Laser desorption time-of-flight mass spectra of peptides and proteins, as well as of lower molecular weight analytes, have been obtained by using a pulsed nitrogen UV laser (337 nm) to irradiate mixtures of 2-150 microns graphite particles and solutions of the analytes in glycerol. Protonated analytes as well as abundant alkali cation adducts were observed. Carbon cluster ions, Cn+, typically had a low abundance but dominated the mass spectrum at elevated laser powers. In spectra of a cytochrome c tryptic digest, all but one of the tryptic peptides were easily observed. Spectra of low molecular weight analytes dissolved in glycerol are very similar to FAB spectra of the same glycerol solution with added alkali salts. However, in many peptide and protein spectra, glycerol ion abundances are very low, and the alkali ions dominate the spectra at low mass. These spectra may correspond to wet and dry surface desorption conditions, respectively. The best spectra of the larger molecules were observed under dry conditions. In these initial experiments, we have obtained a sensitivity in the pico- to nanomole range and a mass resolution of about 300. The signal intensity is as good as that in conventional MALDI, and under optimal conditions, few background peaks appear, even at low mass.

A new method of electrospray-assisted laser desorption/ionization (ELDI) mass spectrometry, which combines laser desorption with post-ionization by electrospray, was applied to rapid analysis of solid materials under ambient conditions. Analytes were desorbed from solid metallic and insulating substrata using a pulsed nitrogen laser. Post-ionization produced high-quality mass spectra characteristic of electrospray, including protein multiple charging. For the first time, mass spectra of intact proteins were obtained using laser desorption without adding a matrix. Bovine cytochrome c and an illicit drug containing methaqualone were chosen in this study to demonstrate the applicability of ELDI to the analysis of proteins and synthetic organic compounds.

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The sensitivity of atmospheric pressure Ionization mass spectrometry to a range of analytes was studied with a SC I EX TAG A 6000E mass spectrometer. The reagent gas was air containing 5 Torr water. Analytes with gas-phase basicities (GB) greater than ca. 200 kcal/mol have uniformly very high and nearly equal sensitivities determined by fast kinetics for proton transfer from the reagent 30+( 20) (kinetic control). Most analytes (B) with GB less than ca. 200 kcal/mol have sensitivities that are determined by the thermal equilibrium distributions of BH+(H20)6 and H30+(H20)b (thermodynamic control). For these mostly oxygen bases, the sensitivity increases with the gas-phase basicity of B. Several analytes, notably sulfur and carbon bases, have much lower sensitivities than expected from their GB's. The hydrates BH+(H20)e of this group are distinguished by very low sta-