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Inductively Coupled Plasma Mass Spectrometric Determination of Trace Elements in Surface Waters Subject to Acidic Deposition J. M. Henshaw’ Lockheed-Engineering and Sciences Co., 1050 E . Flamingo Suite 120, Las Vegas, Nevada 89109 E. M. Heithmar* a n d T. A. H i n n e r s
US. Environmental Protection Agency, Environmental Monitoring Systems Laboratory, Las Vegas, Nevada 89193-3478
More than 250 water samples from lakes in the Eastern US. were analyzed by inductively coupled plasma mass spectrometry ( ICP-MS) for 49 elements. Standard callbrations were used for 21 elements, and surrogate standards were used for 28 elements. The system detection Ilmlts, evaluated by uslng fleld blanks carrled through the entire sampling and pretreatment process, were less than 0.2 pg/L for most elements. Contamination during sampling and pretreatment was often the limiting factor. The accuracy of the determlnatlons, as determined from the anaiysls of NBS SRM 1643b samples and by recoveries for splked water samples, was typically better than f10% for the elements determlned by uslng standard callbration and better than f25 % for the elements determined by uslng surrogate standards. The long-term (12 months) preclslon was generally better than f10 %, expressed as relative standard devlation, for both methods of determination. The use of surrogate standards and lnterference corrections Is discussed In detail.
INTRODUCTION Increases in the concentrations of several trace elements in natural waters and sediments have been reported for many North American and Scandinavian watersheds (1-4). It has been shown that increases in trace-element concentrations in natural waters and sediments coincide with increases in human activity (5) and that atmospheric deposition is related to the increased concentrations (6). Increased levels of many of these elements pose a threat to both the environment and human health. Trace elements have also been used to determine sources of pollution (7,8). Methods for monitoring the concentrations of a broad range of trace elements in aquatic systems are therefore desirable. Many techniques are used for the determination of trace element concentrations in environmental samples. These techniques include graphite furnace and flame atomic absorption spectrometry (2, 4, 8), neutron activation analysis (I), and inductively coupled plasma atomic emission spectroscopy (5). However, these techniques are not well suited for trace multielement analysis of large numbers of samples in an environmental survey. Graphite furnace atomic absorption spectrometry exhibits sufficiently low detection limits, but it is a single-element technique and lacks the necessary sample throughput. Neutron activation analysis is also relatively slow and impractical for the routine analysis of large numbers of samples. Inductively coupled plasma atomic emission spectroscopy is a rapid multielement technique, but Present address: Department of Chemistry, BG-10,University of Washington Seattle, WA 98195. 0003-2700/89/0361-0335$01.50/0
it does not provide the detection limits required to measure many elements a t the concentrations present in uncontaminated surface waters. Inductively coupled plasma mass spectrometry (ICP-MS) is a multielement technique with sub-part-per-billion detection limits for many elements. Although the use of ICP-MS has been reported for the determination of trace elements in environmental samples (9, lo), the applications have focused on relatively small numbers of target analytes. Quantitative or semiquantitative information on over 50 elements is attainable by scanning over the mass-to-charge range 6 to 238, but studies to date have not fully utilized this capability of ICP-MS. Also, the viability of ICP-MS as a technique for the analysis of large numbers of samples over extended time periods has not been demonstrated. This paper describes the application of ICP-MS for the analysis of lake water samples collected as part of the U.S. Environmental Protection Agency’s Eastern Lakes Survey, Phase 11, a large water-chemistry study conducted as part of the National Acidic Precipitation Assessment Program. The goal of our part of the survey was to evaluate the feasibility of using ICP-MS to establish trace-element “fingerprints” of the lakes. For this purpose, it was desirable to determine the concentrations of as many elements as possible. If ICP-MS could be shown to provide reliable data, they could be used to study the relationships between trace-element concentrations and factors such as water-chemistry parameters (e.g., pH, dissolved organic carbon, etc.), geographic location, and watershed characteristics. Two degrees of quantification were employed in this study. The concentrations of 21 elements were quantitatively determined by the use of calibration standards. Semiquantitative estimates of the concentrations of 28 elements were obtained by using surrogates selected from the first 21 elements. The practicality of estimating element concentrations by the use of surrogates is examined in detail in this report. The performance of the method in the survey, including instrumental and system detection limits, accuracy, and short-term (within-day) and long-term precision, is presented. Since several interferences are known to affect ICP-MS analyses (11-13), although they can often be corrected by various means, the effects of interferences present in lake water matrices are evaluated in this study. Alternative methods for mitigating these effects are presented. Finally, the viability of ICP-MS as a quantitative and semiquantitative mdtielemental analysis method in a large-scale surface-water monitoring project is assessed.
EXPERIMENTAL SECTION Standards and Solutions. All solutions were prepared with ASTM Type 1water. All nitric acid used was ultrapure (Ultrex) grade. 49Tirefers to isotopically enriched titanium (96.25%49Ti). The 49Tistock solution was prepared from isotopically enriched 0 1989 American Chemical Society
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titanium oxide provided by the Oak Ridge National Laboratory. The titanium oxide was dissolved by carbonate fusion followed by refluxing with Ultrex hydrochloric acid. The final solution was made up to 100mg T i / L in 6% hydrochloric acid. All other elemental standards were prepared from natural-abundance atomic-absorption standards (1000 mg/L). In order to improve the precision of the analyses, four internal standards, 49Ti,In, Tb, and Bi, were selected, based upon their atomic masses, representative ionization energies, and scarcity in natural surface waters. Two internal standard spiking solutions were used in the preparation of samples and standards. A 10 mg/L solution of indium was prepared in 5% nitric acid. The second solution contained the remaining internal standards. The final concentrations of this solution were 10 mg 49Ti/L, 10 mg Tb/L, and 20 mg Bi/L. The use of two independent internal standard solutions allowed the detection of internal standard spiking errors. Two dilutions of a single calibration standard were prepared daily from a stock solution. A calibration blank was also prepared daily. All working solutions contained 0.7% nitric acid and internal standards at concentrations of 100 pg In/L, 100 pg Bi/L, 50 pg 49Ti/L, and 50 pg Tb/L. The calibration standards included the following elements: Be, Al, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Cd, Sb, Ba, Ce, T1, and Pb. Two standards were prepared at concentrations of 1and 10 pg/L for all elements except Al, Ca, Mn, Fe, Zn, Sr, and Ba. The two standards contained Ca at 500 and 5000 pg/L, A1 and Fe a t 30 and 300 pg/L, Mn at 10 and 100 pg/L, and Zn, Sr, and Ba a t 5 and 50 pg/L. The higher concentration standard was used for all calibrations. The lower concentration standard was analyzed twice daily to verify linearity. An independently prepared calibration check solution a t a concentration of half that of the high standard was analyzed daily. A calibration check solution a t half of the concentration of the low standard was analyzed to check the calibration linearity a t very low concentrations. National Bureau of Standards SRM 1643B (trace elements in water) was used as an additional independent calibration verification solution. Since the certified concentrations of some elements in this reference material are higher than the calibration range, the solution was diluted 5-fold prior to analysis. The diluted solution was prepared daily, including internal standards. The undiluted reference material was also analyzed periodically. A separate aliquot of one sample from each analytical batch was spiked with standard stock solution so that the added concentration equaled half of the high-calibration standard solution. These samples were analyzed immediately following the unspiked aliquot, and the results were used to determine percent spike recovery. Several quality assurance solutions included as part of the National Surface Water Survey were also analyzed. These included deionized water blanks prepared in the processing laboratory and the field, natural adult samples, and duplicate samples collected during the same lake visit. Sample Handling. The samples were collected in polypropylene Cubitainers from van Dorn samplers which had been filled at a depth of 1.5 m below the lake surface. The Cubitainers were kept a t 4 "C and shipped via overnight express to the processing laboratory in Las Vegas, NV. Upon receipt, the samples were immediately filtered through 0.45-pm polycarbonate filters and acidified to pH
