Advances in Direct Quantification Analysis by Mass Spectrometry

Direct quantification analysis by mass spectrometry refers to the technique that sample is quantified directly by mass spectrometry without chromatographic separation before ionization. Compared to the traditional chromatographic methods, this technique could significantly shorten analytical time, and then further to meet the rapid, online monitoring and in situ analysis demands. At present, new publications have continuously appeared and widened the application areas. This review provided a snapshot of the field and mainly focused on the mass spectrometric techniques and sample pretreatment strategies. From the technical aspect, ambient desorption ionization MS has made a great breakthrough for direct quantitative analysis. Various samples could be directly desorbed and ionized in the open air conditions of the laboratory or in their natural environment. It typically required little or no sample preparation and offers a much=simplified work flow prior to MS analysis. Certainly, ambient MS techniques have been successfully coupled to mass spectrometers equipped with different high=resolution mass analyzers， such as time of flight (TOF), Orbitrap and ion cyclotron resonance (ICR). The ability to provide accurate mass values and isotope patterns makes feasible the unequivocal identification of compounds from MS data. Sometimes, in order to extract the maximum information, hybrid instruments such as triple-quadrupole mass analyzer (QQQ) and quadrupole-TOF possessing multiple scan modes for further identification and quantification analysis is necessary. From the pretreatment aspect, there are two main strategies for direct quantification analysis by MS. The first one is trying to improve the signal response in the mass spectrometer by utilizing the derivative reagent. At the same time, derivatization using stable isotope labeling such as N-alkylpyridinium isotope quaternization could also introduce the internal standard for the accurate quantification. The second strategy is the extraction technique significantly reducing matrix interference and improving the sensitivity of detection. Trace amounts of semi-volatile or non-volatile compounds can be efficiently extracted from the complex matrix by micro-extraction and QuEChERS method, etc. The effective and selective characteristics of the extraction technique facilitate a perfect combination of ambient MS technique for further high-throughput and accurate analysis. Up to now, direct quantification analysis by MS has been applied for mass spectrometry imaging, single cell analysis and online reaction monitoring, etc. although the feasibility for the quantification analysis of multiple real samples remains to be verified. Accompanied by the enhancement of instrument performance and the improvement of sample pretreatment, it can be easily foreseen that this technique will become a more reliable means for quantitative analysis in the near future.