United States Environmental Protection Agency Environmental Monitoring Systems Laboratory Las Vegas, NV 89193-3478 Research and Development EPA/600/S4-89/043 Apr. 1990 Project Summary Pre-Concentration Method for Inductively Coupled Plasma- Mass Spectrometry J. T. Rowan and E. M. Heithmar A semi-automated system is used to pre-concentrate Ti, V, Mn, Fe, Co, Ni, Cu, Cd, and Pb prior to analysis by inductively coupled plasma-mass spectrometry (ICP-MS). The pre- concentration system accepts digests with a broad range of acid concentrations, neutralizes them, and loads them onto a macroporous imlnodlacetate resin. Alkali and alkaline earth metals, along with deleterious anions such as chloride, are washed off the resin before the concentrated analytes are eluted with nitric acid. Measurement of a total of 13 Isotopes of the analytes, as well as two Internal standard elements added to the eluent stream, indicates that the technique enhances the ICP- MS response of the target metals. Investigation of the nature of the blank signals suggests that the detection limits of several of the isotopes could benefit by much larger pre-concentration factors, while those of copper, cadmium and lead are currently limited by reagent purity. Method performance data is presented for several simple synthetic matrices, synthetic sea water, two waste waters and a natural surface water. This report was submitted in fulfillment of contract number 68-03- 3249 by Lockheed Engineering and Sciences Company under the sponsorship of the U.S. Environmental Protection Agency. This report covers a period from October 1, 1988 to September 30, 1989. Work is on-going. This Pro/ect Summary was developed by EPA'a Environmental Monitoring Systems Laboratory, Las Vegas, NV, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction The analysis of environmental samples for trace elements poses two major problems. First, the regulatory action levels of several metals are in the low part-per-billion range, and reliable quantitation requires analytical methods with detection limits 5-10 fold lower than these threshold levels. Second, the matrices encountered in environmental samples are extremely diverse and often highly complex. Because of the need for low detection limits, graphite furnace atomic absorption spectrometry (GFAAS) and, more recently, inductively coupled plasma-mass spectrometry (ICP-MS) have been employed in environmental analysis. ICP-MS has the advantage over GFAAS of being a multi-elemental technique, but there is incomplete performance data for ICP-MS analyses of target elements in all matrices of interest to the U.S. Environmental Protection Agency. One of the major drawbacks of ICP-MS is the interferences often exhibited with the complex matrices encountered in environmental analysis. Pre-concentration can be used to separate analytes from in- terferences prior to analysis. Separation is effected by complexation of analytes to an immobilized form of an iminodiacetate chelating agent. The functional group is covalently bonded to a highly cross- linked divinyl benzene, polystyrene copolymer resin. The iminodiacetate strongly complexes transition metals and certain main group metal ions, such as lead, while showing very little affinity for alkali and alkaline earth metals which can ------- interfere with subsequent analyses. The analytes can then be released by changing the ionic form of the resin. This approach allows the development of semi-automated methods that make use of resin-packed columns. Procedure Acidified samples are buffered to pH 5.5 by on-line mixing with 2-M ammonium acetate and then fill a sample loop of appropriate volume for the degree of pre-concentration desired. Prior to the pre-concentration step, the resin is placed in the ammonium form using 2-M ammonium acetate. When the pre- concentration program is initiated, a high pressure pump sweeps the sample to the resin column with 2-M ammonium acetate at 3 mL/min, pre-concentrating the analytes, and passing the rest of the sample matrix, including alkali metals and anions, to waste. The ensuing wash-out with additional ammonium acetate replaces chelated Mg2+ and Ca2+ with NH4 + . The high pressure pump then switches to 1-M nitric acid, passing the eluted trace metals to the ICP-MS. After the analyte peaks have passed, 3 mL/min of 2-M ammonium acetate is passed through the column for 1 minute. This is followed by successive 3-mL rinses with nitric acid, ammonium acetate, nitric acid, and finally ammonium acetate, again, each at a rate of 3 mL/min Mass spectra are obtained using the "Multiple Elements" program of the Perkin-Elmer ELAN ICP-MS. The measurement routine allows the observation of each peak every 0.67 seconds, and a temporal resolution of 2 seconds. The spectrum is converted to an ASCII file and automatically sent via the Kermit communications protocol to an 80386-based microcomputer for processing. The spectrum is placed in a commercial spreadsheet by a macro that automatically finds the analyte peaks and integrates each over the optimal detection period. Results and Discussion Peak areas were used for quantitation. Peak widths, and therefore peak heights, sometimes varied considerably during the day. Peak areas, conversely, were very reproducible. Calibration curves were determined for each of the analytes from 0.3 ^g/L to 30 iig/L using a 2.5-mL sample loop, and from 0.3 to 10 pg/L using a 10-mL sample loop (the iron calibration curves were determined using ten times these concentrations). All isotopic calibrations were linear, with the correlation coefficients ranging from 0.998 to 0.9999. Generally, the long-term precision of the method was good, but the sensitivity for lead tended to drift during the day. Lead calibrations and analyses used the internal standard intensity of bismuth to correct for this drift. Only in the cases of copper, cadmium and lead, as well as 49Ti, would more rigorous reagent purification and stringent clean-room conditions significantly improve detection limits over those obtained in this study. Conversely, the detection limits of the other isotopes would definitely improve with larger sample sizes. Even in the cases of copper, cadmium, and lead, some improvement with increased sample size is still expected. The efficiency of the pre-concentration system was studied by comparing total integrated signals for 10-^/L solutions of each of the analytes, directly aspirated into the ICP-MS, with the integrated signals after pre-concentration. Ammonium nitrate, which was present in the elution front, apparently caused a pronounced enhancement of signal that gave higher sensitivities than would be expected by pre-concentration alone. The on-line buffering technique adequately buffered standard solutions, ranging from 0.8% to 1.4% (v:v) nitric acid, to a pH of 5.4 ± 0.2. Analyte recoveries from these solutions ranged from 90% to 103% relative to the recoveries from a standard solution in 1 % nitric acid. The on-line mixing of samples and buffer caused only a 20% dilution of the samples, and eliminated hydrolysis problems which can be caused by prolonged exposure of analytes to non- acidic conditions. In early work, some analytes exhibited elevated apparent concentrations in the first blank after a high concentration standard. It is not clear why vanadium, and to a lesser extent, titanium, cobalt and copper exhibited memory. It was later determined that alternating the resin between the hydronium and ammonium forms three times between runs of the pre-concentration program reduced the effect. This cycling of ionic forms was therefore added to the prescribed procedure. The degree of residual interferences from molecular ions of alkali and alkaline earth metals, as well as chloride, was examined as a function of pre-elution wash-out time. At a concentration of 2000 mg/L, only calcium produced residual spectral interferences when the washi times were at least two minutes. The o spectral interferences remaining from synthetic sea-water matrix after a tv minute wash-out were relatively SIT residual 4«ca + and CaOH+ peaks. Mi spike recoveries for high concentrati salt matrices fell between 85% a 115%. A major drawback to pre-concentrati methodologies is the low recover! obtained in the presence of certi organic chelators. Titanium, coppi cadmium and lead showed very pc recoveries in the raw waste wate included in this study. A simp microwave digestion procedure, howev was sufficient to liberate the bou metals and restore full recoveries. Conclusions and Recommendations On-line pre-concentration with ; iminodiacetate resin minimizes seve spectral and physico-chemic interferences in ICP-MS. Interferenc from sodium, potassium, and magnesii are largely eliminated, and that frc calcium is greatly attenuated. Anio which can cause severe spectr interferences, such as chloride, are al removed. Native organic chelatii agents, which might reduce pr concentration recoveries, are eliminat from samples by a nitric acid digestion. The efficiency and cost-effectivene of this method could be improved w expansion to an automated, multip column system, wherein many sampl would be in various stages of analysis a given time. Unfortunately, the curre status of ICP-MS software mak< complete automation of the pr concentration techniques problemati Not only is satisfactory support f transient signals lacking, but so is tl hardware and software which would mal interfacing a less daunting task. Work on the pre-concentration metru should continue, especially to extend i application to ICP-AES. Further effc should be expended to reduce tl memory exhibited by some analyte Alternate chemistries which might mal possible the pre-concentration of arser and selenium should be investigate Finally, a multilaboratory evaluatic should be conducted to assess tt routine performance of the pn concentration method coupled with bo ICP-MS and ICP-AES. ------- J. T. Rowan is with Lockheed Engineering and Sciences Company, Las Vegas, NV 89119 £. M. Heithmar is the EPA Project Officer (see below).. The complete report, entitled "Pre-Concentration Method for Inductively Coupled Plasma-Mass Spectrometry," (Order No. PB 90-181348 ;Cost: $17.00 subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Environmental Monitoring Systems Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268Las Vegas, NV 89193-3478 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 ^GWx U'S-OF^]AL_MAIL -<\\ 'ENALTY 0 ,2 5 V Official Business Penalty for Private Use $300 EPA/600/S4-89/043 000065833 PS 8"ElioE! "I " »«•« ------- |