Neutral desorption sampling of biological surfaces for rapid chemical characterization by extractive electrospray ionization mass spectrometry

Extractive electrospray ionization mass spectrometry (EESI-MS) allows the real-time, direct analysis of complex gaseous and liquid samples without any sample pretreatment under ambient conditions. By using a neutral desorption (ND) sampling gas beam to gently impact a surface, the analyte present on a surface can be efficiently sampled and then transported to the EESI source for soft ionization without any previous work-up. By separating the sampling process and the ionization process in both space and time, ion suppression effects are significantly decreased in ND-EESI-MS. Therefore, virtually any surface can be gently sampled in real time, allowing in vivo analysis of living objects while maintaining the native conditions of the sample. This greatly simplifies the procedure for the characterization of complex surfaces (e. g., plants, food, skin) and prevents potential chemical contamination, providing a unique platform for wide applications in multiple disciplines such as chemistry, biology and the life sciences. Here we present a complete description of a protocol for ND-EESI-MS for in vivo analysis of living objects, including equipment setup, reagent preparation, data acquisition and analysis steps. The data collection can be completed within a few minutes even though the time required for the entire analytical process, which largely depends on the preparation of samples and materials, takes slightly longer.