Electrospray mass spectrometry and tandem mass spectrometry of synthetic multicomponent peptide mixtures: determination of composition and purity.

Electrospray (ES) mass spectrometry and tandem mass spectrometry were used to evaluate the composition and purity of synthetic peptide mixtures ("peptide libraries") consisting of 48 peptides in equimolar amounts. For the structurally similar components of the mixtures little mass discrimination effects were found. The ion intensity distribution of the protonated molecular ions was found to be independent of the concentration. The peak heights in the ES mass spectra reflected the number of isobaric peptides in the mixture. By-products formed by incomplete removal of side chain protecting groups were detected fast and conveniently by tandem mass spectrometry. Daughter ion scans were used to identify common structural features of the peptides and the by-products. Parent ion and constant neutral loss scans allowed the detection of all peptides modified with the same protecting group.