Characterization of unknown compounds from stainless steel plates in matrix-assisted laser desorption/ionization mass spectrometry

Peaks originating from unknown compounds on stainless steel plates used in matrix-assisted laser desorption/ionization (MALDI) mass spectrometers are observed around m/z 304.3, 332.3, 360.4, and 388.4 regardless of the matrix and/or solvent, and are even observed with bare plates. These peaks were characterized using three different types of MALDI-MS instrumentation: MALDI-TOF MS, MALDI-TOF/TOF MS, and MALDI-FTMS. The fragmentation data from MALDI-TOF/TOF MS and accurate mass determination by MALDI-FTMS enabled identification of the chemical formulae and structures. The unknown compounds are, in fact, likely benzylalkylmethylammonium salts, as confirmed by closely matching fragmentation patterns with a commercially available benzalkonium chloride.