A Comparative Study of Nitrogen versus Helium Direct Analysis in Real Time Mass Spectrometry for the Analysis of Polar Compounds

摘要

Helium (He) has been commonly used for direct analysis in real time (DART) as theoretically it gives the best sensitivity. However, in consideration of the looming helium shortage, nitrogen can be the best options as it is inexpensive and more readily available. In our present study, the ionization capability of N2 DART has been systematically studied and compared to He DART. Our results showed that N2 DART generated a cleaner background and caused less ion fragmentation than He DART because of its less energetic metastable species. Unlike He DART, the generation of [M +H]+ by N2 DART was mainly through direct Penning ionization followed by self-protonation because it was unable ionized the common solvents like ambient water, methanol and acetonitrile efficiently. Besides, N2 DART showed about one order of magnitude less sensitive in generating [M+H]+, but more sensitive in generating [M+NH4]+ than He DART. Generation of [M+NH4]+ was most likely through the formation of ammonium from the ambient ammonia followed by hydrogen bonding with the analyte molecules which have weaker proton affinities (PA) than ammonia. N2 DART has been concluded to be feasible in the forensic analysis of illicit drugs as it showed an excellent performance in term of speed, sensitivity and accuracy in the analysis of commonly abused drugs.