Studies on the positive-ion mass spectra from atmospheric pressure chemical ionization of gases and solvents used in liquid chromatography and direct liquid injection

Abstract

Detailed studies have been made using different source gases and solvents in a Micromass Quattro mass spectrometer under positive ion atmospheric pressure chemical ionization conditions. The major background ions from nitrogen, air, or carbon dioxide were investigated by tandem mass spectrometry, followed by similar studies on solvents commonly employed in normal- and reversed-phase high-performance liquid chromatography, namely, water-acetonitrile, acetonitrile, and dichloromethane, with nitrogen, air, or carbon dioxide; hydrocarbon solvents were studied using nitrogen. Spectra were interpreted in terms of the gases, solvents, and their impurities. The acetonitrile spectra provided clear evidence for both charge exchange and proton transfer, the former being facilitated by the introduction of some air into a flow of nitrogen. Radical cations of acetonitrile dimers, trimers, and tetramers were observed, as were protonated dimer and trimer species. Examination of the analytical response of four polycyclic aromatic hydrocarbons in various hydrocarbon solvents, with nitrogen gas, showed that the sensitivity of detection for an analyte and its ionization mechanism are dependent on both the analyte structure and the solvent, with pyrene showing the highest sensitivity, phenanthrene and fluorene being intermediate, and naphthalene having the lowest sensitivity. The degree of protonation followed the same trend. Signal intensity and degree of protonation were dependent on the alkane solvent used, with isooctane providing the best overall sensitivity for the sum of protonated molecules and molecular ions. The ions observed in these studies appeared to be the most stable ions formed under equilibrium conditions in the source.