Critical comparison of two standard digestion procedures for the determination of total mercury in natural water samples by cold vapour atomic absorption spectrometry

377

Analytrca Chrmrca Acta, 236 (1990) 377-384 Elsevler Science Pubhshers B V., Amsterdam

Critical comparison of two standard digestion procedures for the determination of total mercury in natural water samples by cold vapour atomic absorption spectrometry DOUGLAS Department

C. BAXTER

* and WOLFGANG

FRECH

of Analytrcal Chemrstry, Unwersrty of (/med. S-901 87 Lime6 (Sweden) (Received

5th March

1990)

ABSTRACT

Two pretreatment procedures for total mercury determmatlons m natural water samples were compared The hrst. the Swechsh Standard method (DPl), Involves dIgestIon of water m the presence of concentrated mtrlc acid at 120 o C and under pressure for 30 mm. In the West German Standard method (DP2). small volumes of mtrlc and sulphunc acids, permanganate and peroxodlsulphate are added to the sample, and dlgestlon proceeded at 50 o C m an ultrasomc bath Mercury was determmed after both digestIon procedures usmg a modltied cold vapour atomic absorption spectrometnc method, In which mercury generated on addltlon of a reducmg agent 1s collected and subsequently atonnzed m a platinum-hned graphite furnace The efficacy of the two dIgestion procedures was tested usmg various standard organic mercury compounds and It was found that only Df?1 provided quantltatlve recoveries Purlticatlon of the reagents reqmred by DP2 was achieved usmg a mercury-sefectlve Ion-exchange resin, Chehte S, resultmg m blank levels below 1 5 ng Hg I-’ Both methods were apphed to the determmatlon of total mercury m an unpolluted marsh water sample, glvmg 2 0 ng Hg I-’ (DPl) and 2 7 ng Hg I-’ (DP2) The West German Standard dIgestIon procedure (DP2) 1s recommended for the determmatlon of total mercury m natural water samples Keywords

Mercury,

Waters

A great variety of digestion methods have been proposed for the pretreatment of water samples for total mercury determmatlons. These typically involve combmatlons of strong acids, oxidants, elevated temperatures and pressures and UV Irradiation for long time periods [1,4,6-10,13-151. As the total mercury concentrations m unpolluted, natural waters are typically about 1 ng 1-l [4,16181, the addition of large amounts of reagents will obviously present a potentially serious source of contamination [4,9,13,14]. Further concern exists regarding the ability of the various pretreatment procedures to provide accurate total mercury concentrations, some methods having been shown to give low recoveries [6]. Here, a comparison 1s made between two dlgestlon procedures for the preparation of natural

The determmatlon of total mercury concentrations is important for assessing water quality [l]. For this purpose, most laboratories routinely use cold vapour atomic absorption spectrometry (CVAAS) [2] combined with an amalgamation/ preconcentration step to attain the necessary sensitivity [3-lo]. As yet, however, there is no umversally accepted method to convert all mercury species present in water samples to a form amenable to reduction by tm(I1) chloride or sodium tetrahydroborate. Some reports claim that sodium tetrahydroborate m combination with copper will allow the determination of total mercury without prior oxidative treatment [11,12], an observation not borne out by others [13,14]. Hence a digestion procedure is generally considered mandatory before the reduction step [1,4,6-lo]. 0003-2670/90/$03

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0 1990 - Elsevler

Science Pubhshers

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DC

water samples for total mercury determmattons. One IS the current Swedish Standard method [15] and the other has been presented by Sinemus and Stable [19] and will be adopted as one of the West German Standard methods [20]. The latter requires only small amounts of added reagents (leading to low blank levels) and involves dtgestion at 50 o C m an ultrasonic bath. It is shown that only the latter method is capable of providing complete recoveries of various organic mercury compounds added to distilled and marsh water matrices, as determined by CVAAS using a platinum-lined graphite furnace for m situ preconcentration [21]. Procedures are described for purifying the necessary reagents and cleanmg the vessels, permitting the determination of total mercury m unpolluted water at low ng 1-l levels.

BAXTER

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W

FRECH

EXPERIMENTAL

furnace mcorporating an integrated contact (IC) tube [22] lined with platinum gauze was used for the collection of the generated mercury. The furnace was mounted on the optical bench of a research spectrometer system based on a modified Varian Techtron AA-6 atomic absorption spectrometer [23]. A Tecmar Labmaster with 12-btt ADC was used to interface the spectrometer with an Ericsson PC equipped with a 20 Mb hard disk and Facit 4513 A4 matrix printer. Acquisition and processmg of atomic and background absorbance data were achieved using graphite furnace AAS software obtained from B. Radziuk (Bodenseewerk Perkm-Elmer, F.R.G.). Instrumental parameters are listed m Table 1, those given for the mercury generator having been optimized by response surface methodology, as described previously [21]. The characteristic mass for mercury was 49 pg/O.O044 A s for the conditions m Table 1.

Instrumentation Mercury vapour is generated and collected using the apparatus shown in Fig. 1. The mercury generator consists of a 100-ml Pyrex round-bottomed flask with a side-arm for mtroduction of the reducing agent. A laboratory-constructed graphite

Procedure Water samples or standard solutions are pipetted into the reaction vessel (1) (see Fig. l), then reducing agent (1 ml of 10% SnCl, m 16% H,SO, or 3% NaBH, in 1% NaOH) is inJected from the disposable plastic syringe (2). Argon is

A

T 7

-

Fig 1. SchematIc diagram of the apparatus used for the generatlon and collection disposable plastic syringe,, 3 = argon carrier gas Inlet, 4 = goId filter, 5 = smtered-glass gas outlet, 7 = Integrated contact graphite tube hned wth platmum gauze

of mercury 1 = Reactlon vessel, 2 = l-ml fnt, 6 = mercury vapour and argon carrier

DETERMINATION

TABLE

OF MERCURY

IN NATURAL

WATER

SAMPLES

379

1

Instrumental

parameters

Step/parameter

Ttme (s)

Dry

100-200 10 15 10 3

Ash I Ash II Atonuze Clean-out

Temperature a

( o C)

100 100 100 b 750 c 1000

Purge gas

Argon,

300 ml mu-’

spectrometerd. Wavelength (nm) Spectral band width (nm) Hollow-cathode lamp

253 7 0.5 Beckman

Mercury generator Argon flow-rate (ml mu-‘) Sample volume (ml) Purge time (mm)

Hg (4 5 mA)

540 10 (50-60) 5 (15) e

e

a Drymg ttme increased for larger sample volumes b Basehne offset correctlon performed ’ Read command selected, heatmg rate ca 1500“ C s-l -d-B_ckground correctlon usmg Vanan H, lamp. e Repeated five times for the analysis of marsh water samples

introduced (3) passing first through a gold mercury adsorber (4) and then out via the sintered-glass frit (5) through the sample. The generated mercury vapour is carried by the argon flow along the PTFE and glass tubing (6) and directed into the platinum-lined IC tube (7). Once collection is complete (longer times are required for larger sample volumes, see Table l), the tubing is removed from the furnace and the heatmg programme is started. To remove condensed water vapour from the furnace, a drying step is included before atomization.

TABLE Standard

Reagents and materials Inorganic mercury solutions were prepared in 1% (w/v) HNO, from a 1000 mg 1-l standard (Coleman Instruments, U.S.A.). Acids were generally taken from an all-quartz sub-boiling still (Acidest, F.R.G.), and water purified m a Milli-Q system (Milhpore) and contammg no detectable mercury was used for dilutmg solutions. Merck analytical-reagent grade HNO, without further purification was found to be suitable for sample digestion according to the Swedish Standard method [15] (see below) when higher mercury con-

2 orgamc

mercury

compounds

and solvents

used

Compound

Formula

Mol. wt (g mol-‘)

Suppher

Solvent

Methyl mercury chlonde Mercury(I1) acetate

CH,HgCI Hg(CH,C4),

25108 318 70

Merck Baker and Adamson

Mercurescem sodmm Sodium ethylmercunttiosahcylate Phenylmercury chlonde

C,,HsBr,HgNa&&

750 66

Rledel de-Haen

10 ml CH,OH 20 ml CH,OH + 5 ml CH ,COOH 10 ml CH,OH

C,H,HgNaO,S C,H,HgCl

404 82 313 15

Fluka Merck

10 ml CH,OH 0 2 ml dusobutyl

a Ddutton

to 1 1 with Mdh-Q

water.

b Dllutton

to 1

I with CH,OH

d

ketone

’

DC

380

centrations were to be determined (2 100 ng 1-i). No significant contribution to the blank was observed at this concentration level and so a considerable saving o’f distilled HNO, was possible. Reducing agents (10% SnCl, in 16% H,SO, and 3% NaBH, in 1% NaOH) were purified by bubbling a stream of argon for 30 min through the solutions in the reaction vessel shown m Fig. 1. Stock solutions (100 mg Hg 1-l) of five organic mercury compounds were prepared according to the prescriptions given m Table 2 using appropriate amounts of compounds. All chemicals used were of the highest available purity. A stabihzing solution was made by mixing (1 + 1) distilled HNO, and 1.0% K,Cr,O, (dissolved in Milli-Q water). This preservative was added (10 ml) to the marsh water samples (1 I), see below, to prevent losses of mercury. For sample digestion according to the procedure described by Sinemus and Stable [19], the following reagents were also required: potassium permanganate (50 g KMnO, in 1 1 of Milh-Q water), potassium peroxodisulphate (40 g K,S,O, in 1 1 of Mini-Q water) and hydroxylammonium chloride (10 g HONH,CI m 100 ml of Milli-Q water). Entire IC tubes were manufactured from single pieces of RWO quality graphite and coated with pyrolytic graphite. The IC tube is of 5 mm square mside diameter, 0.6 mm wall thickness, 25 mm long and has an injection port diameter of 1.8 mm. Platinum gauze to line the tubes and gold turnings used to adsorb mercury from the SRgrade argon (see Fig. 1) were obtained from Johnson Matthey Chemicals (Great Britain). Nalgene FEP bottles (1 1) were used for the collection of marsh water samples. For the digestion of water samples according to the method of Smemus and Stable [19], 200-ml Nalgene PSF flasks were employed. For reagent purification, a mercury-specific, ion-exchange resin, Chelite S (Serva, Sweden), was used. Details of reagent purification and vessel cleanmg procedures are given under Results and Discussion. Sample collectron Two water samples were collected sedge type marsh near Svartberget

from a lowForest Re-

BAXTER

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search Area in the north of Sweden on the morning of December 12th, 1989. The ice covermg the marsh was first penetrated and then the precleaned l-l PTFE containers were removed from sealed plastic bags and immersed and filled to the mark. A lo-ml volume of the preservative agent was immediately added and the bottles were closed and returned to the plastic bags. After transport to the laboratory, the samples were digested by the followmg procedures.

DIgestIon procedure 1 Digestion procedure 1 (DPl) was done following the Swedish Standard method [15]. This mvolves adding 50 ml of the sample plus 10 ml of HNO, to a colourless 100-ml borosilicate glass autoclave bottle, fitted with a screw-cap. The bottle is then placed in an autoclave and digestion proceeds at 200 kPa (120°C) for 30 min. Owing to the low total mercury concentration in the marsh water, five sub-samples were digested and, after cooling, the contents of one bottle were poured into the reaction vessel shown in Fig. 1. Mercury vapour was generated and collected on the platinum gauze-lined IC tube and the next sub-sample was taken. Hence a total marsh water sample volume of 250 ml was used for the determination. Blanks consistmg of Mini-Q water were similarly treated with preservative and digested. For these solutions, the 10% SnCl, in 16% H,SO, reducing agent was used as lower blank values were obtained.

Dtgestlon procedure 2 In this procedure (DP2) [19,20], 100 ml of the sample are poured mto a 200-ml PSF bottle and then 1 ml of 5% KMnO,, 1 ml of distilled HNO,, 1 ml of distilled H,SO, and 2 ml of 4% K,S,O, are added m sequence. The sample bottle is placed m an ultrasonic bath filled with water and thermostated at 50°C. After somcation for 30 mm, the samples are allowed to cool and 0.4 ml of 10% HONH,Cl is added (to reduce excess of oxidant) immediately before analysis. For the purposes of determining total mercury in the marsh water, the process was scaled up and two digestions of a 125-ml sample were performed. A total sample volume of 250 ml was

DETERMINATION

OF MERCURY

IN NATURAL

WATER

used, the mercury generatton and collection step being carried out m five batches usmg SnCl, as reducing agent and the conditions listed m Table 1. Mini-Q water blanks underwent the same procedure.

RESULTS

AND

DISCUSSION

Purlfrcatlon procedures

Owing to the extremely low total mercury concentrations m relatively unpolluted waters, it is of paramount importance that the digestion procedure used does not contribute greatly to blank levels. Churchwell et al. [9] noted that 5 ng of mercury were extraneously added to water samples when digested accordmg to the U.S. Environmental Protectton Agency protocol (SW-846 Method 7470 [24]). For a l-l sample, the blank level obtamed would thus be higher than the endogenous total mercury concentratton m an unpolluted water sample. To purify the reagents used in DP2, sufficient ion-exchange resin, Chehte S, to half-cover the bottom of each reagent storage bottle (KMnO,, K,S,O, and HONH,Cl) was added. The same method was used to remove mercury contamination from the potassium dichromate portion of the stabthzmg solution. It should be noted that this ion-exchange resm cannot be used to purify acids, smce at pH I 1 bound mercury 1s released.

TABLE

Bottles used for the digestion of samples by either procedure were first cleaned by filling with 10% HNO, and heating at 50” C for 3 h. The bottles were then thoroughly rinsed with Mini-Q water and filled with 5 ml of KMnO,, 10 ml of HNO,, 1 ml of H,SO, and 2 ml of K,S,O, per 100 ml of solution (the remainder being Milli-Q water). Next, the bottles were placed m the ultrasome bath and sonicated for 2 h at 50” C, and fmally rmsed again with Mtlli-Q water. Blank levels were determined using SnCl, as reducing agent for three 250-ml distilled water samples pretreated according to each of the two digestion procedures. These experiment resulted in blank values of 1.38 * 0.41 and 1.43 k 0.48 ng Hg 1-l for DPl and DP2, respectively. Similar experiments for three undigested distilled water samples (treated only with preservative) yielded a blank level of 0.58 f 0.20 ng Hg 1-l. Effect of sample matrix and pretreatment procedure on recovery of added mercury compounds

To assess whether or not a digestion procedure is actually necessary to determine total mercury in water samples, the followmg experiments were done. Five organic mercury compounds were indtvtdually added to separate 50-ml ahquots of dtstilled water and marsh water, the latter having been treated with preservative. The concentratton of mercury in these samples was equivalent to 500 ng 1-l. Blank solutions and samples spiked with

3

Recovery Added

381

SAMPLES

of mercury

compound

a

compounds

added

to dlstllled Recovery Dlstdled SnCI,

CH,HgCI Hg(CH,CO,), C,,H,Br,HgNa,O, C,H,HgNaO*S C,H,HgCI HgCl2

1 48 I 4 17 96

’

and marsh

water samples,

wlthout

dIgestIon

(%) b water NaBH, 101 21 24 104 21 99

d

Marsh water NaBH, d

e

89 8 7 97 59 104

A Compounds added to gve 500 ng Hg I-’ ’ Recovery given by 100 (blank corrected peak area for spiked sample/peak area for mercury standard) ‘ 1 ml 10% SnCI, m 16% H,SO, added to 10 ml of sample d 1 ml 3% NaBH, m 1% NaOH added to 10 ml of sample e Water samples treated with preservative

DC

382

an inorganic mercury standard were also prepared. Mercury was then determmed m these solutions using both reducing agents, and the results are presented in Table 3. From Table 3 it can be seen that the SnCl, reducing agent only gtves total recoveries for inorganic mercury, although it is not completely selective. Also, NaBH, is not capable of reducing completely all the organic mercury compounds, which agrees with recent observations by Ping and Dasgupta [14]. It is interesting that in only one case (phenylmercury chloride) does NaBH, give a higher recovery in the marsh water matrix. This is probably due to the presence of actdic preservative m the marsh water, as NaBH, is known to recover mercury from organic compounds more efficiently in acidic media [11,14]. Nevertheless, the recovery of mercury is not quantitative in all mstances and there is a clear effect of the sample matrix, confirming the need for a digestion procedure prior to the determmation of total mercury. As noted in the Introduction, a variety of sample pretreatment methods have been proposed for total mercury determinations [1,4,6-10,13-15,19, 201. The Swedish Standard method [15] (DPl) was chosen for study as it is fairly rapid (30-min digestion period) and mvolves the addition of a single reagent, HNO,, which can be readily punfied by sub-boiling distillation. For comparison, the West German Standard procedure [19,20] (DP2) was selected as this requires the addition of minimum volumes of reagent (4.4 ml per 100 ml of sample), a short dtgestion period (30 min) and avoids high-temperature sample treatment. To test the applicabihty of these two digestion procedures, orgamc mercury compounds were added to distilled water samples that had been treated with the stabtlizmg solution. Results of total mercury determinations following digestion by both methods are given m Table 4. Mean values of three separate digesttons are reported, except for the results with NaBH, as reducmg agent in combination with DPl, where only duplicate digestions were made. It IS seen that DP2 [19,20] gives excellent recoveries for all the compounds tested, whereas low values are obtained following DPl. Some improvement 1s possible m the latter mstance when the stronger reducmg agent,

TABLE

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4

Recovery of mercury compounds samples (treated with preservative), dIgestton procedures Added

BAXTER

compound

d

added to dlstdled water after usmg each of the two

(% ) ’

Recovery DPI 1151

CH,HgCl Hg(CH,CO,), GOHsBr2HgNa& C,H,HgNaO,S C,H,HgCI HgCl z d Compounds given by 100 peak area for H,SO, added NaOH added

SKI z ‘

NaBH,

6 68 55 67 8 100

95 67 65 96 86 99

’

DP2 [19.20] SnCl z ‘ 103 100 99 98 101 101

added to give 100-500 ng Hg I-’ ’ Recovery (blank corrected peak area for spiked sample/ mercury standard). ’ 1 ml 10% SnCI, m 16% to 10 ml of sample ’ 1 ml 3% NaBH, m 1% to 10 ml of sample

NaBH,, is used mstead of SnCl,, but quantitative recoveries of mercury are not generally posstble. The low recoveries cannot be explained by sample losses durmg autoclaving, as morgamc mercury is retained, and no significant difference m the weight of the bottles was noted after digestion. Nevertheless, autoclavmg is disadvantageous owing to the potential risks of mercury losses [14]. Subsamples of the marsh water were also spiked with the various mercury compounds and digested

TABLE

5

Recovery of mercury compounds ples (treated with preservative) dIgestIon procedures

added to marsh after usmg each

Added

(S) b

compound

a

Recovery DPl [15]

CH,HgCI Hg(CH,CO,), C,,H,Br,HgNa,O, C,H,HgNaOzS C,H,HgCl HgCI,

SnCl z ’

NaBH,

21 70 32 83 78 52

105 113 99 114 96 100

d

water samof the two

DP2 [19,20] SnCl, ’ 108 107 88 99 105 101

a Compounds added to gve 500 ng Hg I- ’ h Recovery gven by 100 (blank corrected peak area for spiked sample/peak area for mercury standard) ’ 1 ml 10% SnCl, m 16% H,SO, added to 10 ml of sample dl ml 3% NaBH, m 1% NaOH added to 10 ml of sample.

DETERMINATION

OF MERCURY

IN NATURAL

WATER

SAMPLES

by both methods. The results of the total mercury determinatrons gave the recoveries shown in Table 5. Surprisingly, the results followmg DPl and usmg SnCl, as reducing agent are generally better than those m Table 4 for the distilled water matrix, but the recoveries are not quantitative. The use of NaBH,, however, yields fairly good results (the large devlatlons are probably due to the larger and less reproducible blank values obtained using NaBH, rather than SnCl, as reducing agent), but we can offer no explanation for the apparently better recoveries from marsh water (Table 5) compared with dlstrlled water (Table 4). Table 5 also demonstrates again the effectiveness of the West German Standard digestion procedure, DP2 [19, 201. The better performance of DP2 can be explained by the action of the reagents used, KMnO,, HNO, and H,SO, for oxtdlzing sulphrdes in the sample, and K,!$O, for oxidrzmg organic compounds [9]. For the determination of total mercury, NaBH, appears to be the reducing agent of choice, at least when higher concentrations are present in the samples. However, the use of NaBH, results in higher blank values, and so for total mercury determinatrons below about 10 ng l-‘, SnCl, is to be preferred. This means that all mercury present m the sample must be present m the inorganic form so that SnCl, can work properly, requiring the use of an efflclent digestion procedure. Applrcatlon to the analysis of marsh water The total mercury concentration in a marsh water sample was determined using SnCl, as reducing agent after digestion by both procedures, the results being 2.00 ng Hg 1-l (DPl) and 2.67 ng Hg I-’ (DP2). Only one measurement was made followmg each pretreatment method so the uncertainty m the results could not be directly assessed. The results are m quahtative agreement with the data in Table 4, and the dtfference IS probably not statistically slgmflcant. One determination was also made wtthout any sample digestion, yielding a mercury concentratron of 0.85 ng 1-l. Mercury determmed m this fashion IS usually termed the “reactive” fraction, mcludmg the halides, hydroxIdes and complexes wtth organic acids [13,25].

383

0 05

a Tube

1

0 05

temp

750°C

/

1

0

7

35 Time

(s)

Fig 2 SIgnal traces for 250-ml samples of M&-Q water blanks (dotted lines) and marsh water samples (sohd hnes) (a) Undigested samples, blank peak area 0 007 A s, marsh water 0 032 A s (b) Samples dlgested by DPl, blank 0031 A s, marsh water 0 076 A s (c) Samples dlgested by DP2; blank 0 032 A s, marsh water 0 092 A s.

Figure 2 depicts mercury atomic absorption signals for blanks and marsh water samples obtained using the platmum-lined graphite furnace. The marsh water sample IS characterized by low pH (ca. 4) and salinity (s = O.OOS%), but 1s rich in strongly complexmg agents, e.g., humic substances [26]. A recent study using computatronal methods to develop an equilibrium speciation model for several metals m such a chemical environment Indicated that the bulk of the mercury would be present as organic complexes [26]. This is m agreement with the results of our measurements. Conclusrons The results of this study allow the recommendation of the West German Standard method [20] used here for the preparauon of water samples for total mercury determmatrons on the following grounds: quantitative recoveries of both orgamc and inorganic mercury compounds from distilled and marsh water matrices; (ii) low blank

DC

384

levels for the digestion after purification of reagents and cleaning of vessels by the prescribed methods; (ih) rapid sample preparation, only 30min sonication at 50°C being required, which represents a considerable savmg in time over many other digestion procedures [1,6,7]; and no high temperature or pressure 1s necessary, avoiding potential risks of losses of mercury from the samples [14]. The authors thank H.W. Sinemus (BodenseeWasserversorgung, F.R.G.) for valuable dlscusslons concerning the digestion of water samples for total mercury determinations. This work was supported by the Swedish Centre for Environmental Research.

REFERENCES AnalytIcal Quahty Control (Harmomsed Morutonng) Comnuttee, Analyst, 110 (1985) 103 W R. Hatch and W L Ott, Anal. Chem ,40 (1968) 2085. 0 I Joensue, Appl. Spectrosc.. 25 (1971) 526. N S. Bloom and E.A Crecehus, Mar. Chem., 14 (1983) 49. B Welz, M. Melcher, H.W &emus and D. Maier, At. Spectrosc , 5 (1984) 37 E U&no, T Kosuga, S Komsh and M N&umura, Envxon Scl. Technol , 21 (1987) 920 R Ahmed, K May and M Stoeppler, Fresemus Z Anal Chem , 326 (1987) 510 G A Gill and W F Fitzgerald, Mar. Chem , 20 (1987) 227

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9 M E. Churchwell, R L. Llvmgston, D L Sgontz, J D Messman and W.F. Beckert, Envxon Int , 13 (1987) 475 10 N Mlkac, Z Kwokal, K May and M Braruca, Mar Chem , 28 (1989) 109 11 J Toffalettl and J. Savory, Anal Chem , 47 (1975) 2091 12 S Margel and J Hirsch, Chn Chem , 30 (1984) 243 13 A Iverfeldt, Mar. Chem , 23 (1988) 441 14 L. Pmg and P K. Dasgupta, Anal Chem , 61 (1989) 1230 15 Swedish Standard, Metal Content of Water, Sludge and Sediment Determmed by Flameless Atonuc Absorption Spectrometry - Special Guidelines for Mercury, SS 02 81 75, Standarchsermgskomrssionen I Svenge, Stockholm, 1989 16 P Fnemann and D Schrmdt, Freseruus’ Z. Anal Chem., 313 (1982) 200. 17 Y H Lee, Int J Environ Anal. Chem., 29 (1987) 263 18 G. Kmewald, Z Kwokal and M Bramca, Mar Chem , 22 (1987) 343 19 H W Smemus and H-H Stabel, poster presented at the 5th Colloqtuum Atomspektrometnsche Spurenanalytik, Konstanz, Apnl 3-7, 1989 of Water, 20 German Standard Methods for the Exammatlon Waste Water and Sludge, CatIons (Group E), Determmation of Mercury (E12). Deutsches Instltut fhr Normung, Berhn, 1990 21 DC Baxter and W Frech, Anal Cium Acta, 225 (1989) 175 22 W. Frech, D.C Baxter and B Hutsch, Anal. Chem, 58 (1986) 1973 23 E Lundberg and W Frech, Anal Chem , 53 (1981) 1437 24 U S EnvIronmental ProtectIon Agency, Test Methods for Evaluatmg Sohd Waste, Physxal/Chemxai Methods, SW846, Otflce of Sohd Waste and Emergency Response, Washmgton, 2nd edn., 1982. 25 W H Schroeder, Trends Anal Chem , 8 (1989) 339 and S SJoberg, Nord Hydrol , m press 26 L Gunnenusson