Anal. Chem. 1995, 67,2020-2025
Capillary Electrophoresis Coupled On-Line with Inductively Coupled Plasma Mass Spectrometry for Elemental Speciation Yan Liu and Viorica LopecAvila* Califomia Operations, Midwest Research Institute, 625-8Clyde Avenue, Mountain View, Califomia 94043
Jim J. Zhu and Daniel R. Wederin CETAC Technologies Inc., 5600 South 42nd Street, Omaha, Nebraska 68107
Wemer F. Beckert Environmental Monitoring System Laboratory, U.S. Environmental Protection Agency, 944 East Harmon Avenue, Las Vegas, Nevada 891 19
A novel interface to connect a capillary electrophoresis (CE) system with an inductively coupled plasma mass spectrometric (ICPMS) detector is reported here. The interface was built using a direct injection nebulizer (DIN) system. In this interface, the CE capillary was placed concentrically inside the fused-silica DIN sample introduction capillary. A makeup liquid was pumped at a flow rate of 15 pWmin into the fused-silica DIN sample transfer capillary via a connector. The makeup liquid flowing outside the CE capillary served two purposes: (a) it established continuous and stable electrical contact at the exit (grounding) terminus of the CE capillary, and (b) it allowed the DIN system to be optimized independently of the CE system for efficient sample introduction to the ICPMS system. The makeup liquid along with the liquid from the electroosmotic flow generated inside the CE capillary was nebulized by the DIN system directly into the ICP torch. The CE/DIN-ICPMS systemwas evaluated using test samples containing selected alkali, alkalineearth, and heavy metal ions, as well as selenium [Se(IV) and &(VI)], and various inorganic and organic arsenic species. The preliminary results show that the CE/DINICPMS system can be used to determine metal species at parts-per-trillionto parts-per-billionlevels. Capillary electrophoresis (CE) has become a powerful analytical technique for the separation of a variety of analytes ranging from small inorganic ions to large biomolecules such as proteins and nucleic The development of highly sensitive and selective detectors for CE has been one of the most important and challenging prerequisites for the growth of CE. Oncolumn UV and fluorescence detectors are commonly used to determine the analytes separated by CE. However, these detectors are often not very sensitive; the analyte concentrations that can be detected (1) Jorgenson, J.; Lukacs, K D. Anal. Chem. 1981,53,1298-1302. (2) Jorgenson, J.; Lukacs, K D. Science 1983,222,266-272. (3) Ewing, A G.; Wallingford, R A; Olefirowicz, T. M. Anal. Chem. 1989,61, 292A-303A (4) Monnig, C. A; Kennedy, R. T. Anal. Chem. 1994,66,280R-314R
2020 Analytical Chemistry, Vol. 67,No. 13, July 1, 1995
with these detectors are on the order of 10-5-10-6 M.5 The use of other detection techniques such as mass spectrometry, laserinduced fluorescence, amperometry, and conductometry has been investigated to provide more sensitive and selective detection of the target analytes separated by CE.5-12 Some of the most sensitive and selective techniques for the determination of metals include plasma atomic spectrometry techniques such as microwave-induced plasma atomic emission spectrometry NIP-AES) , inductively coupled plasma atomic emission spectrometry (ICP-AES) , and inductively coupled plasma mass spectrometry (ICPMS). These detection techniques have been successfully combined with separation techniques such as gas chromatography, liquid chromatography, and supercritical fluid ~hromatography.'~-'~ However, relatively few studies have been published on the use of plasma atomic spectrometry or mass spectrometry techniques as a means of detection in CE separat i ~ n s . ' ~ JIn* 1993, a CE system was interfaced with a microwaveinduced helium plasma atomic emission detector (MIP-AED) and it was demonstrated, in principle, to be useful for element-specisc detection of organotin compound^.'^ The results also indicated, however, that the helium plasma of the MIP-AED system did not have sufficient energy to vaporize and atomize the analytes contained in the aqueous buffer solution. The detection limit was (5) Smith, R D.; Wahl, J. H.; Goodlett, D. R.; Hofstadler, S. A Anal. Chem. 1993,65,574A-584A. (6) Smith, R D.; Udseth, H. R.; Barinaga, C. J.; Edmonds, C. G. J. Chromatogr. 1991,559,197-208. (7) Olivares. J. A ; Nguyen, N . T.;Yonker, C . R; Smith, R D. Anal. Chem. 1987, 59,1230-1232. (8) Gassman. E.; Kuo, J. E.; Zare, R N. Science 1985,230,813-814. (9) Wallingford, R A ; Ewing, A G. Anal. Chem. 1987,59,1762-1766. (10) Huang, X.; Pang, T.-KJ.; Gordon, M. J.; &re, R N.Anal. Chem. 1987,59, 2747-2749. (11) Dasgupta, P.K; Bao, L. Anal. Chem. 1993,65,1003-1011. (12) Albin, M.; Grossman, P. D.; Moring, S. E. Anal. Chem. 1993,65,489A497A (13) Donard, 0. F. X.; Martin, F. M. Trends Anal. Chem. 1992,1 1 , 17-26. (14) Vela, N. P.; Olson, L. K; Camso, J. A Anal. Chem. 1993,65,585A-597A (15) Hill, S. J.; Bloxham, M. J.; Worsfold, P. J. J. Anal. At. Spectrom. 1993,8, 499-515. (16) Lobinski, R ; Adams F. C. Trends Anal. Chem. 1993,12,41-49. (17) Liu, Y.;Lopez-Avila, V. J. High Resolut. Chromatogr. 1993,16,717-720. (18) Olesik. J. W.; Kinzer, J. A; Olesik. S. V. Anal. Chem. 1995,67,1-12.
0003-2700/95/0367-2020$9.00/0 0 1995 American Chemical Society
thus rather poor (about 1mg/mL for trimethyltin). More recently, dmethylarsinic acid [ (CH3)2HAsO2, DMA], disodium selenite Olesik and co-workers reported on interfacing CE with ICPAES (NaZSeOJ, disodium selenate (NaZSeOd, l,lGphenanthroline,and and ICPMS and demonstrated that CE/ICPAES and CE/ICPMS ammonium nitrate were obtained from Sigma Chemical Co. (St. could be used to speciate metals at parts-per-billion levels in Louis, MO) . Ultrex-grade ultrapure nitric acid was obtained from aqueous solutions.18 J. T. Baker (Philipsburg, NJ). Trace metal-grade ammonium Designing an efficient interface to connect the end of the CE hydroxide was obtained from Fisher Scientific. separation capillary with the plasma is critical to the success of A multielement stock solution containing E+,K+, Cs+, Cd2+, any CE/plasma atomic spectrometry or mass spectrometry Co2+, Mnz+, Pb2+, SrZ+,and Tl+ at 10 pg/mL per metal was technique. In the interface reported by Olesik and co-workers, prepared by compositing the individual standard stock solutions the CE capillary exit terminus, which was coated with silver paint and diluting the composite solution with deionized water. Mul(serving as a conductor to complete the electrophoresis circuit), tielement test solutions at lower concentrations were prepared was fitted into the center tube of a concentric, pneumatic nebulizer by diluting the 10 pg/mL multielement stock solution with with a spray chamber. These workers reported that silver deionized water as needed. contamination could occur under some conditions,1aand their Individual stock solutions of NaAsOz, KHAs04, MMA, DMA, results also indicated that the interface caused some band NazSe03, and NazSe04 at 100pg/mL (as As or Se) were prepared broadening (probably due to the dead volume of the spray by dissolving the appropriate amounts of the corresponding salts chamber used in the study). in deionized water. Test solutions containing selenium and arsenic The direct injection nebulizer (TIIN) is a microconcentric species at lower concentrations were prepared by compositing pneumatic nebulizer with no spray chamber;it nebulizes the liquid the individual stock solutions and diluting the composite solution sample directly into the central channel of the ICP t o r ~ h . ~ ~ - ~ ~ with deionized water as needed. Compared with conventional nebulizers, important advantages of For the CE separation of the selected alkali, alkalineearth, and DIN include (a) 100%sample introduction efficiency into the ICP, heavy metal ions, an electrophoresis buffer solution of 10 mM (b) accommodation of low sample introduction flow rates (10ammonium nitrate, 5 mM phenanthroline,and 50 ng/mL Cs+ was 100 pL/min), (c) an extremely small internal volume (
