Sensitivity improvement of bio-molecules using ionic liquid matrix in mega electron volt secondary ion mass spectrometry

Enhancement effect of ionic liquids on analysis of peptides was investigated in mega electron volt secondary ion mass spectrometry (MeV-SIMS). An ionic liquid of tripropylammonium α-hydroxycinnamate (trip-CHCA) was added to Des-Arg⁹-[Leu⁸]-bradykinin (bradykinin) with various molar ratios. The mixtures as well as the neat bradykinin were prepared on Si substrates. The samples were irradiated with 0.84 MeV C⁺ ions at a grazing angle of 1°. Secondary ions emitted from the sample surface were analyzed using a time of flight mass spectrometer. The yields of both intact peptide ions and fragment ions were enhanced by adding trip-CHCA. The enhancement factor increases almost linearly with the molar ratio R of trip-CHCA to bradykinin for the fragment ions which do not contain arginine while it saturates at R > 1000 for the arginine containing ions. These results are attributed to the strong proton affinity of arginine.