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 [M_d + 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 (<i>I</i>_EPI ([M + H]⁺)/<i>I</i>_PI ([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.