Exploring the limits and losses in MALDI sample preparation of attomole amounts of peptide mixtures

We have used a non-contact piezo-electric liquid dispensing workstation to transfer 200 pL aliquots of both a sample solution containing 12 peptides, each at a concentration of 5 or 50 fmol/μL, and dilution buffer onto microcrystalline spots of α-cyano-4-hydroxycinnamic acid for thin-layer preparations on prestructured MALDI sample supports. This approach allowed us to systematically investigate the dependence of detection sensitivity on sample volume, spot size, concentration, as well as sample losses. We demonstrate that 1 amol of the peptide mixture was sufficient to detect all components if it was prepared in a volume of 0.2 nL across a spot of ca. 180 μm diameter. If the sample was diluted on the target to 200 nL and prepared on a spot of 400 μm diameter, 10 amol could be detected and if the sample volume was increased to 1 μL, 50 amol were required to detect all components. If the peptide mixture instead was diluted in a sample vial and 1 μL of that solution was prepared on a spot of 400 μm diameter, 50 amol of the peptide mixture could be detected as well and the signal-to-noise ratios of the analyte signals were comparable to those observed for the on-target dilution experiment.