Development of electrospray ionization mass spectrometry for direct analysis of complex samples and an investigation of ionization mechanism of matrix-assisted laser desorption/ionization mass spectrometry

Abstract

Mass spectrometry (MS) is a powerful tool for analysis of various samples. Conventional mass spectrometric analysis, however, typically requires extensive pretreatment to raw samples. In recent years, ambient ionization techniques have been developed to enable direct analysis of complex samples with little or no sample preparation. Following our previous research on direct ionization analysis of tissue samples, in this study, pipette-tip electrospray ionization mass spectrometry (ESI-MS) was developed for direct analysis of another common sample form, herbal powder. This new technique combines commonly available and disposable pipette tip with a syringe pump for on-line sample extraction and ionization. We demonstrated that various powders of herbal medicines and foods could be directly analyzed with this technique and closely related herbal species could be clearly differentiated based on the mass spectra thus obtained. This technique offered continuous and stable signals, and its capability in direct quantitation of chemical ingredients in powder samples was also demonstrated in this study. The pipette tip of this technique could be replaced with a commercially available C18 pipette tip (Zip-Tip), which contained C18 absorbent in the tip end. In this way, the technique could be used for direct analysis of raw solution samples, including detection of protein solutions containing high contents of sodium chloride or sodium dodecyl sulfate (SDS) and quantitation of abused drugs in urine. The capability of MS for direct analysis of raw protein samples, particularly membrane proteins, was further explored in this study. Membrane proteins play important roles in biological processes and are targets of many drugs. MS analysis of membrane proteins has been a challenging task due to their poor solubility and that detergents commonly used to improve the solubility are not compatible with MS analysis. In this study, for the first time, extractive electrospray ionization mass spectrometry (EESI-MS) was attempted for direct analysis of membrane proteins containing high contents of detergents, and our results indicated that EESI-MS was more tolerant to nonionic detergents than ESI-MS. In this study, wooden-tip ESI-MS, a technique developed in our group, was also developed for direct quantitation of di-(2-ethylhexyl)-phthalate (DEHP), a hazard plasticizer, in beverages such as fruit juice and sports drink. We demonstrated that wooden-tip ESI-MS was a simple and economical method for rapid screening of DEHP in beverages. Matrix-assisted laser desorption/ionization (MALDI) is another commonly used ionization technique in mass spectrometry. The ionization mechanism of MALDI is, however, not completely clear yet. In this study, oil-assisted sample preparation (OASP), a MALDI sample preparation method recently developed in our group, and deuterium-labeled matrix and analyte were employed to investigate the interactions between matrix and analyte molecules. The results revealed that all active hydrogens of analyte molecules could be replaced by hydrogens from matrix. The exchange tendency of active hydrogens from different functional groups of analyte and different conformational peptides was investigated. These results allowed us to get new insights into the MALDI ionization process.