Acta Chim. Slov. 2003, 50, 409-418. 409 DETERMINATION OF TRACE METALS IN SEA WATER BY ICP-MS AFTER MATRTX SEPARATION Chandra K. Sekhar,* Sreedhar N. Chary, Kamala C. Tirumala, and V. Aparna Analytical Chemistry & Environmental Sciences, Indian Institute of Chemical Technology, Hyderabad, (A.P), India, 500 007 Received 04-02-2003 Abstract The analysis of ultra trace elements in sea water samples is one of the most difficult analytical tasks in the field of environmental monitoring, as extremely low detection limits for elements buried in a highly saline matrix is required. The use of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for direct sea water analysis is currently limited by spectral and non-spectral interferences caused by the sea water matrix. Sample dilution is always a way out at the expense of inadequate sensitivity after such dilution especially for open ocean sea water. In order to approach this complex analytical task by ICP-MS there are two common strategies studied so far: first the use of pre-concentration technique and matrix removal and second the use of double focussing ICP-MS instruments. The use of Ammonium tetramethylene dithiocarbamate (APDC)/Methyl isobutyl ketone (MIBK) solvent extraction for matrix removal and direct estimation of trace elements in sea waters by ICP-MS are discussed in this paper. Introduction In recent years the determination of trace elements in seawater has received an increasing attention in the field of environmental monitoring and since most of the heavy metals occur at extremely low concentrations, very sensitive techniques are required. With the ever-increasing use of ICP-QMS as a routine analvtical tool, the demand for the technique to handle a wider variety of sample matrices has increased. Although the technique of ICP-QMS is very powerful by virtue of its selectivity and sensitivity, the use of ICP-QMS for direct seawater analysis is currently limited by spectral and non spectral interferences caused by the sea-water matrix. Suppression of analyte signal in ICP-QMS in the presence of high concentration matrix salts is due to several factors: (a) the ionization of the element from the salt and possibly their total contribution to the electron densities in the plasma, (b) changing aerosol transport efficiency at high salt concentrations and (c) MS sampling conditions. In particular, the effect of oxide/hydroxide adduct ions of alkali and alkaline earth elements, common constituents in sea water can effectively mask the determination of most transition elements as seen C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea JVater... 410 Acta Chim. Slov. 2003, 50, 409-418. in Table 1. Sample dilution is always a way out at the expense of inadequate sensitivity after such dilution, specially for open ocean sea-water. In order to approach this complex analytical task by ICP-QMS, there are two common strategies studied so far: first, the use of pre-concentration techniques and matrix removal (using chelating ion exchange resins, co-precipitation and liquid-liquid extraction) before ICP-QMS detection and second, the use of double focusing ICP-MS instruments (for direct trace metal determination after a simple dilution of the sample). ' By operating at 3000 resolving power (m/Am) most of the spectral interference coming from molecular ion due to sea water matrix elements (eg., Ca, Na, Cl etc.,) can be satisfactorily resolved. Also the sensitivity attainable at low resolving power (m/Am=300) with the ICP-MS is at least 10-100 times better than with ICP-QMS, due to comparatively low instrumental background and higher transmission than in the ICP-QMS instrument. Currently a lot of work is being done to reduce or eliminate matrix interference in ICP-MS by collision/reaction celi technology. The dynamic reaction celi (DRC) provides an excellent technology for eliminating the interference that are derived from the plasma gas as well as the sample matrices. Unlike other approaches DRC technology completely eliminates interference and produces ultra trace detection in the difficult samples like seawater. The pre-concentration technique applied for the sea water analysis should achieve the following requirements for the interference free estimation: a) Selective removal of most interfering ions from solution, b) elimination of cone blockage problems, c) retention of long term calibration integrity, d) reduction of polyatomic interferences, e) capability to pre-concentrate for further enhancement of detection limits, f) offline sample processing for maximum productivity. ' Pre-concentration techniques such as solvent extraction, ion exchange and carrier precipitation have been extensively studied with reference to sea water analysis. ' Pre-concentration is usually accomplished by solvent extraction using pyrrolidine N-carbodithioate (APDC), chloroform and combined ammonium tetramethylenedithiocarbamate (APDC), diethylammonium diethyldithiocarbamate (DDDC) or APDC/MIBK techniques or by ion exchange with a resin such as chelex-100 or adsorption on immobilized 8-hydroxyquinoline (I-8-HOQ). In the present work seawater samples were pre-concentrated following the ammonium pyrolidine C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea JVater... Acta Chim. Slov. 2003, 50, 409-418. 411 dithio carbamate (APDC)/methyl isobutyl ketone (MIBK) solvent extraction technique prior to analysis by ICP-QMS. Table 1. Major interferences of alkali and alkaline earth adduct ions on transition metal isotopes in a typical seawater matrix. Isotope V51 Cr52 Cr53 Mn5 Fe* Fe5' Ni5S Co5 Ni6C Cu6 Zn6 Cu6 Zn6 Cl Ca CIO 5C10H 37C10 CaOH 42CaO C102 CaO CaOH, CaO 44CaO, 43CaOH CaO 8CaOH Na Na02 ArNa K 39KO 9KOH Ar ArC ArNH ArO ArOH Experimental Instrumentation and Calibration An Ultra Mass 700 ICP-MS (Varian, Australia) was used for the determination of trace metals in seawater and the instrument parameters (RF power 1.4 kW, Nebulizer gas flow 0.81 L/min, Plasma flow 18 L/min, Spray chamber temperature -10°C) were set according to the manufacturers instructions. Standardization of the instrument was based on a two-point calibration procedure, using a multi element standard (J. T. Backer, Inc Phillsburg NJ) as the high standard, and double distilled water (zero metal concentration) as the low standard. Standardization of the instrument was repeated after every three sets of seawater extracts to minimize the effect of instrument drift. Distilled water and a check standard were also included in the analysis sequence. The accuracy of the method was confirmed by analysis of certified reference material CASS-3 (near sea shore water). For interlaboratory testing a Perkin-Elmer (Elan DRC II) ICP-QMS and Varian (Spectra AA 800) GF-AAS present in the Geochemistry laboratory of National Geophysical Research Institute (NGRI) was used. C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea JVater... 412 Acta Chim. Slov. 2003, 50, 409-418. Reagents and Standards Multielement stock standard solutions were prepared from 1000 ppm stock solution (J. T. Backer, Inc Phillsburg NJ) after appropriate dilution. Ali chemicals used were of analytical grade. Glass double distilled water was used for the preparation of ali solutions. Nitric acid used in the present study was of AR grade (Merck, Mumbai). A 1% aqueous solution of APDC (Eastman Chemicals): the solution which was prepared daily was purified by shaking with an equal volume of MIBK, separating the phases and filtering the lower aqueous phase. Since APDC is only slightly soluble in MIBK whereas metal complexes are highly soluble, the reagent is easily purified in this manner. Reagent grade MIBK was used throughout without further purification. Ammonium acetate (Sigma, Mumbai) buffer (5N) was prepared and metallic impurities were removed by adding a few milliliters of APDC solution and extracting with MIBK. Sea Water Source: Sea water samples were provided by Institute of Petroleum Safety and Environmental Management (IPSEM), Oil and Natural Gas Corporation (ONGC), Goa, India as part of their study to monitor the extent of pollution due to various industrial activities. They supplied 200 samples per each season (pre-monsoon and post-monsoon) for the last three years. The values presented in Table 5 is a representative samples of seawater provided by IPSEM. Pre-concentration Procedure: The dissolved metals were simultaneously extracted from filtered water samples by chelation with APDC followed by MIBK extraction according to method described in literature. " Ali experiments were carried out in a pH range of 2-6. Extractions were done in a separatory funnel shaken for 2 min. by hand with 50 mL of aqueous volume, lml of APDC solution and lOml of MIBK. When the highest possible accuracy was required the organic phase was placed in a 10 mL volumetric flask and diluted to volume with water saturated MIBK. This eliminated the error caused by variation in the volume of organic phase lost because of solubility. Ali sample preparations were carried out in a clean laboratory equipped with laminar flow benches and fume cupboards. Ali laboratory wares were cleaned in 1:1 C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea Water... Acta Chim. Slov. 2003, 50, 409-418. 413 HNO3 and rinsed in DIW. Once in use, they were rinsed with DIW between determinations. Results and discussion Pre-concentration procedure The efficiency of separation of matrix elements and pre-concentration by the APDC/MIBK extraction was studied by taking three NIST drinking water standards 1643a, 1643b, 1643c and adding different concentrations of salt matrix. The results are presented in Table 2 which shows very good extraction efficiency, except for the sample NIST 1643d where the recoveries are higher which may be due to the high salt concentration (4%). From the results it is clear that we can adopt APDC/MIBK extraction for our seawater (where the salinity is around 3-3.5%) samples for removing salt matrix and pre-concentration. Seawater standard analysis Having effected the removal of interferences from seawater analysis a test on analytical performance of the method is carried out. In Table 3 results of ICP-QMS analysis of near sea shore seawater reference material CASS-3 by both standard addition and direct analysis are compared with the certified values. For critical assessment of accuracy there is no substitute for seawater standard reference material certified for elemental composition but unfortunately we do not have such standards in our lab. The accuracy of the procedure was therefore tested by preparing three synthetic standards. The concentration of metals were kept similar to CASS-1, CASS-2, NASS-4 standards with a salt matrix of 3%. These three standards were analysed directly after solvent extraction as mentioned above. From these figures it can be seen that direct analysis may not be possible due to the interferences from alkali and alkaline earth metals which are present in high concentrations in sea water matrix (Table 1), even after pre-concentration. To overcome these polyatomic spectral interferences an online elemental correction equation (C.E.) is applied and these values are presented in Table 3. The values obtained are in good agreement with the certified/accepted values which indicates C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea Water... 414 Acta Chim. Slov. 2003, 50, 409-418. that sea water samples can be analysed directly using ICP-QMS after matrix separation and application of C.E. Table 2. Study on the efficiency of APDC/MIBK extraction/pre-concentration method. Elements Certified Analysed by Addition of After APDC/MIBK Extraction value ICP-QMS NaCl matrix and Preconcentration NIST 1643b Mn 27.5 26.8±1.2 27.9=1=1.2 Co 25.3 24.3±2.1 26.1=1=2.0 Ni 47.7 48.4±3.0 46.2=1=1.7 Cu 21.2 22.0±2.1 3% 22.2=1=1.7 Zn 67.1 66.1=1=1.9 68.7=1=2.3 Cd 19.9 18.8=1=2.0 18.7=1=1.1 Pb 24.3 25.7±2.2 26.1=1=2.7 Cr 18.8 19.0±2.0 19.1=1=1.0 NIST 1643c Mn 35.1 36.2±2.1 34.2=1=1.3 Co 23.5 22.7±1.8 24.2=1=1.6 Ni 61.0 62.1±3.1 58.6=1=2.1 Cu 22.3 22.1=1=2.1 3.5% 23.2=1=1.9 Zn 74.0 74.2±1.8 77.1=1=1.1 Cd 12.2 12.8=1=1.2 14.1=1=1.0 Pb 35.3 36.6±1.4 36.1=1=1.1 Cr 19.0 19.0=1=1.7 17.9=1=2.1 NIST 1643d Mn 37.6 38.0=1=2.1 35.1=1=1.2 Co 25.0 25.7=1=2.0 20.2=1=2.2 Ni 58.0 58.2=1=1.8 63.1=1=1.7 Cu 20.5 21.2=1=1.5 4.0% 26.6=1=2.1 Zn 72.5 73.0=1=2.1 80.1=1=3.1 Cd 6.5 6.6=1=2.0 9.9=1=1.2 Pb 18.1 18.8=1=1.7 22.3=1=2.1 Cr 18.5 19.1=1=1.8 23.1=1=1.6 Ali values are in ppb. The values reported are a mean of five readings ± SD. C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea JVater... Acta Chim. Slov. 2003, 50, 409-418. 415 Table 3. Direct analysis and recoveries of seawater standards using ICP-MS and online correction equation. Metals Mn Co Ni Cu Zn Cd Pb Cr CASS-3 Certified/ Accepted 2.51 0.04 0.38 0.51 1.24 0.03 0.01 -± 0.36 ± 0.009 ± 0.06 ± 0.06 ± 0.25 ± 0.005 ± 0.004 Standard Addition ($) 1.98 0.06 0.31 0.50 1.20 0.02 0.01 -± 0.06 ± 0.005 ± 0.03 ± 0.04 ± 0.16 ± 0.008 ± 0.002 CASS-1* Certified/ Accepted 2.11 0.06 0.31 0.29 1.02 0.03 0.37 0.22 ± 1.01 ± 0.007 ± 0.02 ± 0.01 ± 0.11 ± 0.007 ± 0.09 ± 0.06 Direct Analysis 3.56 0.12 0.91 0.56 0.53 0.21 0.48 1.09 ± 1.56 ± 0.05 ± 0.32 ± 0.15 ± 0.09 ± 0.06 ± 0.19 ± 0.25 Standard Addition ($) 1.78 0.04 0.29 0.31 1.01 0.02 0.39 0.23 ± 0.06 ± 0.003 ± 0.006 ± 0.007 ± 0.006 ± 0.002 ± 0.01 ± 0.09 CASS-2* Certified/ Accepted 2.02 - 0.31 0.26 1.88 0.11 0.04 0.22 ± 0.52 ± 0.14 ± 0.12 ± 0.31 ± 0.02 ± 0.007 ± 0.09 Direct Analysis 1.56 - 0.12 0.45 0.85 0.25 0.09 0.36 ± 0.26 ± 0.04 ± 0.25 ± 0.25 ± 0.09 ± 0.06 ± 0.14 Standard Addition ($) 2.14 - 0.30 0.28 1.92 0.20 0.06 0.27 ± 0.12 ± 0.12 ± 0.12 ± 0.28 ± 0.11 ± 0.009 ± 0.11 NASS-4* Certified/ Accepted 0.41 0.02 0.26 0.28 0.19 0.02 0.16 -± 0.12 ± 0.001 ± 0.008 ± 0.09 ± 0.01 ± 0.003 ± 0.007 Direct Analysis 0.58 0.32 0.15 0.32 0.31 0.15 0.32 -± 0.25 ± 0.007 ± 0.06 ± 0.22 ± 0.006 ± 0.05 ± 0.05 Standard Addition ($) 0.40 0.01 0.29 0.25 0.21 0.03 0.18 -± 0.02 ± 0.002 ± 0.004 ± 0.1 ± 0.07 ± 0.005 ± 0.006 Ali values are in ppb. ($): After applying correction equation. *synthetic standards prepared in our laboratory corresponding to the composition of the standards. The values reported are a mean of five readings ± SD. Precision Studies Precision of the method was tested by replicate analvsis of near sea shore sea water SRM CASS-3 and sea water samples 12 times after applving C.E. From the values in Table 4 it can be concluded that the method has a very good precision except for Mn, which may be due to polvatomic interference from NaC»2, KO , ArNH (Table 1). C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea JVater... 416 Acta Chim. Slov. 2003, 50, 409-418. Table 4. Precision studies. Element CASS-1 %RSD Seawater real samples 1.2 %RSD Mn 2.1 8.7 10.1 Co 0.06 1.8 1.6 2.7 Ni 0.3 3.1 20.2 5.4 Cu 0.3 3.9 92.4 3.3 Zn 1.0 3.7 453.5 5.8 Cd 0.03 1.6 58.6 2.0 Pb 0.4 2.3 206.9 2.8 Cr 0.2 1.2 141.2 1.1 Analysis of sea water samples The analytical results for sea water sample was given in Table 5. The sample was analysed directly (after solvent extaction by APDC/MTBK and online C.E) and by standard method. Interlaboratory tests were performed by using two different techniques (ICP-QMS, GF-AAS) in our sister laboratory (NGRI) for quality assurance and quality control checks. The values presented in Table 5 were in good agreement indicating the analysis is accurate. Table 5. Direct analysis of sea water samples and interlaboratory testing ± S.D. Element Direct analysis by ICP-MS Concentration Found After Standard Addition ICP-MS (IICT)* ICP-MS (NGRI) GF-AAS (NGRI) Mn 1.2±0.3 1.5=1=0.1 1.4±0.2 1.4±0.07 Co 1.6±0.2 1.9±0.2 1.9±0.2 1.9±0.08 Ni 20.2±1.9 25.0±0.5 25.2±0.4 25.5=1=0.1 Cu 92.4±6.1 113.6±1.1 114.1±1.0 112.8=1=0.7 Zn 452.5±20.1 580.1±1.3 577.6±4.1 579.2±2.7 Cd 58.6±4.8 69.1±0.9 68.7±1.0 68.4±0.7 Pb 206.9±11.2 280.7±2.7 282.1±2.7 281.7=1=1.8 Cr 141.2=1=7.1 242.6±3.1 241.8=1=2.1 242.7±1.1 *With C.E. The values reported are a mean of five readings ± SD. Conclusions Over the last two decades there has been an increasing need for trace and ultra trace determination of heavy metals in seawater in order to monitor possible pollution of natural environments and also oceanographic studies. The serious matrix effects arising from seawater salts often cause the determinations more difficult. The philosophy of C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea JVater... Acta Chim. Slov. 2003, 50, 409-418. 417 removing the sample matrix prior to analysis by ICP-QMS has many advantages. Interferences are removed, stability is improved and calibration is more straightfonvard. In this paper use of APDC/MIBK pre-concentration technique is used in order to analyse seawater samples directly. The results presented demonstrate the effectiveness of ICP-QMS for the analysis of most difficult environmental samples, seawater overcoming the previous limitation-total dissolved solids. In this manner the previous limitation on total dissolved solids with ICP-QMS is eliminated and the flexibility of the ICP-MS technique can now be fully utilized for this difficult matrix. Acknowledgements Authors are grateful to Dr K.V. Raghavan, Director: Indian Institute of Chemical Technology, for his encouragement and providing ali the facilities for carrying out this work. Authors are thankful to Dr. V. Balaram, Deputy Director, National Geophysical Research Institute for extending GFAAS, ICP-MS facilities and providing SRMs for our study. References 1. M. Plantz, S. Elliott Stability of ICP-MS for the analysis of sodium chloride and 1:1 sea water Instruments at Work, ICP-MS-17, Varian, Australia, 1998. 2. I. Rodushkin, T. J. Ruth, J. Anah At. Spectrom. 1997, 12, 1181-1186. 3. Analysis of Sea water by ICP-MS, CETAC DSX 100, Matrix Elimination/pre-concentration system, 2002. CETAC technologies, Omaha, Nebraska, USA. 4. A. R. Date, A. L. Grey, Spectrochim Acta, Part B 1985, 40B, 115-122. 5. D. J. Douglas, R. S. Houk, Prog. Anah At. Spectros. 1985, 8, 1-18. 6. C. N. Ferrarello, M. M. Bayon, J. I. G. Alonso, A. S. Medel, Anah Chim. Acta 2001, 429, 227-235. 7. M. P. Field, J. Cullen, R. M. Sherrell, J. Anah At. Spectrom. 1999, 14, 1425-1431. 8. V. Balaram, Proč. A. P. Academy of Sciences 2002, 6(4), 265-274. 9. I. M. M. Kenawy, M. A. H. Hafez, M. A. Akl, R. R. Lashein, Anah Sci. 2000, 16, 493-500. 10. J. P. Riley in Chemical Oceanography, 2n Edition, Vol. III, Academic Press, New York, 1975, pp 278-290. 11. M. Hiraide, Y. Yoshida, A. Mizuike, Anah Chim. Acta 1976, 81, 185-189. 12. E. E. Sturgeon, S. S. Berman, A. Desaulniers, D. S. Russel, Talanta 1980, 27, 85-94. 13. C. W. McCleon, A. Otsuki, K. Okamote, H. Haraguchi, K. Fuwa, Analyst 1981, 106, 419-428. 14. S. S. Berman, J. W. McLaren, S. N. Willei, Anah Chem 1980, 52, 488-492. 15. A. P. Mykytiuk, D. S. Russel, R. E. Sturgeon, Anah Chem 1980, 52, 1281-1283. 16. J. W. McLaren, A. P. Mykytiuk, S. N. Willei, S. S. Berman, Anal. Chem 1985, 57, 2907-2911. 17. Shane Elliott, Analysis of organic solvents including naphtha by ICP-MS, Varian Instruments at Work, ICP-MS-19, October 1998. 18. L. Ferrer, E. Contardi, S. J. Andrade, R. Astesuaina, A. E. Pucci, J. E. Marcovecchio, Oceanologia 2000, 42(4), 493-504. 19. R. R. Brooks, B. J. Presley, I. R. Kaplan, Talanta 1967, 14, 809-816. 20. S. R. Koirtyohanm, J. W. Wen, Anah Chem 1973, 45(12), 1986-1989. 21. M. Murakami, H. Tadanu, T. Tokade, Talanta 1992, 39(2), 179-185. C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea JVater... 418 Acta Chim. Slov. 2003, 50, 409-418. Povzetek Določevanje elementov v sledovih v vzorcih morske vode je ena izmed težjih nalog na področju okoljskega monitoringa zaradi visoke koncentracije NaCl v matrici. Ta otežuje uporabo masne spektrometrije z induktivno sklopljeno plazmo (ICP-MS) za neposredno analizo morske vode zaradi spektralnih in nespektralnih motenj. Redčenje vzorca je ena od možnosti, da motnje zaobidemo, a pri tem izgubimo potrebno občutljivost metode. Drugi dve strategiji, ki sta še v uporabi, sta uporaba predkoncentracije in ločitev matrice ter uporaba ICP-MS instrumenta z dvojnim fokusiranjem. Uporaba ekstrakcije z amonijevim tetrametilenditiokarbamatom (APDC)/metilizobutilketonom (MIBK) za ločitev matrice in neposredna določitev sledov elementov v morski vodi je tema tega prispevka. C. K Sekhar, S. N. Chary, K. C. Tirumala, V. Aparna: Determination of Trace Metals in Sea JVater...