Supercritical Fluid Chromatography-Mass Spectrometry with Atmospheric Pressure Chemical Ionization: Negatively Charged Ions of Mobile Phase Components

Supercritical fluid chromatography (SFC) is a promising alternative to conventional chromatographic methods and has several advantages over them in selectivity, efficiency, speed of separation, and environmental friendliness. Its combination with atmospheric pressure chemical ionization (APCI) mass spectrometric detection is one of the central areas of method development and requires an understanding of the APCI specificity in using carbon dioxide-based mobile phases. In the present study, the composition of negatively charged ions formed under APCI conditions for carbon dioxide-organic modifier (methanol, isopropanol, acetonitrile) systems, and also the influence of the mobile phase and ion source parameters (corona discharge needle voltage, temperature) on it are studied using high-resolution mass spectrometry. The results obtained testify to a significant role of carbon dioxide as a direct precursor for the formation of reaction particles (hydrocarbonate and percarbonate anions, carbonate radical anion) and as a reagent capable of shifting the ion-molecular equilibrium in corona discharge plasma and forming compounds with the organic modifier (alkylcarbonic acid anions in the case of alcohols, adduct with a deprotonated acetonitrile molecule). Ion-molecular equilibria involving carbon dioxide result in the formation of a large number of reactive particles that can take part in the ionization processes of analytes in SFC-APCI-MS.