On plate graphite supported sample processing for simultaneous lipid and protein identification by matrix assisted laser desorption ionization mass spectrometry

In this study, the simultaneous identification of lipids and proteins is achieved by matrix assisted laser desorption ionization - mass spectrometry (MALDI-MS) analysis after direct on-plate processing of micro-samples supported on graphite. Taking advantages of large surface area and thermal conductivity, graphite provided an ideal substrate for on-plate proteolysis and lipid extraction without interfering with MALDI analysis. The new protocol was first developed on simple standards as bovine serum albumin to set up the best experimental conditions for proteolysis. Then, its feasibility and performance were demonstrated by processing more complex samples as milk and egg powder for simultaneous lipid/protein analysis. By such a protocol, proteins could be efficiently digested on-plate within 15 minutes, with sequence coverages comparable to those obtained by conventional overnight in-solution digestion. As an additional feature, detection of hydrophilic phosphorylated peptides could be achieved without any further enrichment procedure. Furthermore, lipids could be identified in the same analysis without any additional treatment/processing step. The present strategy is simple and efficient, of large applicability, offering great promise for high-throughput proteo-lipidomics in very small samples since no conventional solvent extraction is required reducing sample loss.