Gas-phase separations of electrosprayed peptide libraries.

High-resolution ion mobility spectrometry has been combined with time-of-flight mass spectrometry for analysis of a combinatorial peptide library that is expected to contain 676 components. In this approach, the components of a mixture of three residue peptides, having the general form (D)Phe-Xxx-Xxx-CONH2 (where Xxx is randomized over 26 residues including 10 naturally occurring amino acids and 16 synthetic forms) were ionized by electrospray ionization. Ion mobility/time-of-flight distributions have been recorded for all ions using a nested drift(flight) time technique. The improvement in resolving power [(t/delta t) = 100-150 for singly charged ions] was illustrated by analysis of a mixture of tryptic digest peptides using high- and low-resolution instruments. The approach allows many components of the library (e.g., structural, sequence, and stereo isomers) that cannot be distinguished by mass spectrometry alone to be resolved. Impurities due to side reactions appear to be minimal, comprising < 10% of the total ion signal. Direct evidence for approximately 60-70% of the expected peptides is found. Variation in ion abundance for different components indicates that there are differences in solution concentrations or ionization efficiencies for the components.