Solid-phase extraction/MALDI-MS: Extended ion-pairing surfaces for the on-target cleanup of protein samples

The surface of a matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) target can be covalently modified so that it behaves like a medium for solid-phase extraction (SPE). These modified targets are capable of binding molecules of interest, but not contaminants, from sample solutions placed on them, This allows the analyte to be cleaned up on the probe surface by simply washing the target to remove the contaminants prior to MALDI-MS analysis. A limitation of the current SPE/MALDI-MS targets is that they have a fairly low binding capacity, since the coating on these targets is based upon self-assembled monolayers (SAMs). To overcome this limitation, we have investigated new surface coatings for SPE/MALDI-MS that will have a higher binding capacity than targets modified with SAMs. Here, we describe the development of new SPE/MALDI-MS surfaces that have very high molecular weight (>300 000) polylysine chains attached to them. Targets modified in this manner are capable of binding peptides/proteins by ion-pairing interactions and have approximately 100 times the binding capacity of the SAM-based targets. Furthermore, these polylysine targets can capture over 60% of a protein from a highly contaminated solution. Consequently, polylysine SPE/MALDI-MS targets offer a practical solution for analyzing very small volumes (<1 mu L) of peptide/protein solutions contaminated with high levels of inorganic salts, buffers, detergents, chaotropic agents, and other solubilizing agents.