Anal. Chem. 2000, 72, 3833-3840
Glow Discharge Ionization Source for Liquid Chromatography/Particle Beam Mass Spectrometry Terri E. Gibeau† and R. Kenneth Marcus*
Department of Chemistry, Howard L. Hunter Chemical Laboratories, Clemson University, Clemson, South Carolina 29634-0973
A detailed evaluation of the analytical characteristics of a liquid chromatography/particle beam-glow discharge mass spectrometry (LC/PB-GDMS) system is described for applications in the area of inorganic (free metals in solution) and organic compound analysis. A highly efficient particle beam interface is used to introduce analyte species into a glow discharge source for subsequent vaporization and ionization. The GD operating current and gas pressure were optimized, with the general responses found to be similar to those obtained previously for particulate matter analysis by this PB-GDMS system. The mass spectra obtained for inorganic species are comprised exclusively of the respective elements’ isotopic patterns, with no evidence of appreciable oxide species formation. Organic species introduced into the discharge through the particle beam interface yield mass spectra that are virtually identical to those from standard electron impact (70 eV) ionization. An analytical response curve for caffeine, using 200-µL (H2O/MeOH) injection volumes, showed less than 5% RSD for replicate injections over a concentration range of 10-500 ppm, with a detection limit of 13 ppb (2.7 ng) obtained for the caffeine molecular ion. Similarly, detection limits for Fe, Ag, and Cs ranged from 5.8 to 6.1 ppb (∼1 ng, each) in the injected volume. As an example of the feasibility of the PB-GDMS system as a detector for liquid chromatography, the separation and identification of the organic constituents in diet soda was performed. Glow discharge (GD) devices are some of the oldest and most widely studied spectroscopic sources.1 The most common analytical applications of the devices lie in the area of metals, alloy, and semiconductor material analyses. Due to their efficient and easily controlled atomization, excitation, and ionization processes, commercial instrumentation is available employing GDs as primary sources in atomic absorption (AA), atomic emission (AE), and mass spectrometry (MS).2 Radio frequency (rf)-powered devices have expanded the application of glow discharge techniques for direct solids elemental analysis to include both conducting and † Present address: Micromass, Inc., 100 Cummings Center, Suite 407N, Beverly, MA 01915-6101. (1) Marcus, R. K. Introduction. In Glow Discharge Spectroscopies; Marcus, R. K., Ed.; Plenum: New York, 1993; Chapter 1. (2) Harrison, W. W.; Barshick, C. M.; Klingler, J. A.; Ratliff, P. H.; Mei, Y. Anal. Chem. 1990, 62, 943A-949A.
10.1021/ac0002691 CCC: $19.00 Published on Web 07/08/2000
© 2000 American Chemical Society
nonconducting materials.3 The combination of cathodic sputtering to effect atomization and gas-phase collisions for excitation and ionization produces a powerful tool for direct solids analysis. In addition, the low-pressure, low-temperature plasma presents a number of positive qualities that make the analysis of solution- or gas-phase-originating samples an attractive possibility. Gaseous sample introduction directly into the discharge is quite facile, permitting analysis of organic and organometallic species by atomic emission and mass spectrometry.4-7 The extension of the GD techniques to the direct analysis of liquid samples is hindered, though, by the inability of the low-pressure/-temperature plasma to effectively desolvate the analyte. In addition, the presence of any solvent vapors (i.e., water) greatly affects the plasma energetics. The desire to make the GD source a multifaceted mass spectrometric analysis tool has led researchers to explore the possibilities of using the source for both inorganic and organic liquid analyses. An obvious extension of such capabilities would be to use the sources as detectors for liquid chromatography separations. The most common method for the introduction of liquid samples for GD analysis to date has been the deposition of the analyte solution onto a conductive substrate, followed by solvent evaporation and subsequent discharge atomization, excitation, and ionization.8-13 In general,
