TIP spray ionization mass spectrometry and its analytical application in direct sample analysis

Abstract

Electrospray (ES)-based ambient ionization techniques have been rapidly developed and became indispensable tools for direct sample analysis using mass spectrometry. In this study, it was found that with the use of porous substrate, analytes present in complex samples could be detected by directly introducing untreated sample on porous tips. Moreover, the specific feature, micro-channels of the porous tips, allows efficient sample loading and facilitates electrospray ionization (ESI), which is beneficial for the direct sample analysis. Systematic study on the influences of the physical and chemical factors of various porous materials (polyester, polyethylene and wood) on the detection sensitivity and ionization efficiency was firstly investigated. It was found that the chemical interaction between the porous materials and analytes could significantly affect the detection sensitivity for analytes. Hydrophobic spraying tips showed better detection sensitivity for polar analytes while better detection for non-polar analytes was obtained when hydrophilic spraying tip was used. Moreover, the electrostatic properties of spraying tips could also affect the detection sensitivity for analytes. Hydrophobic materials (i.e., polyethylene and polyester) which tend to form negative charge on the surface could facilitate the ionization of analytes in negative ion mode. With a clear understanding of the effect of porous spraying materials on detection sensitivity, tip spray ionization mass spectrometric method was developed and successfully applied to direct analysis of pharmaceutical, forensic, clinical and food samples. Rotary sample stages compatible to the ion source of two commercial mass spectrometers were designed and constructed for the automation of tip spray ionization mass spectrometry, which allowed the analysis of 12 samples in less than 15 minutes. In addition, instead of using porous substrates for the sample deposition, the developed technique was further applied to the direct chemical analysis of American ginseng tissue, demonstrating the feasibility of performing direct analysis on raw herbs in solid state.