Proposal and application of nitrogen rules in interpreting gas chromatography negative-ion chemical ionization mass spectrometry spectra

Abstract

Analytical methods based on GC-MS and GC-MS/MS are widely used for the qualitative and quantitative analysis of physiological and non-physiological organic compounds in biological samples. They include chemical derivatization, GC separation of gasous analytes, their inline ionization in the ion-source, separation of ions by MS or MS/MS, their conversion to electrons, and their final multiplication and registration. Negative-ion chemical ionization (NICI) is usually performed by using methane as the reagent gas. For MS/MS analysis, argon is a common collision gas. In many GC–NICI–MS and GC–NICI–MS/MS mass spectra, the negative charge cannot always be assigned to particular atoms. In this article, rules are proposed for the interpretation of GC–NICI–MS and GC–NICI–MS/MS mass spectra. The NICI nitrogen rules (NICI-NR) were derived from GC–NICI–MS and GC–NICI–MS/MS mass spectra reported in the literature, and their utility is demonstrated exemplarily for N-containing analytes. The NICI-NR say that ions with even m/z values and odd number of N atoms, and ions with odd m/z values and even number of N atoms have a definitely assignable negative charge. In all other cases, including analyte derivatives that contain stable isotopes including ¹⁵N and are radicals, the negative charge is hidden. In those cases, the negative charge is associated with a reduction of a particular C atom, such as that in carbonyl groups, by the uptake of one methane-derived secondary electron. In NICI, carbonyl functionalities are introduced into the analytes by means of perfluorinated derivatization reagents such as pentafluoropropionic anhydride and pentafluorobenzoyl chloride that target N atoms of analytes such as amino acids, bioamines and drugs. The relative importance of carbonylic and F atoms in NICI is discussed.