Discrimination and Selective Enhancement of Signals in the MALDI Mass Spectrum of a Protein by Combining a Matrix‐Based Label for Lysine Residues with a Neutral Matrix

Discrimination with a difference: The N-hydroxysuccinimide ester of HCCA (α-cyano-4-hydroxycinnamic acid) was used as a labeling reagent to increase the MALDI MS signal of weakly concentrated peptides of interest in a proteolytic mixture derived from cytochrome c relative to the signals of other peptides. The desired effect was only observed with the neutral MALDI matrix HCCE (the methyl ester of HCCA; see mass spectra).