Laser-Induced Acoustic Desorption/Dichloromethane-Enhanced Vacuum Ultraviolet Photoionization Mass Spectrometry for Analysis of Amino Acids.

Laser-induced acoustic desorption (LIAD) enables the soft volatilization of nonvolatile and thermally labile compounds as neutral molecules from metal surfaces. Concurrently, doping with dichloromethane (CH2Cl2) during vacuum ultraviolet photoionization (PI) has been established as a highly efficient protonation method for oxygenated volatile organic compounds. In this study, an experimental apparatus based on a krypton VUV lamp, a 355 nm pulsed laser, and a time-of-flight mass spectrometer was set up to investigate LIAD/PI and LIAD/dichloromethane-enhanced vacuum ultraviolet photoionization (abbreviated as EPI) mass spectra of five representative amino acids─valine (Val), serine (Ser), aspartic acid (Asp), histidine (His), and glycine (Gly). The obtained LIAD/PI and LIAD/EPI mass spectra revealed that the main ion species of both LIAD/PI and LIAD/EPI for the five amino acids were the protonated molecular ions ([M + H]+) and the fragment ions resulting from the combined loss of H2O and CO (denoted as [Md + H]+). The fragment-to-parent ion ratios of the LIAD/PI for Val, Ser, Asp, His, and Gly were 0.95, 0.45, 0.29, 0.71, and 0.03, while the corresponding ratios of LIAD/EPI were 0.63, 0.26, 0.21, 1.19, and 0.05, confirming that LIAD/EPI is a soft ionization technique comparable to LIAD/PI. In contrast to the LIAD/PI process, the LIAD/EPI signal intensities of the amino acids were remarkably enhanced via doping with dichloromethane. The best observed enhancing factors of the signal intensities (IEPI ([M + H]+)/IPI ([M + H]+)) were 637, 559, 354, 80, and 716 times for Val, Ser, Asp, His, and Gly, respectively, derived from the LIAD/PI and LIAD/EPI mass spectra of the five amino acids mixture. In addition, the protonated dimers of the amino acids ([2M + H]+) were observed in the LIAD/PI and LIAD/EPI processes, along with their fragment ions resulting from the loss of CO, H2O, and NH3 (denoted as [(2M)d + H]+). The key experimental parameters, including the carrier gas flow rate, the doped CH2Cl2 concentration, and the LIAD laser intensity, were investigated and reported in the paper. This study demonstrates that the LIAD/EPI method is a significantly more efficient ionization method for amino acids compared with the conventional PI method.