Nanoliter-volume selective sampling of peptides based on isoelectric points for MALDI-MS.

A simple way to selectively isolate peptides based on their isoelectric points (pI) for MALDI mass spectral analysis is described. An applied voltage was used to electromigrate peptides into a capillary. The capillary was modified with a zwitterionic surfactant, 1,2-dilauroyl-sn-phosphatidylcholine (DLPC), to suppress the electroosmotic flow (EOF) during injection. Hence, either the cationic or the anionic peptides were introduced, depending on the voltage polarity. By controlling the pH, selective loading of peptides was performed to isolate trace components from a mixture. The injected sample plugs were subsequently spotted in nanoliter volumes for MALDI-MS analysis. No significant sample losses resulting from selective sampling were detected. Low attomole-level detection of peptides (adrenocorticotropic hormone fragment 18-39, pI 4.25) was achieved from a mixture containing other peptides (angiotensin I, pI 6.92, and bradykinin, pI 12.00) at 100 000-fold higher concentrations.