerator and the Merlin Atomic Fluorescence Detector.These instruments are controlled using an IBM com-patible PC.

Figures 2 and 3 show the schematic arrangement of thevapour generator and the transfer of the mercuryentrained in an argon carrier gas into the Merlin detector.The switching valve ensures a steady transfer from reagentblank to sample and this minimizes the inherent noise onthe signal. A typical signal response for the continuousflow approach is shown in figure 4. The steady state signalis produced by a 10 ppt standard and from this it is easyto show a detection level below ppt without preconcen-tration. The peak shape is specific to the sample type andthe presence ofinterferents can be recognized should thispeak shape deviate from the norm. Table shows acomparison between continuous flow and batch analysissystems. Typically, the measurements can be made overseven orders of magnitude. The discrete sample injectionapproach allows the system to cope both with high concen-trations of mercury; matrix interference effects can alsobe masked.

0142-0453/96 $12.00 (C) 1996 Taylor & F is Ltd.53

P. B. Stockwell et al. The role of discrete sample injection in trace mercury analysis by atomic fluorescence spectrometry

Figure 1. Merlin Plus system.

DryerSnC,)_ _] k = .... Gas Dr"- - I-" Out GasYlnB ..[:"’)’=>] [--’- 01 21 Argon 1 ISamplNOh-:---- Carrier 11 nl ToDetectorGas \,] ]’,Rotameter J] .....Waste

Gas/LiquidSeparator(B-pe)

Figure 2. Hydride valve configuration for sampling.

DryerSnCl2_l_/’(xLL-,,2a__ / ] GasI-’( j..t "r uryer

/ Gas InBlank O

I,’;’, Argon LF ToSample......: Carrier "Jl E! Detector

Gas I11 V IP.....RotameterJ "’WasteGas/LiquidSeparator(B-Type)

Figure 3. Hydride valve configuration for blank.

Reading Std 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 4. Typical signal response for the continuous flowapproach.

Sensitivity* Mercury continuous flow. Typical Instrument L.O.D. <1.00 ppt.

* Further enhancement can be obtained by using the goldtrapping technique, with typical improvements of greater thanx l0. This is achieved by using the PSA Galahad Pre-concentration Unit.

154


Table 1. The advantages and disadvantages of continuous and batch systems.

Continuous flow Batch analysis

Advantages

Disadvantages

Precise control over reaction conditionsConstant generation of hydrogenExperienced operators not requiredPrecisions of approx. 1 easily obtainable in linear rangeLarge sample volume requiredLong analysis time (60 s)

Small sample requirementEconomical reagent usageInexpensive equipment

Operator intensivePrecision is function of injection techniqueIntermittent production of hydrogenTime consuming

Further reduction of the detect levels has recently beenrepeated by Cossa et al. [6] using an additional concen-tration step onto a gold/platinum trap. Figure 5 showsthe instrumental configuration required for this.

Table 2 sets out the advantages of atomic fluorescence.These basically relate to selectivity and sensitivity,especially the wider linear dynamic range which canextend across many orders of magnitude.

Table 3 sets out the few limitations ofthe technique. Whenspecifically looking at the situation with mercury theseare significantly overcome using the P S Analytical designconcepts. The presence of self-absorption at high concen-trations and the possibility ofmatrix effects in, for example,contaminated land samples can be seen as a problem area.This paper shows how the introduction of small discretesamples into the flowing steam can, ifproperly controlled:

(1) Extend the dynamic range of the analyses.(2) Effectively eliminate matrix effects by dilution.(3) Provide the basis of a flexible approach to on-line

analyses.

Fluorescence techniques have typical limits of detectionbelow 10 ng 1-1 with linearity to 100 ng ml-1. The linearcalibration range stretches over four orders of magnitudewhich is obviously beneficial in view of the wide range ofmercury concentrations found in the environment.

Table 2. Advantages of atomic fluorescence spectrometry.

Sensitivity attainable is controlled by the intensity of the lightsource.

Equipment can be less complex than that needed for AAS orAES.

High sensitivity attainable into the far UV (AAS and AES areinsensitive).

Good linearity.Low spectral interference.High selectivity.Analytical line summation.

Table 3. Disadvantages of atomic fluorescence spectrometry.

Quenching from gaseous species in atom cell.Scattering from light source.Self absorption at high concentrations.Poor sensitivity for elements which absorb and emit in the visible

region compared to AES.

Samples with concentrations exceeding the linearity aresusceptible to self-absorption. This process is best explainedusing a standardized fluorescence cell like that shown infigure 6.

This theoretical model assumes that the light beams areparallel and that there is uniform atomic concentrationand temperature. At high concentrations, incidentradiation passing through A1 may be lost by absorptionbefore excitation can occur. Useful fluorescence may alsobe lost by reabsorption in the region AL. In an idealsituation these regions would be infinitely small, therebyminimizing self-absorption. Figure 7 shows a typicalprofile obtained using the continuous flow approach fora 2000 gg 1-1 mercury solution and the self-absorptionprocess is clearly evident. As the concentration increases,there is a rapid rise in signal until the concentration hasreached a level where self-absorption occurs. At this pointthe signal begins to fall, in severe cases to zero. When thesample is removed the concentration begins to decline andthe signal begins to rise once more. Carry-over timesbetween samples can be up to 5 minutes depending onthe concentration of mercury present.

The atomic fluorescence signal magnitude can be reducedwith the use of alternative carrier gases, such as nitrogenor air. These gases have been found to reduce thefluorescence signal by eight and 30 times, respectively,due to quenching. This is basically radiation-less deacti-vation of excited atoms due to collisions with foreignspecies present in the cell. The effectiveness of this processis dependent on the rate at which collisions occur, thetype of non-radiative process involved and the effectivecross-section of the quenching species. The fraction ofabsorbed photons actually re-emitted as fluorescenceradiation is known as the fluorescence yield, b.This is defined as:

Total probability per second of de-excitation

where BO. is the Einstein coefficient for fluorescenceemission. The total probability of de-excitation is thesummation of BO. with the rate of all non-radiativeprocesses contributing to quenching. The quenchingprocess occurring with mercury in the presence ofnitrogenor air is due to inelastic collisions involving transfer ofenergy. The process for nitrogen is thus:

Hg* + N2 --- Hg + N2 155


(a)

SnCI

Blank

ArgonCareerRotameter

Gas/LiquidSeparator(B-Type)

Out DryerGas In

HygroscopicMembrane

Waste

ValveWaste C

Trap

Valve Valve

A

orMerlin

Sample ArgonGas/LiquidSeparator

MxlinMercury Level

Balance

Vapour Generator

Trap

Galahad

(b)

SnCI2-3.5 ml miri 1:8 ml rnin-1Sample ArgonCareer GasRotameterGas/LiquidSeparator(B-Type)

ValveC Merlin Mercury Level

Vapour Generator

GasOut Dryer IF! [1 TrapGasln # l.., e ITI Galahad

Hygroscopic orMerebrane

SampleMerlin

Argon"’’;

Gas/LiquidWaste Separator

(c)

ArgonCarrier GasRotameter

Figure 5. Instrumental configuration.

DryerGasOut Dryer

Gas In

HygroscopicMerrbrane

Gas/LiquidSeparator(B-Type)

where the superscript * is used to indicate the excitedstate. The rate, r, ofeach collision is defined as the numberof excited mercury atoms quenched per second per unitvolume and can be expressed in the form:

where k is the rate constant for the process. The probabilityof an excited mercury atom being quenched is thereforer/[Hg*]. Hence the fluorescence yield factor for mercury

ValveMerlinC

Valve ValveTrap

Waste

MerlinSample Argon

Gas/LiquidSeparator

Mercury Level

Generator

Galahad

with quenching caused by nitrogen will be:

BlokiN2] + B10

where B10 is the Einstein coefficient for the excited stateto the ground state transition. It therefore follows thatthe maximum value of b is unity where no quenchingoccurs. This, however, is unlikely to occur.

156


ExcitingLight Beam

ObservedFluorescenceRadiation

Figure 6. Standardized fluorescence cell.

Ileadi.9 Ztd Itu.-2 ilaseLi.e= -0.75

10 20 30 40

Peak area" 32Gfl.56zxec

Peak Height" 17U.95z

""150 60 70 80 90 10 120

Figure 7. Typical peak shape, illustrating the process of self-absorption, using the continuous-flow approach for a 2000 #g l-lsolution of mercury.

Although a reduction in signal is clearly observed, thequenching process has no relation to linearity because theself-absorption process is dependent on the atomic concen-tration and the atom cell dimensions. The reduction insignal from quenching therefore has no practical use inthis application. The analytical response curve for argonand nitrogen is shown in figure 8 for continuous flowvapour generation.

Discrete sample analysis typically uses volumes between50 and 200 gl. Although not as sensitive as the continuousflow approach, it is less suceptible to self-absorption andmatrix interference. A schematic arrangement for adiscrete sample analyses is shown in figure 9. Thisapproach has been subsequently superseded by using thestandard P S Analytical vapour system configuration forthe 10.004 model. With this instrument all the time cyclesof the vapour generator are programmed by the computersoftware. The discrete volume is therefore determined asa fraction of flow rate and the time of valve opening: Thelimitation on this effect is the dead volume within theswitching value itself. This allows the upper limit of thecalibration range to be increased. Figure 10 shows threeanalytical response curves corresponding to 75, 100 and200 gl loop sizes. The smaller volumes gave higher upperlimit calibration ranges, with slightly less sensitivity. Anestimation of the sensitivity is again obtained from theslope of the curve at the point where deviation fromlinearity occurs. Table 4 summarizes the effect of samplevolume on linearity.

Samples containing levels of mercury exceeding the linearrange are still susceptible to self-absorption; a typicalprofile is shown in figure 11. The profile corresponds toa 1000001agl- solution of mercury, and the self-absorption is clearly observed. However, this is not assevere as that for continuous flow and the carry-over timesbetween samples with high levels is negligible. This allowsthe analysis of total samples to proceed with minimaldelay.

]oooooo

100000

1OOOO

1000

100

10

.001 .01 10 100

Concenation pgt1000 10000 100000

Argon -"- Nitrogen

Figure 8. Analytical response curves for continuous-flow atomic fluorescence spectrometry using both argon and nitrogen carrier gases.

157


2% m/V SnCI //3.5 ml/min

1% V/V HNO7.5 ml/min

Sample

3.5 rnl/min Waste O (C)

Rotarmters

- ShieldCarrier_,Gas/Liquid

Separator

[] [][] [][] []

Merlin Computer

Printer

Figure 9. Schematic arrangement for a discrete sample analysis.

E+07

1000000

100000

10000

1000

O0

10

Graph

t/" 100 lal loop75 lal loop

O0 1000 10000 100000 1000000Concentration (gg

Figure 10. Three analytical response curves corresponding to 75,100 and 200 #l loop sizes.

10 20 100 110 120 13030 40 50 60 70 80 90

Time Secs

Figure 11. Typical peak shape, illustrating the process of self-absorption, for the flow-injection approach for a 100 000 #g l-1solution of mercury.

Table 4. The effect of sample volume on linearity.

Sample volume Upper limit calibration range(tl) (mg 1-1 Slope

75 10.5 3.7100 10 4.42OO 7 8.4

To assess the validity of the flow-injection cold vapour-atomic fluorescence spectrometry (CV-AFS) technique, arange of certified reference materials and zinc batteryanodes has been analysed for mercury. These resultsare shown in table 5. Table 5 shows that accurate,precise quantitative measurements can be made using the

Table 5. Determination of mercury in certified reference materialsand battery anodes.

Certified reference Expected/certified Concentra Weightmaterial concentration found dilution

NIST SRM 1641b 1.52 __+ 0.04 1.41

__0.04 0

(mercury in water) (gg ml-) (lag ml-)NBS SRM 3133 10.00 __+ 0.01 9.89 __+ 0.20 2500(spectrometric solution) (lag ml-1) (lag ml-1)Zinc Anode A 1000 1060 + 30 200

(lag ml- (lag g-)Zinc Anode B 0 4.11 + 0"29 200(lag g-X)

Zinc Anode C 1200 1150 + 43 200(lag ml-’) (lag g-l)

158


o

o

o

No TAG Ref ippm RunBaseline=0.100%

Peak Area: 1892%secPeak Height: 157.4%

No TAG Ref blank Run 1

Baseline=0.100x

Time-136secs Signal>0. 300% Standby

Mo

Range 10

Peak Area: 6.253%secPeak Height: 0.500%

Range i0 Filter 16 Filter 16 Time-136secs Signal>0.100% Standby

(a) (b)

Figure 12. Analysis of 1 ppm followed immediately by blank--the signal trace for this is running along the baseline. Levels higher than1 ppm will automatically be reset to blank when the software senses that the signal level will go @scale.

flow-injection CV-AFS approach. The advantage of thissystem is that minimal sample dilution is required, whichconsiderably reduces the sample preparation time anderrors involved in large serial dilutions. One furtheradvantage is that matrix interference is reduced becausethe analyte is separated from the matrix by generation ofthe gas and because small volumes are utilized.

Another major advantage of the discrete approach is thatthere is little interference or carry-over from one sampleto another. This allows linear calibrations up to 10 ppm(as shown in table 4). However, it is possible to analysesamples up to 100 ppm with little or no carry-overbetween the high and low sample. The signal from100 ppm will provide a detector overload, but the selectionvalve in the vapour generator will quickly switch to theblank/standby situation thereby returning the signal tothe baseline. The next sample to be analysed can beaccurately determined. Figure 12 illustrates this point withresults for a sample greater than 100 ppm followed by ablank.

The application of the discrete sample injection and thecapabilities of the continuous flow approach can beeffectively married together using the method chainingapproach developed by P S Analytical. The detector hasa pre-amplifier allowing selection of gain ranges betweenand 1000. In standard operation mode, the gain range is

pre-selected for different concentration ranges. One gainrange will allow a calibration span of two orders ofmagnitude. This provides the most accurate and precisemethods of analysis. Samples which contain concentrationsabove the calibration range are normally diluted manuallyafter the analytical run. The method chaining facilityallows different methods with different gain ranges to bechanged together, so that no manual dilutions for samplesthat are above the calibration range are required.

The discrete sample mode can be used to assign samplesto the appropriate calibration ranges. With a samplingrate of 80 samples per hour, the samples can be quickly

screened to estimate levels. The reproducibility of thediscrete injection mode is illustrated in figure 13, whichshows replicate analyses in the ppb region for a mercurystandard solution containing lag 1-1.

In method chaining, up to five different methods, eachwith a unique calibration, can be coupled together. Thiscan be illustrated by reference to two calibration methods.In the first, the calibration range is set between 0-1 lag 1-1;ifsamples above lag 1-1 are analysed offscale, recognitionwill switch the solution to the reagent blank, thusconserving the sample and minimizing carry-over. Afterthe autosampler programme is complete the system isrecalibrated at a higher range, such as 0-100 lag 1-1 andthese samples, which previously went offscale, arere-analysed. All conditions on the vapour generator areset automatically by the software programme. Theoperational sequence of the method chaining is activatedfrom the autosampler programming mode.

On-line applications

Discrete sampling has a major advantage when dealingwith complex matrices, especially with concentrated acidsor alkalis. The effective dilution step has been extremelybeneficial when combined with the extremely lowdetection capabilities of the detector to analyse suchsamples. For on-line process analyses this benefit hasfurther advantages: sample volume is reduced; the risk ofcontamination between corrosive materials and the instru-mentation is minimized; and the response time to changesin sample concentration is also reduced.

In comparison to laboratory analyses, the chemistryinvolved must be more complex in order to cope with thedigestion of all forms of mercury to mercury(II), priorto the tin(II) chloride reaction. The authors have directedtheir research to a number of chemical regimes, but inthis paper reference is only made to the application toconcentrated sulphuric acid.

159


Reproducibility of Atomic Fluorescence (lppb)Discrete Analysis over 80 Minutes

lOO

80

40

0 20 40 60 80 100 120 140

Run Number

Figure 13. Response curve--the method chaining facility allows the user to run up to five different analytical methods in sequence.

VALVE B

VALVE C

VALVE D

VALVE E

7 t!’ "[- "l Perma Pure,--".e-+ tI:]"’’’’’’2 Dryer

/ i ,l/’ Mixing Manifold

L2 3 4 ,Der Gas

1 ’-’ S ".- s,a@ etector

Six Port > t Sm;l Valve C=al

Sensor Unit Wste Mix4 6 7 8Carrier GLS

Figure 14. Schematic arrangement of the chemical manifold required for the determination of mercury at low levels inconcentrated sulphuric acid.

Table 6 shows the specific considerations that requireattention prior to translating a laboratory instrument toon-line applications. Reagent consumption is a primeconsideration since it is desirable that little, if any,maintenance is required at rates greater than one week.

Figure 14 shows the schematic arrangement of thechemical manifold required for the determination ofmercury at low levels in concentrated sulphur aciddeveloped by Brahma et al. [7-1. The oxidation or digestion

Table 6. Specific considerations required to translate laboratoryinstrumentation to process applications.

Conversion of all mercury species to divalent mercury.Low reagents consumption and reagent stability.Stable and rugged detection system.Reliable interface between sample stream and on-line system.Fault diagnostics with feedback system.Data processing via CPUs.

160


Do ne244

S"t LI

RuI; 7 Juri 961 1. 4i 35" 22

T ,,.: U ,:: h.".’., t ,:..) il :

DELAY NEASURE RESET%30sec 200see ZOOsecLoop" 100u F RaLe" 1 rnl/mn

PSA Merlin Calibration Setdp"RANGE 10 FINE 10.0 ZERO Au, toRal]g.., & Ru.rrlil,9 1/4s et": aul::.ornaticall’/

SENSOR1: Blank 2: Lo,. Standard3" Hgh Sd 4" Sample5: 20% HCL 6: 0. BY/BY037: .OHNH3C 8: 2 5nC2N,,B,,A.[1 PLump L"es RED-RED

F1-Help F2-Keep Displayed F3-PT int. F4-Ed it_ Es:c- Me.nu

Figure 15. Methods page from the computer software.

S

C

n

H9 ]>9 Online R.F.. Fit Least gares gtrJght Line$1ope:BT.41@91 ermZ:@.l]BB@ 7er3--@.@@BI]8 No ReslopeStd Conc Output Fit Runs1 @,BBB 1,Z44.8,@EH 12 8.543 7,64 -8,883 1.886 %.17 8.@@4 1Linear Corr [email protected] Intercept= i].89

Printed ro ?oucltone 23 tla9 96

Figure 16(a). Simple response for a repeat sampling sequence.

step is provided by the reaction ofpotassium permanganate.This is then followed by a conventional tin II chloridereduction to form mercury vapour which is introducedinto the detector. Figure 15 shows the methods page fromthe computer software. To minimize matrix inteferenceeffects and excessive heat generation, the system is usedin a discrete sampling mode. Reagent flow rates are keptat a level of0.5 ml min to conserve reagents and maximizethe time between reagent changes in the instrument. Theanalytical cycle performed is to analyse a blank, a 100 ppbstandard, a 200 ppb standard and then the sample streamfour times prior to repeating the analytical sequence tofit the analytical needs at the time. The inherent sensitivityof the atomic fluorescence detector allows air (which

provides a 30-fold quenching of the signal) to be used asa transfer gas. For process applications, this provides thesite managers with a considerable comfort factor becausethe dangers of asphyxiation due to other carrier gases areovercome.

The analysis is operated in a similar manner to thelaboratory instrument, which is somewhat different toconventional laboratory analyses. Repeat cycle discreteanalyses are performed using the software facilities tocontinuously update the calibration and blank values; thismeans that the results are continuously updated. Figure 16shows a simple response for a repeat sampling sequenceand a calibration graph over a 48-hour period.

161


0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

H concentration (ml/i)in Sulphuric acidMean 0.59ppm Std Dev 0.06ppm RSD 9.54%

Time

Figure 16(b). Calibration graph over a 48-hour period.

Conclusions

The sensitivity of the continuous flow vapour generationsystem coupled to atomic fluorescence provides extremelylow detection levels. The combination with discretesample introduction extends the linear dynamic rangeof the instrumentation to encompass seven orders ofmagnitude. In addition, this mode of operation extendsthe capabilities of the system to handle complex matricesand also to provide extremely versatile on-line processinstrumentation.

References

1. JoN.s, R., JAFFE, R. and AZAAM, A., Journal of High ResolutionChromatography, 17 (1994), 745.

2. THOMPSON, K. C. and GODDV., R. G., Analyst, 100 (1975), 544.3. GODDE, R. G. and STOCIWWLL, P. B., Journal of Analytical Atomic

Spectrometry, 4 (1989), 301.4. CORS, W. T., EBDO, L. C., HILL, S. J. and Sroc:WFLL, P. B.,

Analyst, 117 (1992), 717.5. CORNS, W. T., Em)o, L. C., HILL, S. J. and Sa’ocI,:WFLL, P. B.,

Journal of Automatic Chemistry, 13 1991 ), 267.6. COSSA, O., SANJUAN, J., CLOUD, J., STOCKWELL, P. B., and

W. T., Journal of Analytical Atomic Spectrometry, 10 (1995), 287.7. BRAHMA, N., CORNS, W. Y., EVANS, E. H. and STOCIWFaA, P. B.,

Private communication.

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Vol. Therole of discrete sample injection in trace mercury ...JournalofAutomaticChemistry,Vol. 18, No. 4 (July-August 1996), pp. 153-162 Therole ofdiscrete sampleinjection in trace