Sodium salts effect on the time of flight mass spectra of some amino acids in the direct-laser desorption ionization and matrix-assisted laser desorption/ionization

One of the problems in the matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) of small molecules is the overlap between the peaks originated from the matrix and analyte. The MALDI and the direct laser desorption ionization (direct-LDI) spectrum of the phenylalanine amino acid (FAA) has been recorded and assigned in the absence and presence of the salts, including NaF and NaOH. It was seen that the peaks originated from the matrix were suppressed in the MALDI spectrum of the FFA in the presence of salt. In addition, the salt effect on the TOF mass spectrum of the FAA, recorded by the direct-LDI, was investigated. The mass spectrum of the FFA, in the presence of NaF, showed a relatively strong and sharp peak assigned to [FAA-H + 2Na](+) species in both the direct-LDI and MALDI. The direct-LDI spectrum was less crowded than the MALDI spectrum. The same results were obtained when NaOH was used instead of NaF. The peak related to [Amino acid-H + 2Na](+) species was considered a characteristic peak in both the direct-LDI and MALDI of the selected amino acids in the presence of NaF and NaOH.