Determination of Trace Metals in Seawater by Inductively Coupled Plasma Mass Spectrometry After Off-line Dithiocarbamate Solvent Extraction GRANT J. BATTERHAM, NIELS C. MUNKSGAARD AND DAVID L. PARRY* School of Mathematical and Physical Sciences, Northern T erritory University, Darwin 0909, NT , Australia An oV-line solvent extraction procedure with inductively technique that utilises the inherent advantages of ICP-MS analysis is yet to be reported. The development of such coupled plasma mass spectrometric (ICP-MS) determination was developed for the determination of heavy metals in sea- a technique would significantly reduce analysis times and improve detection limits owing to the simultaneous water at ultra-trace levels.The procedure was adapted from a dithiocarbamate–diisobutyl ketone solvent extraction system multi-element capability and high sensitivity of ICP-MS. Recently, we developed a rapid solvent extraction technique with Hg back-extraction, which was previously validated using electrothermal atomic absorption spectrometry.The to determine Cd, Co, Cu, Fe, Ni, Pb and Zn in sea-water with ETAAS analysis.13 The technique complexes trace metals in simultaneous ICP-MS determination significantly reduced the analysis times and improved the detection limits (Cd 0.2, Co sea-water with DTC, extracts these complexes into a retained diisobutyl ketone phase and then back-extracts the complexed 0.3, Cu 3, Fe 21, Ni 2, Pb 0.5 and Zn 2 ppt).The rapid single extraction procedure was quantitative and external standards metals into aqueous solution by exchange with Hg, which has a much greater DTC stability constant (of the order of 1040).14 were used for ICP-MS calibration. The method gave good reproducibility with precisions at the 100 ppt level better than In this work, we examined whether this solvent extraction system could be modified and coupled with ICP-MS for the 5% (n=4), except for Fe (7%).The method was validated by accurate analysis of CASS-3 Nearshore Sea-water and determination of trace metals in sea-water. NASS-4 Open Ocean Sea-water Standard Reference Materials. EXPERIMENTAL Keywords: Heavy metals; sea-water; preconcentration; solvent Reagents extraction; dithiocarbamate; inductively coupled plasma mass Ultrapure water was obtained from a Permutit HI-PURE spectrometry system (Sydney, Australia) fed with reverse osmosis water and was used for the blank and to make up all solutions.Chemicals In the field of trace metal analysis of natural waters, inductively were of analytical-reagent grade unless indicated otherwise. coupled plasma mass spectrometry (ICP-MS) has become The solvent phase was diisobutyl ketone (DIBK) (95% accepted as one of the most sensitive, reliable and accurate 2,4-dimethylheptan-6-one; Ajax, Auburn, Australia) and was techniques. Regulatory agencies have adopted this method for dispensed from a Fortuna 10 Optifix solvent dispenser (Walter compliance monitoring of natural waters in relation to their Graf u.Co, Main, Germany). The DTC complexing agent water quality criteria.1 However, for sea-water, the high salt was 0.5% each of sodium diethyldithiocarbamate (Ajax) content and low levels of trace metals invariably preclude and ammonium pyrrolidinedithiocarbamate (Ajax). The comdirect analysis by ICP-MS. The salt matrix produces lower plexing agent was made up in a 10 ml calibrated flask immedianalyte signals due to ionisation suppression2 and even with ately prior to use.The solution was cleaned once by adding sample dilution, polyatomic isobaric interferences still aVect a 1 ml of DIBK, shaking for 1 min, then discarding the upper number of analytes such as 40Ar23Na on 63Cu and 44Ca16O on DIBK phase. Stock ammonium acetate buVer solution (3 M 60Ni. Hence the analysis of sea-water by ICP-MS generally ammonia–2 M acetic acid) was prepared from Suprapur 25% requires a preliminary matrix separation. ammonia solution (Merck, Darmstadt, Germany) and Aristar Traditionally, matrix separation was performed using either acetic acid (BDH, Poole, Dorset, UK). A stock 100 ppm Hg organic complexing agents with solvent extraction3 or chelating back-extracting solution was prepared from HgNO3 (Ajax) ion-exchange resins.4 These techniques all utilised electrother- and was acidified to 0.02 M HNO3 with Suprapur acid (Merck) mal (graphite furnace) atomic absorption spectrometry and spiked (10 ppb) with Ga, In and Tb (APS solutions, Alpha (ETAAS) for analysis.ICP-MS was first introduced for sea- Resources, Stevensville, MI, USA) to act as internal standards. water analysis by McLaren et al.,5 who used an oV-line column of silica immobilised 8-hydroxyquinoline to chelate trace Extraction Procedure metals, prior to elution and analysis in dilute HCl–HNO3 solution. Chelating ion-exchange techniques have evolved The solvent extraction procedure was described in detail by Batterham and Parry.13 The extraction procedure was under- considerably in recent times, with many researchers developing sophisticated on-line ICP-MS systems using both taken in a class-100 laminar flow cabinet.All laboratory ware was acid-washed to an ultraclean standard. In addition, a 8-hydroxyquinoline and iminodiacetate columns.6–10 These techniques simultaneously separate the salt matrix and precon- small acid bath (5% HNO3) and a high purity water bath were maintained in the laminar flow cabinet to rinse polyethyl- centrate trace metals by a factor of 5–10, achieving low ppt detection limits with an analysis time of 10–15 min per sample.ene automatic pipette tips immediately prior to use. These baths and other dispensing glassware were maintained under Many researchers still employ dithiocarbamate (DTC) solvent extraction techniques based on the work of Danielsson plastic wrap (Gladwrap) when not in use.For the extraction procedure, 80 ml of acidified filtered and co-workers3,11 and Magnusson and Westerlund12 to determine trace metals in sea-water by ETAAS. A solvent extraction (<0.45 mm) sea-water were placed in a 125 ml screw-capped Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1277–1280) 1277polypropylene separating funnel (Nalgene, Rochester, NY, Table 2. The 111Cd analyte mass includes a correction for a possible MoO interference as recommended by the USEPA.1 USA) and adjusted to approximately pH 4.5 with ammonium acetate buVer.The amount of buVer required was determined External standards (1, 10 and 100 ppb) were prepared using the stock Hg solution (APS solutions, Alpha Resources). The on a separate representative non-extracted sample. A 100 ml volume of DTC complexing solution and 5 ml of DIBK were internal standards in the Hg back-extraction solution were used to correct for ionisation suppression and instrument drift; added (Optifix dispenser).The solution was shaken for 10 min with a mechanical wrist-shaker (Lab-Line, Melrose Park, IL, In was used for Cd, Tb was used for Pb and Ga was used for the remaining elements. USA), followed by a 5 min rest for phase separation. The lower sea-water phase was drained and 4.5 ml of the DIBK were removed by an adjustable pipette (Gilson, Worthington, OH, Method Validation USA) fitted with a long tip, and placed in a 10 ml screwcapped polypropylene, tapered centrifuge tube.A 1 ml volume CASS-3 Nearshore Sea-water and NASS-4 Open Ocean Seaof Hg back-extraction solution was then added and the tube water Standard Reference Materials (SRMs) were obtained was shaken by hand for 2 min for back-extraction. The upper from the National Research Council of Canada (Ottawa, DIBK phase was completely removed from the centrifuge tube Canada) to validate the method. CASS-3 was extracted and using an adjustable pipette fitted with a polypropylene tube analysed by both ICP-MS and ETAAS for comparison, and extension on the pipette tip.NASS-4, which contains lower levels of trace metals, was For analysis by ETAAS, about 0.9 ml of the aqueous phase analysed only by ICP-MS. was transferred into an autosampler cup and analysed immediately as described by Batterham and Parry.13 For analysis by RESULTS AND DISCUSSION ICP-MS, the centrifuge tube was re-capped and refrigerated at 4 °C prior to analysis.These refrigerated solutions were On-line column ICP-MS techniques that have involved inadfound to be stable for at least 3 d. equate washing of the column to remove residual salts or To examine whether the relatively high concentration of Hg co-elute retained alkaline metals have been reported to be significantly suppressed the ionisation of the lighter elements subject to isobaric interferences such as 40Ca16OH on 57Fe, with ICP-MS analysis, a series of solutions were prepared 42Ca16OH and 43Ca16O on 59Co, 44Ca16O on 60Ni and containing Hg concentrations from 0 to 200 ppm and equival- 40Ar23Na on 63Cu.6,15–17 Similarly, the use of a solvent extracent concentrations of trace metals (32 ppb Zn, Fe and Cu; 16 tion technique with ICP-MS requires the eVective elimination ppb Pb, Ni and Co; 10 ppb Ga, Tb and In; and 3.2 ppb Cd).of the salt matrix. The solvent DIBK meets an important The respective ICP-MS count rates were then recorded for the criterion in this regard, having a low water miscibility trace metals at each diVerent Hg concentration.(0.06±0.02 ml per 100 ml).18 This and the non-complexing of alkali metals by DTC provide a relatively interference free matrix for the determination of trace metals by ICP-MS. Instrumentation Solvent extraction techniques generally back-extract the The ETAAS instrument was a Varian (Mulgrave, Australia) DTC complexed metals retained in the solvent into an aqueous SpectrAA40 with a GTA-95 graphite tube atomiser and auto- phase using nitric acid decomposition of DTC.We have found sampler. Pyrolytic graphite-coated platform ETAAS tubes and the acid decomposition to be very ineYcient for Cu and Co, deuterium background correction were used. Ammonium dihy- which form very strong and stable DTC complexes, respectdrogenphosphate (0.5%) was used as a chemical modifier and ively. Acid back-extraction also entails a large final backthe operating parameters and furnace programmes were as extraction volume to bring the acid concentration to 2% for specified by Batterham and Parry.13 ICP-MS analysis.Metal-exchange back-extraction, however, The ICP-MS system was an Elan 6000 with an AS90 displaces DTC complexed metals with a metal of greater autosampler (Perkin-Elmer, Norwalk, CT, USA) fitted with complex aYnity. Mercury has a very high DTC stability an MCN-100 microconcentric nebuliser (CETAC, Omaha, NE, constant and gives quantitative back-extraction of all trace USA).The AS90 autosampler was enclosed inside a purpose metals examined in this study in 2 min, into a 1 ml volume. built, sealed acrylic housing, and the sample uptake tubing This enables a high preconcentration factor (72) to be achieved was replaced with the MCN-100 capillary tubing. The instru- from a small volume of sea-water (80 ml ). Metal-exchange ment was operated in accordance with the manufacturer’s back-extraction is essentially non-destructive, with the specifications and the operating conditions are given in Table 1.DTC–Hg complex retained by the solvent, and hence not The nebuliser gas flow and ion lens voltage were optimised for contributing to any polyatomic interferences. The typical conmaximum sensitivity prior to analysis. centration of Hg remaining in an aqueous back-extract after The analyte masses and elemental corrections are given in back-extraction was approximately 50 ppm.The ionisation suppression of the relatively high Hg concentration on the lighter metals was examined and found to be very small over Table 1 Instrument conditions for Elan 6000 ICP-MS system with an MCN-100 microconcentric nebuliser Power 1000 W Table 2 Recommended elemental equations Argon plasma gas flow rate 17 l min-1 Argon auxiliary gas flow rate 1.2 l min-1 Element Elemental equation Argon nebuliser gas flow rate 1.0 l min-1 Sample rinse 20 s at 48 rpm Fe 57M Co 59M Sample uptake 20 s at 48 rpm 60 s at 24 rpm Ni 60M Cu 63M Scan mode Peak hopping Sweeps per reading 15 Zn (64M)-(0.035313) (60M) Ga 69M Replicates 3 Dwell time 100 ms Cd (111M)-(1.073) ((108M)-(0.712) (106M)) In (115M)-(0.016) (118M) Integration time 1500 ms Analysis time per sample 65 s Tb 159M Pb (206M)+(207M)+(208M) Total volume used per sample 300 ml 1278 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 3 EVect of Hg concentration in the back-extraction solution on the ionisation suppression of other metals Metal/103 counts s-1 Hg, ppm 57Fe 59Co 60Ni 63Cu 64Zn 69Ga 111Cd 115In 159Tb 208Pb 0 23 224 49 226 185 140 11 345 490 395 50 20 209 52 233 178 150 12 376 490 420 100 21 213 48 230 185 126 13 365 530 420 150 20 204 42 200 160 132 13 370 504 410 200 20 200 48 190 150 133 13 345 462 426 the range 0–100 ppm Hg (Table 3) and could be compensated masses for Cu (63 and 65) and Zn (64 and 66) and found no interferences, with identical results being obtained for NASS-4 for by the use of the internal standards.The results obtained for CASS-3 and NASS-4 SRMs are at both analyte masses for each metal. Only the results for the most abundant isotope in each case are presented in Table 4. given in Table 4. The results were within the 95% confidence limits of the certified values for all metals examined using To investigate the versatility of our technique, we quantitatively extracted sea-water samples spiked with 25–200 ppb of either ETAAS or ICP-MS analysis.The blanks and detection limits achieved with both methods are given in Table 5. The Zn or Cu. Of the metals we examined, Zn forms one of the weakest DTC complexes and Cu forms the strongest (after use of ICP-MS analysis improved the detection limits by up to an order of magnitude (Co, Pb), with all metals 3 ppt, Hg). This indicates that our procedure can determine total dissolved metal concentrations to the stoichiometric equivalent except Fe (21 ppt), compared with ETAAS analysis.The detection limit of 2 ppt for Zn (Table 5) is diYcult to achieve on a of 200 ppb of Zn or Cu. However, one exception was an upper limit of approximately 3 ppb for the quantitative back- routine basis given the relatively high Zn blank (61 ppt), but a detection limit of better than 10 ppt is typically attained. extraction of Co. Although Co has a lower extraction constant than Cu, it is transformed into a Co(DTC)3 complex which is These detection limits are lower than those of other solvent extraction techniques that until now have relied upon ETAAS kinetically stable20 and therefore diYcult to back-extract.Generally, even contaminated sea-water contains sub-ppb analysis. The detection limits are also generally lower than those previously reported for on-line column ICP-MS analy- levels of Co. For higher Co levels, additional Hg solution and/or an extended back-extraction time can be used.sis.6–10 On-line techniques generally only utilise a preconcentration factor of 5–10 to limit analysis times, whereas our oV- We previously reported that a spike correction may be necessary for Zn determined by ETAAS using our extraction line preconcentration technique uses a factor of 72. Ultrapure water for the blanks was extremely important with such a high method,13 attributing this to incomplete extraction with aged NaDDC. ICP-MS analysis has shown that the Zn recoveries preconcentration factor when determining low ppt metal levels.The precision obtained for our technique with ICP-MS was are indeed quantitative and that our ETAAS spike correction has been acting as a standard additions calibration, compensat- better than 5% at the 100 ppt level, except for Fe (7%). This precision is also generally better than that previously reported ing for an unknown matrix interference. This has only occurred since changing sources of NaDDC and it was likely that an for on-line column ICP-MS analysis.6–10 Few on-line column ICP-MS procedures determine Fe and unknown contaminant or breakdown product in the original NaDDC source acted as a chemical modifier in the ETAAS those which do have very poor precision owing to high Fe blanks and ArO isobaric interferences.6,19 Our method gives analysis.The interference does not aVect ICP-MS determination and can be decreased in ETAAS analysis by greater better precision, but still has a relatively high detection limit (21 ppt) in comparison with the other metals (3 ppt). This sample dilution.One advantage of on-line column ICP-MS systems is that was due to the high background isobaric interferences on 54Fe and 57Fe from 40Ar14N and 40Ar16OH, respectively. Even the potential for exposure and airborne contamination of samples is reduced. This must be balanced by the meticulous though the low concentration of nitric acid (0.02 M) used in our Hg back-extraction minimises the 40Ar14N interference, we attention necessary to limit contamination from the column assembly and reagents, particularly with regard to poor Fe found that this interference had a greater fluctuation than 40Ar16OH. This fluctuation can become significant when meas- and Zn blanks.6,8,10,15 Our solvent extraction system requires only a single quantitative extraction that entails minimal uring Fe at low ppt levels.We also examined both analyte Table 4 Analysis of CASS-3 and NASS-4 SRMs (ppt) Method and sample Cd Co Cu Fe Ni Pb Zn ETAAS, CASS-3* 33.2±0.8 47±6 532±7 1210±20 390±14 11±4 1200±40 ICP-MS, CASS-3* 31.0±0.2 43±2 537±5 1160±20 384±11 9.1±0.9 1250±20 Certified† 30±5 41±9 517±62 1260±170 386±62 12±4 1250±240 ICP-MS, NASS-4* 17.3±0.1 8.6±0.2 223±2 121±8 231±1 9.0±0.5 106±4 Certified† 16±3 9±1 228±9 105±16 228±11 13±5 115±18 * Precision expressed as standard deviation of four replicates.† Precision expressed as 95% confidence interval.Table 5 Comparison of blanks and detection limits (DL) obtained with ETAAS and ICP-MS analysis (ppt) Parameters Cd Co Cu Fe Ni Pb Zn Blank, ETAAS BDL* BDL 19 BDL BDL BDL 69 Blank, ICP-MS 0.2 0.3 24 38 5 6 61 DL (3s), ETAAS 1.0 15 6 30 18 8 10 DL (3s), ICP-MS 0.2 0.3 3 21 2 0.5 2 * BDL=below detection limit. Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1279exposure. Of the reagents used in our procedure, only the Council for a grant allowing the purchase of the ICP-MS system. DTC solution requires minimal pre-cleaning.Most of our blank (Table 5) was attributed to technical grade DIBK. These blank levels are generally lower than that obtained with on-line REFERENCES column ICP-MS procedures.6–10 1 Method 1638: Determination of T race Metals in Ambient Waters Unlike on-line techniques, where the signal is transitory, by Inductively Coupled Plasma-Mass Spectrometry, USEPA, OYce longer analysis times can be used with oV-line techniques to of Water, Washington, DC, 1995, EPA 821-R-95–031.allow isotopic analyses to be undertaken. We used this tech- 2 Beauchemin, D., McLaren, J. W., Mykytiuk, A. P., and Berman, nique to study Pb isotope ratios at low ppt levels in sea-water, S. S., Anal. Chem., 1987, 59, 778. 3 Danielsson, L.-G., Magnusson, B., and Westerlund, S., Anal. using sample analysis times up to 3 min to determine the Chim. Acta, 1978, 98, 47. potential impact of Pb–Zn ore concentrates in a marine 4 Kingston, H.M., Barnes, I. L., Brady, T. J., Rains, T. C., and environment. Champ, M. A., Anal. Chem., 1978, 50, 2064. With our system, a dozen samples can be extracted simul- 5 McLaren, J. W., Mykytiuk, A. P., Willie, S. N., and Berman, S. S., taneously in about 1 h and back-extracts can be stored prior Anal. Chem., 1985, 57, 2907. to analysis. The CASS-3 ICP-MS results (Table 4) were 6 McLaren, J. W., Lam, J. W. H., Berman, S. S., Akatsuka, K., and Azeredo, M.A., J. Anal. At. Spectrom., 1993, 8, 279. obtained on back-extracts that had been stored at 4 °C for 3 d. 7 Bloxham, M. J., Hill, S. J., and Worsfold, P. J., J. Anal. At. This allows many extraction runs to be undertaken prior to Spectrom., 1994, 9, 935. analysis, with the internal standards compensating for any 8 Bettinelli, M., and Spezia, S., J. Chromatogr., 1995, 709, 275. changes in the sample viscosity and instrument drift. Taylor 9 Nelms, S. M., Greenway, G.M., and Koller, D., J. Anal. At. et al.10 employed internal standards in the elution acid for Spectrom., 1996, 11, 907. on-line column ICP-MS. This would be particularly important 10 Taylor, D. B., Kingston, H. M., Nogay, D. J., Koller, D., and Hutton, R., J. Anal. At. Spectrom., 1996, 11, 187. for on-line column techniques given that the ICP-MS instru- 11 Danielsson, L.-G., Magnusson, B., Westerlund, S., and Zhang, K., ment is in continual operation, generally taking 10–15 min per Anal.Chim. Acta, 1982, 144, 183. sample. Our overall instrumental analysis time per sample was 12 Magnusson, B., and Westerlund, S., Anal. Chim. Acta, 1981, 2.75 min (including rinse cycles), giving a major reduction in 131, 63. the ICP-MS running time. The overall preparation and analysis 13 Batterham, G. J., and Parry, D. L., Mar. Chem., 1996, 55, 381. time in our procedure, for at least 10 samples, would be 14 Lo, J. M., Yu, J. C., Hutchison, F. I., and Wal, C. M., Anal. Chem., 1982, 54, 2536. comparable to those in existing on-line procedures. Our pro- 15 Beauchemin, D., and Berman, S. S., Anal. Chem., 1989, 61, 1857. cedure is relatively simple and cheap to set up and run in 16 Heithmar, E. M., Hinners, T. A., Rowan, J. T., and Riviello, J. M., comparison with existing on-line column ICP-MS pro- Anal. Chem., 1990, 62, 857. cedures.6–10 The microconcentric nebuliser uses about 300 ml 17 Huang, K. S., and Jiang, S. J., Fresenius’ J. Anal. Chem., 1993, of back-extract per analysis, allowing for the repeated analysis 347, 238. 18 Bone, K. M., and Hibbert, W. D., Anal. Chim. Acta, 1979, 107, 219. of samples if required. In addition to the inherently better 19 Seubert, A., Petzold, G., and McLaren, J. W., J. Anal. At. detection limits obtained using ICP-MS analysis, this has also Spectrom., 1995, 10, 371. shortened the analysis times by at least 6 h in comparison with 20 Stary, J., and Kratzer, K., Anal. Chim. Acta, 1968, 40, 93. analysis by ETAAS. Paper 7/04309K The authors acknowledge the financial support of McArthur Received June 19, 1997 Accepted July 22, 1997 River Mining Pty. Ltd. and thank the Australian Research 1280 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12