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 spectrome try (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 soluti ons placed on them, This allows the analyte to be cleaned up on the probe s urface by simply washing the target to remove the contaminants prior to MAL DI-MS analysis. A limitation of the current SPE/MALDI-MS targets is that th ey have a fairly low binding capacity, since the coating on these targets i s 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 describ e the development of new SPE/MALDI-MS surfaces that have very high molecula r weight (>300 000) polylysine chains attached to them. Targets modified in this manner are capable of binding peptides/proteins by ion-pairing intera ctions and have approximately 100 times the binding capacity of the SAM-bas ed 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 volume s (<1 mu L) of peptide/protein solutions contaminated with high levels of i norganic salts, buffers, detergents, chaotropic agents, and other solubiliz ing agents.