New development of electrospray ionization mass spectrometry

Abstract

Electrospray ionization mass spectrometry (ESI-MS) is widely used in many fields including chemistry, biology, medicine and pharmacy, and is still under fast development. Conventional ESI uses a capillary for sample loading and ionization. Non-capillary ESI can avoid the clogging problem, allow easy sample loading, and create new analytical features. In this study, ESI on solid substrates has been developed and their analytical properties and mechanism have been investigated. We developed ESI on wooden tips (wooden toothpicks) and demonstrated that wooden-tip ESI-MS was applicable for qualitative and quantitative analysis of various samples, including samples that cannot be directly analyzed by conventional ESI techniques, e.g., slurry samples and powder samples. Wooden-tip ESI-MS with modified wooden-tip surfaces was found to allow selective and sensitive detection of analytes in mixtures, and direct coupling of ESI-MS with commercially available solid-phase micro-extraction (SPME) fibre was developed for enhanced detection of analytes in raw samples. Other materials, including bamboo, fabrics, sponge, polyester, and aluminium (Al) foil and thin-layer chromatography (TLC) plate have also been successfully employed as solid substrates for ESI-MS analysis. Particularly, Al-foil ESI-MS allowed on-target sample extraction, cleanup and heating, extending ESI device from usually only for sample loading and ionization to including sample processing; TLC-ESI-MS could be used for direct analysis of samples containing salts and detergents, and rapid detection and quantitation of target analytes in raw samples. Solid-substrate ESI-MS has been extended to direct ionization analysis of sample materials including plant tissue and animal tissue. Furthermore, by avoiding direct connection of samples with high voltage, field-induced direct ionization was developed to much facilitate sample analysis and allow in vivo and real-time monitoring of secondary metabolites of living organisms by mass spectrometry. In conclusion, replacement of capillary with solid substrates generates many new possibilities for ESI-MS and allows us to get more insight into the fundamentals of ESI-MS. The new techniques developed in this study enable direct analysis of raw samples with no or little sample pretreatment and can be applied in various sample analysis including bioanalysis, food quality control, pharmaceutical analysis, environmental analysis and forensic analysis. The motion of molecules on porous substrates under ESI conditions was found to be size and shape-dependent, suggesting that solid-substrate ESI-MS could be further developed for investigating molecular structures and protein conformations in solutions.