We show results from experiments designed to determine the relative importance of gas phase processes and solution phase processes into ionization suppression observed in biological sample extracts. The data indicate that gas phase reactions leading to the loss of net charge on the analyte is not likely to be the most important process involved in ionization suppression. The results point to changes in the droplet solution properties caused by the presence of nonvolatile solutes as the main cause of ionization suppression in electrospray ionization of biological extracts.

A post-column infusion system was developed in order to analyze suppression of electrospray ionization (ESI) tandem mass spectrometry response in the presence of endogenous plasma interferences. By enabling direct detection of these interfering components, this experimental system was used to analyze the ability of several common extraction procedures to remove endogenous plasma components that cause changes in the ESI response of model drug substances. Methyl-t-butyl ether (MTBE) liquid-liquid, Oasis and Empore solid-phase, and acetonitrile (ACN) protein precipitation sample preparation methods were tested using the post-column infusion system. In all cases, ACN protein precipitation samples showed the greatest amount of ESI response suppression while liquid-liquid extracts demonstrated the least. In addition, the three test compounds, phenacetin, caffeine, and a representative Merck compound, demonstrated that ESI response suppression is compound dependent. Suppression was greatest with caffeine, the most polar analyte, and the smallest for the Merck compound, the least polar analyte. Copyright 1999 John Wiley & Sons, Ltd.

Characterization of S-linked conjugates of the endogenous tripeptide glutathione (gamma-glutamyl-cysteinylglycine, GSH) represents a valuable indirect approach for the identification of chemically reactive, electrophilic intermediates formed during the metabolism of both foreign compounds and endogenous substances. In most cases, GSH adducts generated in vitro or excreted in the bile of animals are detected by the use of liquid chromatography-tandem mass spectrometry (LC-MS/MS), employing survey scans based on characteristic fragmentations of this class of conjugates. However, a limitation of current LC-MS/MS approaches, which typically employ electrospray ionization with analysis of positive ions, is that no single survey scan exhibits broad utility in the detection of unknown GSH adducts, since different structural classes of conjugate (aromatic, benzylic, aliphatic, thioester, etc.) behave differently upon collision-induced dissociation (CID) of the respective [M + H]+ parent ions. In the present study, we evaluated MS/MS in the negative ion mode as an alternative approach and report herein that the spectra obtained by CID of the [M - H]- ions of a number of representative GSH adducts, as well as GSH itself, are dominated by fragments originating from the glutathionyl moiety of the tripeptide. In particular, the anion at m/z 272, corresponding nominally to deprotonated gamma-glutamyl-dehydroalanyl-glycine, was abundant in the negative ion spectra of free GSH and all GSH conjugates examined, suggesting that scanning for precursors of this ion may provide a generally applicable technique for the detection of adducts of unknown structure. The utility of this novel detection strategy was demonstrated in a series of in vitro and in vivo experiments where compounds known to undergo metabolic activation were examined for their propensity to form conjugates with GSH. In all cases, scanning for precursors of m/z 272 in the negative ion mode revealed the presence of the expected adducts and in some instances revealed additional conjugates that had not been reported previously. Positive ion MS/MS, on the other hand, was more useful than the corresponding negative ion scans in providing information on the molecular structure of GSH conjugates.

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The goal of this work is the development of a rapid and objective matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) method for the quantitation of peptides and proteins in human plasma suitable for use in the Good Laboratory Practices (GLP) environment, where the analytical method, validation and pharmacokinetic parameters derived from concentration data will be scrutinized by global regulatory agencies. Electrospray deposition has traditionally been used to prepare thin, uniform samples for a number of techniques, including Cf-252 plasma desorption and secondary ion mass spectrometry. Here the electrospray process of sample application is used to reduce the segregation of analyte from matrix during the sample drying step. The small droplets formed during the electrospray process are found to significantly improve the homogeneity of the sample surface prepared. Experiments comparing the traditional air dried and electrosprayed methods of sample preparation show that the increase in sample homogeneity from electrosprayed samples decreases both the within-sample spot and between-sample spot variability, resulting in a decrease in percent coefficient of variation (%CV) for the recorded MALDI mass spectra. The increase in sample homogeneity permits a more objective use of MALDI-TOFMS as a quantitative analytical method and has led to the development of an assay for the determination of desamino-[8-D-arginine] vasopressin (DDAVP) using arginine vasopressin (AVP) as internal standard in human plasma. The range of quantitation observed (2.0-10 micrograms/mL) is of limited value for bioanalytical application; however, the analysis of neat standards shows lower quantitation limits are easily achieved.