Direct REE determination in fresh waters using ultrasonic nebulization ICP-MS Ludwik Halicz, Irina Segal and Olga YoVe Geological Survey of Israel, 30 Malkhe Israel Street, 95501 Jerusalem, Israel. E-mail: Ludwik@mail.gsi.gov.il Received 29th October 1998, Accepted 7th January 1999 Fifteen samples of fresh spring water from northern Israel were analyzed for extremely low REE concentrations. The concentration of LREE was between 0.02 and 13 ng l-1 and that of HREE was between 0.01 and 3 ng l-1.The limit of detection was between 0.005 and 0.05 ng l-1 depending on the element. The eYciency of ultrasonic nebulization (USN) is dependent on the total dissolved solids in the samples and therefore the depression of the signal varies from sample to sample as a function of their chemical composition. This problem was solved by using rhenium as an internal standard. The recoveries (between 85 and 120%) found for the determination of REE in spiked natural samples together with the good agreement with known data for SRM SLRS-3 riverine water indicated the accuracy of the direct determination using the developed USN-ICP-MS procedure.The chemistry of REE makes them particularly useful in Instrumentation studies of water and marine geochemistry. The REE concen- The measurements were performed with a Perkin-Elmer trations in terrestrial waters, such as groundwaters, surface SCIEX (Concord, Ontario, Canada) ELAN 6000 ICP-MS waters and lakes, have been examined as potential tracers of system.This instrument has been described in detail by processes aVecting their geochemical evolution and of rock– Denoyer14 and Tanner15 and was equipped with a CETAC water interactions.1,2 However, REE analysis of these waters 6000AT ultrasonic nebulizer (CETAC Technologies, Omaha, by ICP-MS is still diYcult. The major restrictions are the NE, USA). The operation was fully controlled by a computer extremely low concentrations of the REE, which in many cases with a Windows-NT driven dedicated software package.The are below the instrumental detection limits. To eliminate the ICP-MS and USN instrumental operating conditions and the above problems, the determination of REE in waters by mass spectrometer acquisition settings are summarized in ICP-MS or other techniques is commonly preceded by oV- or Table 1. on-line preconcentration and matrix separation.2–13 These established methods have two disadvantages: (a) time consum- Reagents and standard solutions ing procedures (even in the case of on-line procedures a single A blank solution of 0.1 M HNO3 was prepared from J.T. repetition takes about 10 min); and (b) the high likelihood of Baker (Phillipsburg, NJ, USA) Instra-Analyzed Reagent nitric contamination of samples with concentrations in the pg l-1 acid and doubly de-ionized water (DDW) obtained by passage range. These disadvantages [in the case of fresh water samples of purified water through a NanoPure water purification containing up to a few hundred mg l-1 total dissolved salts system (Barnstead, Dubuque, IA, USA). An REE stock (TDS)] can be overcome by using an enhanced method of standard solution (High-Purity Standards, Charleston, SC, sample introduction such as ultrasonic nebulization (USN), USA; ICP-MS Multielement Standard B) was diluted with which results in an order of magnitude improvement in the 0.1 M HNO3 to 100 ng l-1.LODs of the REE.The problem of matrix eVects due to relatively high (for USN) TDS contents in natural samples can be resolved by using Re as an internal standard. The good recovery values found for spiked natural samples indicate the Table 1 Operating conditions and ICP-MS and USN settings for the accuracy of direct determination using the USN-ICP-MS direct determination of REE in fresh waters procedure. ICP-MS operating conditions— Rf power 1050 W Nebulizer gas flow rate 0.98 l min-1 Auxiliary gas flow rate 0.8 l min-1 Experimental Plasma gas flow rate 15 l min-1 Lens setting AutoLens Sample collection and processing Interface cones Nickel Mass spectrometer acquisition settings— The water samples used for this study were collected in Galilee, Dwell time 70 ms the northern part of Israel, in early June 1998.The samples Number of sweeps 11 were filtered in the field through Millipore 0.45 mm filters and Number of readings 1 immediately acidified with nitric acid (to pH about 1) and Number of replicates 5 stored in the acid pre-cleaned, low density polyethylene bottles. Scan mode Peak hopping Rhenium was added as an internal standard (to the blank, MCA channels per peak 1 USN operating conditions— standard and samples), resulting in a final Re concentration Desolvating temperature: of 1.0 ng ml-1. The TDS of the water samples ranged from Cooling -5 °C 0.012 to 0.053%, which is relatively high for USN with Heating 120 °C continuous mode aspiration. J.Anal. At. Spectrom., 1999, 14, 1579–1581 1579Table 2 Selected REE isotopes and their relative abundances, blanks, sensitivities and detection limits Abundance Blank/ Sensitivity/ Detection limit/ Analyte Mass (%) cps Mcps ppm-1 ng l-1 La 139 99.9 6 485 0.04 Ce 140 88.5 11 498 0.06 Pr 141 100 3 663 0.015 Nd 143 12.2 2 81 0.1 Sm 147 15 3 103 0.06 Eu 151 47.8 2 344 0.02 Tb 159 100 2 730 0.01 Gd 160 21.9 2 169 0.04 Dy 163 25 1 181 0.04 Ho 165 100 4 717 0.01 Er 166 36.4 1 243 0.03 Tm 169 100 1 734 0.005 Yb 174 31.8 2 242 0.03 Lu 175 97.4 2 720 0.01 Table 3 REE range in Galilee fresh water samples and results for selected spring samples and SRM SLRS-3 riverine water Range of SLRS-3/ng l-1 analytical results/ Dan spring/ Shamir spring/ Analyte ng l-1 ng l-1 ng l-1 This work Brenner et al.17 La 0.4–13 5.62 4.61 250 210 Ce 0.7–12 3.55 10.7 293 250 Pr 0.09–3.0 1.27 1.05 61 53 Nd 0.3–13 5.79 4.36 239 200 Sm 0.09–2.5 1.09 0.82 43 39 Eua 0.02–0.7 0.26 0.21 6.5 6.6 Tb 0.03–0.5 0.20 0.11 4.5 3.6 Gd 0.08–3.3 1.28 0.75 39 28 Dy 0.08–3.6 1.29 0.60 22 19.8 Ho 0.015–0.8 0.28 0.13 4.9 3.8 Er 0.06–2.5 0.86 0.35 14 11 Tm 0.01–0.3 0.12 0.06 1.6 1.5 Yb 0.04–2.0 0.70 0.35 12 9.4 Lu 0.01–0.3 0.10 0.06 1.6 1.4 aAfter correction for molecular interference of 135BaO.Results and discussion Blanks, sensitivities and detection limits Selected REE isotopes and their relative abundances, the absolute blanks [in counts s-1 (cps)], sensitivities in 106 cps ppm-1 (Mcps ppm-1) and detection limits are given in Table 2.The detection limits are based on calculation of the uncertainty involved in measurement of the blank, using 3.29 times the standard deviation,16 and are at least an order of magnitude better than those obtained with a conventional nebulizer. High blanks for La and Ce resulted in relatively poor detection limits for these elements. Matrix eVects and internal standard To test the overall recovery and matrix eVects, five water samples (with diVerent TDS) were spiked with 1, 5 and 10 ng l-1 of REE.The recovery was TDS dependent and varied from 60 to 85%. After recalculation using an internal standard, the recovery was fairly good and varied for a 1 ng l-1 spike from 85 to 120% and for a 10 ng l-1 spike from 97 to 107%. Results of water analysis The range of analytical results is given in Table 3. The precision of analysis (RSD) in general varied as a function Fig. 1 REE patterns of representative water samples and associated of concentration and at the 1–10 ng l-1 level was about 5% rocks: (a) Shamir spring and basaltic rocks;17 (b) Dan spring and limestone rocks.16 or better. Since no suitable standard reference materials of 1580 J. Anal. At. Spectrom., 1999, 14, 1579–1581fresh water exist for this level of REE, the accuracy of the References results was validated by (a) comparing our data for SRM 1 P. Smedley, Geochim. Cosmochim.Acta, 1991, 55, 2767. SLRS-3 riverine water with those obtained by Brenner et al.17 2 G. E. M. Hall, J. E. Vaive and J. W. McConnell, Chem. Geol., (Table 3) and (b) normalizing our data to North American 1995, 120, 91. Shale Composite (NASC). The latter is a convenient pro- 3 H. Elderfield and M. J. Greaves, Nature (London), 1982, 296, 214. cedure conducted in geochemistry to characterize an REE 4 H. J. W. DeBaar, P. G. Brewer and M. P. Bacon, Geochim. pattern.18 This pattern is an important tool for understanding Cosmochim.Acta, 1985, 49, 1943. 5 H. J. W. DeBaar, M. P. Bacon and P. G. Brewer, Geochim. the geochemical processes and also to detect anomalous data Cosmochim. Acta, 1985, 49, 1961. that can be due to natural processes, contamination (anthro- 6 A. Masuda and Y. Ikeuchi, Geochem. J., 1979, 13, 19. pogenic in field or laboratory) or analytical error. We found 7 D. G. Piepgras and G. J. Wasserburg, Earth Planet. Sci. Lett., two types of pattern which are represented by the Dan and 1980, 50, 128.Shamir springs (Table 3, Fig. 1). Water from the Dan spring 8 G. Glinkhammer, H. Elderfield and A. Hudson, Nature (London), gives the typical sedimentary carbonate pattern,19 whereas 1983, 305, 185. 9 M. B. Shabani, T. Akagi, H. Shimizu and A. Masuda, Anal. water from the Shamir spring corresponds to that obtained Chem., 1990, 62, 2709. from rocks found in the basaltic province on Golan Heights20 10 M. B. Shabani, T.Akagi and A. Masuda, Anal. Chem., 1992, which lie to the east of the Galilee. All samples give patterns 64, 737. corresponding to one of these two types, which suggests good 11 J. R. Jezorek and J. Freiser, Anal. Chem., 1979, 51, 366. analytical accuracy. 12 B. K. Esser, A. Volpe, J. M. Kenneally and K. Smith, Anal. Chem., 1994, 66, 1736. 13 L. Halicz, I. Gavrieli and E. Dorfman, J. Anal. At. Spectrom., 1996, 11, 811. 14 E. R. Denoyer, Int. Lab., 1995, 8. Conclusion 15 S. D. Tanner, J. Anal. At. Spectrom., 1995, 10, 905. It has been demonstrated that USN-ICP-MS with Re as an 16 D. W. Medley, R. L. Kathren and A. G. Miller, Health Phys., 1994, 67, 122. internal standard is a rapid and accurate method for determin- 17 I. B. Brenner, M. Liezers, J. Godfrey, S. Nelms and J. Cantle, ing REE in fresh water at the sub-ng l-1 level. Using this Spectrochim. Acta, Part B, 1998, 53, 1087. approach, it is possible to overcome serious problems of 18 H. R. Rolinson, in Using Geochemical Data: Evaluation, contamination and the time-consuming procedures connected Presentation, Interpretation,Wiley, New York, 1993, pp. 133–142. with preconcentration and separation processes. The REE 19 A. Bellanca, D. Masett and R. Neri, Chem. Geol., 1997, 141, 141. patterns of the water samples reflected underlying water–rock 20 Y. S. Wenstein, PhD Thesis, Institute of Earth Sciences, Hebrew University, Jerusalem, 1998. interactions. The proposed method is an excellent tool for the geochemical interpretation of REE data even at concentrations at sub-ng l-1 levels. Paper 8/08387H J. Anal. At. Spectrom., 1999, 14, 1579–1581 1581