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Anal. Chem. 1996, 68, 243-249
Electrochemical Modulation of the Sample Stream in Mass Spectrometry Hong Ren, John Szpylka,† and Larry B. Anderson*
Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210-1173
Sampling methods are described, suitable for mass spectrometric identification of volatile species in aqueous solutions. Submillimolar concentrations of oxygen and carbon dioxide were sampled during electrolysis and identified within a second of the time they were electrolyzed. The electrolysis process modulates solution concentrations near a 100 µm thick silicone rubber membrane sampling device, which transmits the information to the mass spectral source as a periodic variation in the partial pressure of the analyte. Consequently, ion currents vary at the same modulation frequency. Correlation analysis quantitates a phase shift between the mass spectral response and the electrochemical excitation as a function of modulation frequency. The time required for various molecules to diffuse through the membrane is derived from these correlation spectra. The silicone membrane thus becomes an ultrashort chromatographic column, interfacing a condensed phase sample stream with a sensitive and discriminating mass spectrometric detector. Mass spectrometry offers a sensitive and specific method for identifying and determining the structures of molecular species participating in electrochemical reactions.1-14 However, before mass spectral analysis can be carried out on an electrochemical solution, the analyte molecules must be separated from each other and from the solvent matrix and injected into the moderate vacuum of the MS source. † Present address: General Mills, Inc., James Ford Bell Technical Center, 9000 Plymouth Ave. North, Minneapolis, MN 55427. (1) Volk, K. J.; Yost, R. A.; Brajter-Toth, A. Anal. Chem. 1992, 64, 21A. (2) Bittins-Cattaneo, B.; Cattaneo, E.; Konigshoven, P.; Vielstich, W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, pp 181-218. (3) House, S. D.; Anderson, L. B. Anal. Chem. 1994, 66, 193. (4) Dupont, A.; Gisselbrecht, J. P.; Leize, E.; Wagner, L.; Dorsselaer, A. V. Tetrahedron Lett. 1994, 35, 6083. (5) Xu, X.; Nolan, S. P.; Cole, R. B. Anal. Chem. 1994, 66, 119. (6) Anastasijevic, N. A.; Baltruschat, H.; Heitbaum, J. Electrochim. Acta 1993, 38, 1067. (7) Hambitzer, G.; Heinz, P. P., Stassen, I.; Heitbaum, J. Synth. Met. 1993, 55-57, 1317. (8) Volk, K. J.; Yost, R. A.; Brajter-Toth, A.; Freeman, J. A. Analysis 1992, 20, 421. (9) Reynolds, J. D.; Cook, D.; Burn, J. L.; Woods, C. J. Am. Soc. Mass Spectrom. 1992, 3, 113. (10) Volk, K. J.; Yost, R. A.; Brajter-Toth, A. Anal. Chem. 1989, 61, 1709. (11) Getek, T. A.; Korfmacher, W. A.; Hinson, J. A J. Chromatogr. 1989, 474, 245. (12) Bruckenstein, S.; Gadde, R. R. J. Am. Chem. Soc. 1971, 93, 7933. (13) Bruckenstein, S.; Comeau, J. Faraday Discuss. Chem. Soc. 1973, 56, 285. (14) Bruckenstein, S.; Grambow, L. Electrochim. Acta 1977, 22, 377.
Permeable membrane systems, composed of a thin, semipermeable silicone membrane15,16 or microporous Teflon membrane12-14 situated between the electrode and the MS vacuum, have been suggested for use in single-stage extraction procedures for sampling electroactive molecules from polar solvents. The permeant from such a membrane usually contains several molecular species, and a mixed-mass spectrum is observed, which must be deconvoluted to obtain interpretable information on individual analytes.3,16 In this study, we have applied a periodically changing electrochemical current to an electrode attached to a silicone rubber membrane; this periodically changes the concentration of permeable species in the solution adjacent to the sampling membrane. The flux of molecules released into the MS source thus has been modulated at the frequency of the electrochemical current applied. The first consequence is that electrochemically modulated ion currents can be readily distinguished from background and separated from the ion currents produced by constant levels of solvent or electrolyte. In addition, we have investigated the phase angle between the MS ion current response and the periodic electrochemical excitation. While the application described here is electrochemical in nature, the principle of modulation of the ion current is quite general. THEORY Permeable membrane mass spectrometry (PERMS) employs a thin silicone rubber membrane in intimate contact with a minigrid electrode as a means to separate volatile species from the electrochemical solution matrix (Figure 1).15,16 A complete solution of the mathematical problem describing mass transport of analyte to the electrode, from the electrode to the membrane, and through the membrane to the MS source has not been attempted, but a semiquantitative description of this complex mass transport system is possible, by invoking several plausible assumptions.17,18 The first assumption for the minigrid electrode in Figure 1 is that the diffusion layer thickness in the solution phase, δss, is large compared to the hole dimension of 12.5 µm (1000 lines/in.) during most electrochemical experiments. Thus, the aqueous solution bathing the membrane at x ) 0 becomes virtually homogeneous in the y- and z-directions in
