Sample morphology effects in matrix‐assisted laser desorption/ionization mass spectrometry of proteins

Millimetre‐sized crystals of 2,5‐dihydroxybenzoic acid and 3,5‐dimethoxy‐4‐hydroxycinnamic acid (sinapinic acid) were grown from a cytochrome c /matrix solution. Effects of sample exposure to UV laser irradiation on the matrix‐assisted laser desorption ionization mass spectra are reported for such single crystals. The exposure was varied by irradiating the same sample spot with different numbers of UV‐laser pulses. The ion yield and mass resolution were monitored as a function of the sample exposure. Further, the minimum deposited laser energy needed for protein ion production (the threshold energy) for single crystals was determined for incidence angles between 35° and 60° with respect to the target surface normal. The results were compared with those obtained for polycrystalline samples containing micrometre‐sized crystals.