Desorption Electrospray Ionization (DESI) Analysis of Tryptic Digests/Peptides

The analytical utility of desorption electrospray ionization (DESI) is such that it can be applied to qualitative proteomics research in the same way as matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) methods, although little work has yet been reported in this regard. Because DESI is a surface analysis technique and easily automated, it can be implemented for high-throughput applications, which include the analysis of chromatographic fractions of digested proteins. The analysis of tryptic peptides follows the same protocols as in typical MALDI or ESI methods, except that the mixture is spotted directly onto an insulating surface, allowed to dry, and analyzed directly without adding matrix compounds (as in the case of MALDI methods). The spectral characteristics are similar to those of ESI in that both singly and multiply charged analyte ions are detected. Spectra are highly similar to electrospray spectra of tryptic digests with regard to the overwhelming presence of multiply charged ions of peptides. DESI-mass spectrometry (DESI-MS) is an emerging technique with great promise, but its application range is still being investigated. Therefore, the protocol for DESI-MS analysis of tryptic digests/peptides presented here provides general procedures used for the applications that have been investigated so far. Optimal ion source parameters and surface types may vary, depending on the application.