Effects of the water content in the sample preparation for MALDI on the mass spectra

Ion abundances in the MALDI TOF mass spectra of the model peptides (bradykinin, alpha-melanocyte stimulating hormone, and melittin) change significantly as water is added to the solution used for dried droplet sample preparation. Changes in the surface tension of the solution can be invoked to explain some of the observed effects on the mass spectra. For example, at low surface tensions, the solution droplet spreads over a larger surface area, resulting in a dilution effect and thereby lowering the [M + H]+ ion yields. Analyte ion yields also have a dependence on matrix crystal morphology. The faster drying samples (low water percentage) promote better inclusion of the analyte into the matrix crystals as compared with the slower drying samples (high water percentage). More efficient inclusion of the analyte in the matrix crystal leads to a better matrix-analyte interaction and hence to higher [M + H]+ ion yields. We present new data that suggest that analyte conformation also influences the MALDI ion yields. The suggestion of conformation affecting MALDI ion yields is based on solvent composition dependence for MALDI H/D exchange data and circular dichroism spectra.