Use of Flow Injection Cold VapourGeneration and Preconcentration onCoated Graphite Tubes for theDetermination of Mercury in PollutedSeawaters by Electrothermal AtomicAbsorption Spectrometry†

PILAR BERMEJO-BARRERA*, JORGE MOREDA-PINEIRO,ANTONIO MOREDA-PINEIRO, ADELA BERMEJO-BARRERA

Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Chemistry, University of Santiago de Compostela, Avenida de las Ciencias,s/n E-15706-Santiago de Compostela, Spain

Methods have been developed for the trace determination of utilized successfully for hydride forming metals. The use ofdifferent coating reagents such a palladium,21–24 platinum,21total and inorganic mercury in natural waters by cold vapour

generation–trapping and atomization in a graphite furnace by rhodium,21 rutenium,21 zirconium,25–27 tungstate,25 thalium25 ,molybdeno25, silver28 and palladium–iridium29 has been suc-selective reduction with NaBH4 and SnCl2. Iridium, tungstate

and zirconium coated graphite tubes were investigated for the cessful for metals such as lead,26 arsenic,21,24 antimony,21,24selenium,21,24,28 bismuth,21,29 germanium,23 tellurium28 andpreconcentration of the mercury vapour. The carrier solution

was 0.4 m of HCl, and 0.2% NaBH4 and 1.0% SnCl2 were tin.21,22,27For adsorption of the mercury cold vapour, the use ofused as reducing solutions for total and inorganic mercury,

respectively. Using iridium coated graphite tubes characteristic gold,30 platinum31 and palladium20 coated graphite tubes hasbeen proposed for preconcentration. Palladium coated graphitemasses of 300 and 240 pg for total and inorganic mercury,

respectively, and detection limits of 90 and 60 ng l−1 for total tubes have been referred to in the literature as those thatpresent a higher adsorption efficiency for volatile hydrides32and inorganic mercury were obtained for a 1500 ml sample

volume. The precision was between 0.7 and 2.1%. Accuracy of and mercury cold vapour20 than graphite tubes with othercoatings. Otherwise, owing to the high volatility of palladium,method was confirmed by the analysis of IAEA/W-4 fresh

water reference material. an injection of this species into the furnace is necessary foreach cycle. In addition, a change in the graphite furnace

Keywords: Electrothermal atomic absorption spectrometry; programme is necessary because of the tube coating and incold vapour; total mercury; inorganic mercury; coated graphite order to take measurements, thus increasing the time necessarytubes; preconcentration; natural waters per cycle. Therefore, the study of other permanent coating

species remains of interest.In the present work, selective mercury cold vapour gener-The concentration of mercury in natural waters is minute,

about 0.1 mg l−1 in fresh water,1 and this concentration is ation using SnCl2 or NaBH4 was carried out.33Preconcentration of mercury cold vapour using iridium, tung-significantly reduced in unpolluted natural waters. In order to

monitor levels of mercury in natural waters a detection limit state and zirconium coated graphite tubes was also carried outand the results obtained were compared.at of least of 10 ng l−1 (preferably 1 ng l−1) is required.

Therefore, sensitive methods are necessary to detect such lowconcentrations.

Owing to the simplicity, high sensitivity and relative freedom EXPERIMENTALfrom interferences, CVAAS, with or without a preconcentration

Apparatusstep before the mercury cold vapour generation, has generallybeen used for the determination of mercury in seawater A Perkin-Elmer (Uberlingen, Germany) FIAS-400 system withsamples.2–7 Continuous flow systems have also been used for a five-port FI valve was used with the cold vapour accessories.mercury cold vapour generation in natural water samples.8–16 The rotation speed of the two multichannel peristaltic pumps

Despite the high sensitivity of CVAAS and owing to the low were programmed and controlled automatically by a separatemercury concentration in un-contaminated waters, variations personal computer. A schematic diagram of the FIAS-400of conventional CVAAS, (introducing a step preconcentrating system is shown in Fig. 1. The FIAS–furnace sample transferthe mercury cold vapour, such as the amalgamation technique tube consists of a quartz capillary (2 cm long×1.3 mmusing different amalgamating media based on copper, silver, od×0.5 mm id) attached to a metal mount with a short piecegold and platinum metals17–19) have been proposed in order of silicone rubber tubing, which in turn is attached to a PTFEto increase the sensitivity of the method. However, the use of tube. The metal mount of the FIAS–furnace sample transferthe amalgamation technique presents several disadvantages, tube was loaded into the spring clip on the end of thesuch as the necessity to clean the amalgamating medium, also autosampler arm (Fig. 2 ). The FIAS mercury cold vapourthe sensitivity is flow rate limited and the reproducibility is generation programme is given in Table 1.affected by slight changes in the flow rate.20 Mercury absorbance was measured with a Perkin-Elmer

Preconcentration on to coated graphite tubes has been Zeeman 4100 ZL atomic absorption spectrometer equippedwith a THGA (transversely heated graphite atomizer) furnaceand an AS-71 autosampler. The source of radiation was a† Presented at the Eighth Biennial National Atomic Spectroscopy

Symposium (BNASS), Norwich, UK, July 17–19, 1996. mercury electrodeless discharge lamp operated at 4 W, which

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Table 2 Furnace temperature programme for mercury determination

Step Temperature/°C Ramp/s Hold/s Ar flow rate/ml min−11 20 0 3 2502 1300 0 3 0 (read)3 1500 1 3 250

Reagents

All solutions were prepared from analytical-reagent gradechemicals using ultrapure water, with a resistivity of 18 MV cm,which was obtained from a Milli-Q water purification system(Millipore, Milford, MA, USA).

Mercury() nitrate stock standard solution (1.000 g l−1 ) wasof analytical-reagent grade and supplied by Panreac(Barcelona, Spain). Methylmercury stock standard solution(1.000 g l−1 ) was prepared by dissolving 58.22 mg of referencematerial from the Commission of the European CommunitiesCommunity Bureau of Reference (BCR) in 50 ml of water.

Iridium chloride (Aldrich, Milwaukee, WI, USA) stock stan-dard solution (5 g l−1 ), zirconium oxychloride stock standardsolution (10 g l−1 ) prepared from ZrOClΩ8H2O (Merck,Darmstadt, Germany) and sodium tungstate stock standardsolution (10 g l−1 ) prepared from Na2WO4Ω2H2O (Merck)were used as trapping solutions.

The reducing agents used were 1.0% m/v tin chloridedihydrate (Merck) dissolved in 0.4 HCl and 0.2% m/vsodium tetrahydroborate (Aldrich) dissolved in 0.5% m/vsodium hydroxide solution (Carlo-Erba, Milan, Italy). Thesesolutions were prepared daily and the solutions were filteredbefore use. The carrier solution used was 0.4 HClprepared from 37% HCl (Merck).

Fig. 1 Schematic diagram of FIAS-400 system; (a) sampling step Synthetic seawater samples of high 72.8‰ (SSWI) and low(valve in fill position) and (b) cold vapour generation and trapping 34.2‰ (SSWII) salinity were prepared according to thestep (valve in injection position). literature31.

Argon of N-50 purity (99.999%) was used as the sheath gasfor the atomizer and as the internal purge gas. This wasobtained from SEO (Madrid, Spain).

Coating the Graphite Tubes

The coating method used involved the injection of five 100 mlaliquots of iridium (1 g l−1 ), zirconium (10 g l−1 ) or tungstate(10 g l−1 ) trapping solutions. Each injection of solution wasdried slowly by heating the atomizer at 150 °C with ramp rateand hold times of 25 and 40 s, respectively. A second dryngstep at 200 °C with ramp rate and hold times of 20 and 30 swere then used. This was followed by a reduction step at2000 °C, applied over 5 s. A tube treated in this manner cancould pass through about 500 firing cycles.

Fig. 2 Schematic diagram of the FIAS–furnace sample transfer tube.

Mercury Cold Vapour Generation and Preconcentrationprovided a 253.7 nm line. The spectral bandwidth used was0.7 nm. A THGA with an integrated platform was used. Volatile species of mercury were generated from a 1500 ml

sample prepared as follows: 9 ml of a natural water sampleIntegrated absorbance signals were used throughout. Thegraphite furnace programme used is given in Table 2. were placed in a 10 ml calibrated flask, with the appropriate

Table 1 FIAS mercury cold vapour generation programme: argon flow rate, 50 ml min−1 ; and trapping temperature, 20 °CFlow rate/ml min−1

Pump 2

Step Pump 1 Carrier solution Reducing solution Time/s Valve Function1 3 2 2 10 Fill Sampling step2 0 0 0 8 Inject Quartz capillary is placed into the graphite tube3 0 — — — Inject Hg cold vapour generation and trapping step4 0 0 0 8 Inject Quartz capillary moves out of the graphite tube5 0 0 0 5 Fill Start of furnace programme

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amount of HCl to obtain a concentration of 0.4 . The tip of addition, it is shown in Fig. 3 that a loss of sensitivity isobtained when using NaBH4 , this is due to differences in thethe quartz capillary tube was inserted, automatically, from the

outlet of the gas–liquid separator at the centre of the graphite mechamism of the reduction reaction in comparison with theuse of SnCl2 .tube. The carrier, 0.4 HCl, and reducing solutions, 1.0%

SnCl2 or 0.2% NaBH4 , were pumped into the generator for60 s at a rate of 3 and 2 ml min−1, respectively, and the evolved

Sample and reaction loopsmercury cold vapour was transferred, using an argon flow rateof 50 ml min−1 , to the furnace tube where it was sequestered Experiments were carried out using different combinations ofonto the coated graphite tube. Mercury was then atomized at sample and reaction loops (200, 500, 1000 and 1500 ml). Results1300 °C over 3 s using maximum power heating and internal showed that an increase in the sample volume involved anargon gas stop. The FIAS mercury cold vapour generation and equivalent increase in the mercury absorbance signal. Anthe graphite furnace programme are show in Tables 1 and 2. increase in the length (110 mm, 100 cm, 200 cm and 300 cm)

of the reaction loops did not produce an increase or decreasein the mercury absorbance signal. Thus, 1500 ml and 200 cm

RESULTS AND DISCUSSION as sample and reaction loops, respectively, were selected.Various experiments using fresh water and seawaters samplesand aqueous standard solutions spiked with Hg2+ (10 mg l−1 )

Flow rate parameters for carrier and reducing solutions, and forand CH3Hg+ (10 mg l−1 ) were carried out to optimize thecarrier gasparameters affecting the efficiency of mercury cold vapour

generation and adsorption onto coated graphite tubes, using Generation of the mercury cold vapour was obtained usingSnCl2 and NaBH4 as reducing solutions. different pump speeds, and the results (Fig. 4) showed an

increase in the integrated absorbance with increasing pumpingspeed up to 50–60 rev min−1 . For higher pump speeds a

Optimization of Mercury Cold Vapour Generation Conditions decrease in the mercury absorbance signal occurred. Thus,flow rates of 3 and 2 ml min−1 for the carrier and reducingCarrier and reducing solutionssolutions, respectively, were selected for further experiments.The optimum concentration of HCl, used as the carrier solution The optimum flow rate of the carrier gas was 50 ml min−1.and NaBH4 and SnCl2 concentrations, used as reducing solu-

tions for inorganic mercury, and total mercury, respectively,obtained for the cold vapour generation are shown in Table 3. Optimization of Mercury Cold Vapour Adsorption Onto Coated

The selective cold vapour generation of mercury species, Graphite TubesHg2+ and CH3Hg+ , by NaBH4 and SnCl2 at the concen-

Reaction and trapping timetrations referred to above are shown in Fig. 3. As can be seen,organic mercury could only be reduced by NaBH4 .33 In The reaction time for mercury cold vapour generation and

trapping time onto the coated graphite tubes was investigated.Results (Fig. 5 ) showed that a reaction and trapping time ofTable 3 Optimum conditions affecting the generation of mercury60 s is necessary for zirconium, tungstate and iridium coatedcold vapourgraphite tubes.

Parameter Optimum condition[HCl] 0.4 NaBH4 0.2% m/vSnCl2 1.0% m/vSample loop 1500 mlReaction loop 110 mmCarrier flow 3 ml min−1Reducing solution flow 2 ml min−1Argon flow 50 ml min−1

Fig. 4 Effect of pump speed on the integrated absorbance of 5 mg l−1of Hg with 0.4 of HCl and 1.0% SnCl2 for Ir (&) , Zr (2 ) and W($) coated graphite tubes. The loop sample was 1500 ml.

Fig. 3 Effect of SnCl2 (1.0% m/v) (dotted lines) and NaBH4 (0.2% Fig. 5 Effect of reaction time on the integrated absorbance of 5mg l−1 of Hg with 0.4 of HCl and 1.0% m/v of SnCl2 for Ir (&), Zrm/v) (solid lines) on mercury cold vapour generation from solution

with various amounts of inorganic and organic mercury. (2) and W ($) coated graphite tubes. The loop sample was 1500 ml.

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T rapping and atomization temperatures

The efficiency of trapping mercury cold vapour onto the coatedgraphite tubes was found to be optimum at room temperaturefor all coated graphite tubes. As can be seen in Fig. 6, thesensitivity decreased when the trapping temperature wasincreased. Thus, an adsorption temperature of 20 °C wasselected for further experiments.

The atomization temperature (using a hold time of 3 s andmaximum power heating) of trace amounts of mercuryadsorbed onto the different coated graphite tubes was alsoinvestigated. The results (Fig. 7) showed optimum atomizationtemperatures of 1300 °C for the use of iridium and zirconiumcoated graphite tubes and 1700 °C for tungstate coated graphitetubes. For atomization temperatures higher than the abovetemperatures a decrease in the integrated absorbance wasobtained (Fig. 7) whilst the peak height was greatly increased.

The mercury cold vapour trapping mechanism could be dueto an intermetallic compound or alloy that is probably formedbetween Hg0 and iridium, tungstate or zirconium, the same asoccurs with palladium.20

Analytical Figures of Merit

The calibration and standard additions equations obtained foraqueous standard solutions, synthethic seawater of high(SSWII) and low (SSWI) salinity and real freshwater andseawaters samples, all spiked with 5, 10 and 20 mg l−1 of Hg2+for inorganic mercury determinations; and 2.5, 5 and 10 ofHg2+ and 2.5, 5 and 10 of CH3Hg+ (as Hg) for total mercurydeterminations, are shown in Table 4. These calibration andstandard additions equations were obtained for iridium, zir-conium and tungstate coated graphite tubes. For each coatedgraphite tube, the slopes of the graphs are identical (t-test fora confidence level of 95%34 ). Therefore, the method is notdependent on the salinity of the natural water.

The characteristic mass (i.e., the mass of analyte whichprovides a defined peak with an absorbance of 0.0044) for a

Fig. 6 Effect of trapping temperature on the integrated absorbanceof 5 mg l−1 of Hg with 0.4 of HCl and 1.0% m/v SnCl2 for Ir ($),Zr (2) and W ($) coated graphite tubes. The loop sample was 1500 ml.

Fig. 7 Effect of atomization temperature on the integrated absorbance

Tab

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Cal

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and

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slope

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s

Cal

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Stan

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usst

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wat

erIn

orga

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mer

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—

Zrco

ated

grap

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QA=

0.01

0+0.

014[

Hg]

QA=

0.01

2+0.

013[

Hg]

QA=

0.01

3+0.

014[

Hg]

QA=

0.01

5+0.

014[

Hg]

QA=

0.01

8+0.

015[

Hg]

Wco

ated

grap

hite

QA=

0.00

8+0.

016[

Hg]

QA=

0.00

8+0.

016[

Hg]

QA=

0.01

0+0.

015[

Hg]

QA=

0.01

1+0.

015[

Hg]

QA=

0.01

7+0.

014[

Hg]

Irco

ated

grap

hite

QA=

0.01

2+0.

028[

Hg]

QA=

0.01

3+0.

027[

Hg]

QA=

0.01

1+0.

029[

Hg]

QA=

0.01

5+0.

027[

Hg]

QA=

0.02

1+0.

029[

Hg]

Tot

alm

ercu

ry—

Zrco

ated

grap

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QA=

0.00

9+0.

011[

Hg]

QA=

0.01

0+0.

010[

Hg]

QA=

0.00

8+0.

012[

Hg]

QA=

0.01

1+0.

012[

Hg]

QA=

0.01

2+0.

011[

Hg]

Wco

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grap

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QA=

0.01

0+0.

009[

Hg]

QA=

0.01

0+0.

011[

Hg]

QA=

0.00

8+0.

010[

Hg]

QA=

0.00

9+0.

011[

Hg]

QA=

0.01

3+0.

010[

Hg]

Irco

ated

grap

hite

QA=

0.01

2+0.

020[

Hg]

QA=

0.01

3+0.

019[

Hg]

QA=

0.01

1+0.

021[

Hg]

QA=

0.01

3+0.

021[

Hg]

QA=

0.01

8+0.

020[

Hg]

of 5 mg l−1 Hg with 0.4 of HCl and 1.0% m/v SnCl2 for Ir (&), Zr(2 ) and W ($) coated graphite tubes. The loop sample was 500 ml.

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Table 5 Sensitivity for different coated graphite tubes 5 Simeonov, V., and Andreev, G., Fresenius’ Z. Anal. Chem., 1983,314, 761.

6 Schmidt, D., and Freiman, P., Freseniuz’ Z. Anal. Chem., 1984,m0* (pg) LOD/ng l−1317, 385.

7 Schintu, M., Kauri, T., Contu, A., and Kudo A., Ecotoxicol.Tube Hg2+ Hg Hg2+ HgEnviron. Saf., 1987, 14, 208.Zr coated graphite 700 900 150 200 8 Goto, M., Munaf, E., and Ishii, D., Fresenius’ Z. Anal Chem.,W coated graphite 450 600 90 120 1988, 332, 745.Ir coated graphite 240 300 60 90 9 Munaf, E., Haraguchi, H., Ishii, D., Takeuchi, T., and Goto, M.,Sci. T otal Environ., 1990, 99, 205.* m0=characteristic mass. 10 Munaf, E., Takeuchi, T., Ishii D., and Haraguchi, H., Anal. Sci.,1991, 7, 605.

11 Munaf, E., Takeuchi, T., and Haraguchi, H., Fresenius’ J. Analsample loop of 500 ml and the detection limits obtained forChem., 1992, 342, 154.each coated graphite tube using both reducing media are

12 Zhan, L., Lu, J., and Le, X., Microchim. Acta. 1993, 111, 207.shown in Table 5. The use of an iridium coated graphite tube13 Hanna, C., Tyson, J., and McIntosh, S., Anal. Chem., 1993, 65, 653.gives better sensitivity than the other coated graphite tubes. 14 McIntosh, S., Baasner, J., Grosser, Z., and Hanna, C., At.In addition, the use of NaBH4 gives poorer sensitivity than Spectrosc., 1994, 15, 161.

the use of SnCl2 . 15 Garcıa, M., Garcıa, R., Garcıa, N., and Sanz-Medel, A., Talanta,1994, 41, 1833.The within-batch precision (RSD for 11 replicate measure-

16 Madrid, Y., Cabrera, C., Perez-Corona, T., and Camara C., Analments at different levels) obtained for the various coatedChem., 1995, 67, 750.graphite tubes is good between 0.7 and 4.0% for all concen-

17 Yamamotto, J., Keneda, Y., and Hikasa, Y., Int. J. Environ. Anal.trations investigated.Chem., 1983, 16, 1.The accuracy of the method was studied by analysing an 18 Bloom, N., and Crecelius, E., Mar. Chem., 1983, 14, 49.

IAEA/W-4 (simulated fresh water) reference material with a 19 Mertens, H., and Althaus, A., Fresenius’ Z. Anal. Chem., 1983,316, 696.certified total mercury concentration of 1.8–2.7 mg l−1 . The

20 Yan, X.-p., Ni, Z.-m., and Guo, Q.-l., Anal. Chim. Acta, 1993,results obtained were 2.2±0.3 and 2.3±0.3 mg l−1 when using272, 105.SnCl2 and NaBH4 as the reducing solutions, respectively,

21 Sturgeon, R., Willie, S, Sproule, G., Robinson, P., and Berman,showing that the value for inorganic mercury determined in S., Spectrochim. Acta, Part B, 1989, 44, 667.the reference material is in good agreement with the certified 22 Tao, G., and Fang, Z., Talanta, 1995, 42, 375.value. 23 Tao, G., and Fang, Z., J. Anal. At. Spectrom., 1993, 8, 577.

24 Zhang, L., Ni, Z.-m., and Shan, X.-Q., Spectrochim. Acat, Part B,1989, 44, 339.

CONCLUSION 25 Iwamoto, E., Shimazu, H., Yokota, K., and Kumamaru, T.,J. Anal. At. Spectrom., 1992, 7, 421.Mercury cold vapour can be effectively adsorbed onto the 26 Yan, X.-P., and Ni, Z.-m., J. Anal. At. Spectrom., 1991, 6, 483.

surface of iridium, zirconium and tungstate-coated graphite 27 Ni, Z.-m., Hang, H.-B., Li, A., He, B., and Xu, F.-Z., J. Anal. At.tubes at room temperature. The iridium coated graphite tube Spectrom., 1991, 6, 385.

28 Ni, Z.-m., He, B., Hang, H.-B., and Li, A., J. Anal. At. Spectrom.,gives the best analytical performance. Thus, this permanent1993, 8, 995.chemical modifier could be used for the preconcentration of

29 Matusiewicz, H., Kopras, M., and Suszka, A., Microchim. Acta,mercury vapour avoiding the problems related to the use of1995, 52, 282.palladium. The inorganic and total mercury concentration of 30 Lee, S., Jung, K., and Lee, D., Talanta, 1989, 36, 999.15 samples can be determined per hour. 31 Baxter, D., and Frech, W., Anal Chim. Acta, 1989, 225, 175.

32 Tsalev, D. L., D’Ulivo, A., Lampugnani, L., Di Marco, M., andZamboni, R., J. Anal. At. Spectrom., 1995, 10, 1003.

REFERENCES 33 Oda, C. E., and Ingle, J. D., Anal. Chem., 1981, 53, 2305.34 Miller, J. C., and Miller, J. N., Statistics for Analytical Chemistry,1 Metals and their Compounds in the Environment, ed. Merian, E., Wiley, New York, 1984.VCH, New York, 1991.

2 Sanemasa, I., Takagi, E., Deguchi, T., and Nagai, H., Anal Chim.Paper 6/05079DActa, 1981, 130, 149.

3 Mandal, S., and Das, A., At. Spectrosc., 1982, 3, 56. Received July 22, 19964 Freiman, P., and Schmidt, D., Fresenius’ Z Anal. Chem., 1982, Accepted October 21, 1996

313, 200.

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