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(16) Davis, D. G. Talanta 1980, 3 , 335-345. (17) Waever, M. J. J . Electroanal. Chem. 1974, 57. 231-244. (18) Gllroy D. J . Electroanal. Chem. 1978, 7 7 , 257-277. (19) Angerstein-Kozlowska, H.; Conway, 8. E.; Sharp W. B. A. J . Nectroanal. Chem. 1973, 43. 9-36. (20) Conway, B. E.; Gottesfeld, S. J . Chem. Soc., Faraday Trans 1 1973, 6 9 , 1090-1107. (21) Cabelka, T. D.; Austin, D. S . ; Johnson, D. C. J . Electrochem, SOC. 1985. 737 1595-1602.
(22) Cabelka, T. D.; Austin, D. S.; Johnson, D. C. J . Electrochem. Soc. 1985, 132, 359-364. (23) Forsberg, G.;O'Laughlin, J. W.; Megargle, R. G.;Koirtyohann, S. R. Anal. Chem. 1975, 47, 1586-1592.
RECEIVED for review December 3,1985.Accepted February 12,1986. The receipt of a Visiting Research Fellowship from the Sciences and Engineering Research Of Canada is acknowledged (L.K.T.).
Evaluation of Dynamic Ion Exchange for the Isolation of Metal Ions for Characterization by Mass and a Spectrometry R. M. Cassidy,* F. C. Miller, a n d C. H. Knight Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada
J. C. Roddick a n d R. W.Sullivan Geochronological Section, Geological Survey of Canada, Ottawa, Ontario, Canada
A 14'La tracer has been used to ldentlfy the sources of cross-contamination when conventlonai hlgh-performance liquid chromatography was used to collect metal ion fractlons for subsequent analysls by a spectrometry or mass spectrometry. Major sources of memory effects ldentlfled were normal band broadenlng, sorptlon onto metal surfaces, and retentlon wlthln sample valves; secondary sorptlon effects wlthln the column and Isotopic exchange wlth sorbed metal Ions were not Important sources of memory effects. On the bask of these results a recommended procedure was developed that gives cross-contamlnatlon levels of 10.006 %. Sample preparation technlques for the a spectrometric and mass spectrometrlc analyses of the collected fractions are discussed, and some examples are given of appllcatlons to anaiytlcal problems In geochemlstry and the nuclear Industry. Good sample recoveries were obtained for the sample range studled (0.1-500 ng).
During the past several years considerable effort has been focused on the development of models for waste management control and for geological and cosmological processes. A variety of instrumental analytical techniques have been used to provide the data needed for the development of these models. Many of these techniques require a separation step prior to an accurate determination of trace amounts of groups of metal ions, stable isotopes, and/or radioactive isotopes. The quality of the final analytical results is dependent on the performance of the chemical separation scheme, which is often based on classical chromatographic or coprecipitation techniques. Such separations can take hours or days, and careful attention is required to maintain reproducibility, cross-contamination, and blanks at the necessary levels. Analysis costs can also be prohibitive due to long analysis times. Recent studies have shown that high-performance dynamic ion exchangers can provide precise and accurate determinations of nanogram amounts of metal ions in complicated matrices (1-3). While such techniques cannot provide information on isotopic composition, they should be able to significantly reduce the time normally required for chemical
separations used in waste management and geochemical analyses. However, analyses in these areas often demand the accurate isolation of nanogram and picogram amounts of metal ions, and some procedures, such as geological dating techniques, can require contamination and memory effects to be maintained at levels considerably less than 0.1%. Such stringent demands are not normally of concern in high-performance liquid chromatography (HPLC), and little has been reported in this area except for some studies of memory effects in sample valves (4). Moreover, the limited studies that have been done refer to organic species, and this information cannot be extrapolated to metal ions. Consequently, before HPLC can be used for the accurate collection of small amounts of metal ions, a number of experimental parameters need to be examined. These include the behavior of small amounts (nanograms to picograms) of metal ions on dynamic ion exchangers, cross-contamination effects, and the effects of eluent components on the subsequent sampling procedures used for solid-source mass spectrometry and a spectrometry. This paper reports the results of an investigation into these areas and briefly describes some applictions of dynamic ion exchangers for the collection of metal ion fractions.
EXPERIMENTAL SECTION Reagents and Materials. All solutions were made from freshly prepared water from a Milli-Q deionizing unit (Millipore, Bedford, MA) that was fed with double-distilled water from a quartz still. Stock solutions of a-hydroxyisobutyric acid (HIBA), ammonium n-octanesulfonate (C8S03-),and 2,7-bis[(o-arsenoacid (Arphenyl)azo]-1,8-dihydroxynaphthalene-3,6-disulfonic senazo 111)were purified by cation ion exchange; the H+ form was used for HIBA and Arsenazo 111and the NH4+form for sodium n-octanesulfonate. The postcolumn reagent, Arsenazo 111, was 1.5 X mo1.L-l and was 0.1 mol.L-* in HNO, and 0.01mol-L-' in urea. All HPLC eluents were filtered through 0.45-rm membranes. Double-distilled and subboiling distilled acids were used for studies of trace amounts of metal ions, and containers were Teflon or polyethylene that had been cleaned in nitric acid. All other chemicals were reagent grade. The zircon sample was obtained from the Branch of Isotope Geology of the U.S.Geological Survey (USGS) (Denver,'CO) where it had been dissolved in HF and HNO,, evaporated, and then redissolved in 1 mol-L-' HNO,. The final sample concentration was 17 mgmL-l. The USGS BCR-1 sample (0.5 g) was dissolved in HF and "OB,
0003-2700/86/0358-1389$01.50/00 1986 American Chemical Society
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evaporated, and then taken up in 6 mo1.L-' HC1. Prior to injection into the chromatograph, this sample was evaporated and then dissolved in dilute "OB. The rare-earth fraction was obtained by elution of the dissolved BCR-1 sample through a 15-mL cation exchange column (Dowex 50W-X8) with 2.5 mo1.L-l HC1 to remove most of the elements; the rare earths were stripped with 6 mo1.L-' HC1. The reversed phases used to remove the C8S03- from collected fractions were Waters Sep-PAK (Waters Associates, Milford, MA) or Chrom-Prep (Hamilton, Reno, NV) cartridges. Apparatus. The HPLC system consisted of a Spectra Physics pump (M8700, Santa Clara, CAI, a Rheodyne sampling valve (M7125, Berkely, CA), a 4.6-mm X 10-cm reversed-phase column (5-pm Supelcosil LC-18, Supelco, Bellefonte, PA), a variablewavelength absorbance detector (Tracor, Austin, TX), and a Spectra Physics SP4270 computing integrator. The eluted metal ions were monitored after a postcolumn reaction with Arsenazo 111,which was added at 0.5 mL-min-' to the eluent with a syringe pump (ISCO M314, Lincoln, NE) via a low-dead-volume mixer (2).A Rheodyne switching valve (M73030) was placed between the column and the postcolumn reactor for fraction collection (2). For some studies a 5-pm styrenedivinylbenzene reversed phase (PRP-1, 4.1 X 150 mm, Hamilton, Reno, NV) was used. The (Y spectrometric measurements were made on a Canberra M8100 multichannel analyzer (Meriden, CT) that was connected to an Ortec surface barrier detector (EG&GInstruments, Ottawa, Ontario). Electrodeposition for a-spectrometric measurements was done with an Anteck Switchmode power supply (Vancouver, BC) operated in the constant-current mode. Procedures. A conventional Ge y-spectrometer with a Nuclear Data pulse height analyzer (Chicago,E)and a Canberra detection system (Meriden, CT) was used to monitor collected fractions for 140La. The liquid samples were contained in small polyethylene bottles, and five measurements were made on each sample at 328 keV, 487 keV, 815 keV, 1596 keV, and a total integral from 100 keV to 1610 keV. All measurements were corrected for relative decay. The 140Latracer was prepared weekly. Approximately 1 mg of La203 was sealed in thin-walled quartz tubing, wrapped in A1 foil, and placed in an A1 irradiation capsule. The sample was then irradiated in the Chalk River NRU reactor for 72 h in a neutron flux of -2 X 1014n.cm%-l. After irradiation the quartz capsule was crushed in a polyethylene bottle containing 50 g of 0.5 mo1.L-l HN03. The dissolved 140f139La(III) was then diluted with the HPLC mobile phase to give a concentration of 1 PgrnL-'. The procedure used for electrodeposition of the samples for a spectrometry was similar to that reported elsewhere (5) for rare earth and transplutonium metals. Samples (1-4 mL) were placed in 10 mL of a 1 mo1.L-l NH4Cl electrolyte and then deposited at 1A (0.36 A.cm-2) for 1h on a 2-cm stainless-steeldisk that had been washed with dilute nitric acid, water, and acetone. The electrolyte was then made basic with ",OH, and the disk was removed and washed with methanol. Mass spectrometric analyses were performed on solutions eluted directly from the HPLC system or an evaporated concentrate of these solutions. A known quantity of between 20 and 50 ng of Nd was loaded on the evaporation filament of a Re double-filament assembly by placing successive 10-pL drops on the filament and drying at 0.8 A. The filament current was then increased slowly until white fumes (of HPLC reagents) evolved at about 1.6 A, and when no more fumes evolved the current was increased until the deposit turned black. The samples were loaded in a MAT 261 mass spectrometer (Finnigan MAT GmbH, Bremen, FRG); the current in the ionization filament was adjusted to 4.0-4.5 A; and the evaporation filament was heated slowly over 30-60 min while monitoring isotopes for the elements Nd, Ba, Ce, La, Sm, Ca, and NdO. At about 1.2-1.5 A of evaporation current the lUNd+ ion beam was about (0.1-0.5) X 10-l' A depending on the quantity loaded. Data were collected over an hour while the ion beam slowly decayed in intensity. Measurements of Nd isotopes were taken in two sequences, either masses 144,146,150, 147 or masses 143,144, 146, 147 with a separate base line at mass 147.5. Mass 147 was used for monitoring possible isobaric 144Sminterference at mass 144Nd.The ion beams were measured with a loll-Q feedback resistor with beam integrations of 2 s with a 3-s delay between readings after switching of mass positions. All unspiked data were normalized to 146Nd/144Ndof 0.7219 to
eliminate instrumental fractionation. Collection of Metal Ion Fractions. The following procedure for the collection of lanthanide ions is based on the results of this study and is intended to reduce cross-contamination levels to
