CF3+ and CF2H+: new reagents for n-alkane determination in chemical ionisation reaction mass spectrometry

Abstract

Alkanes provide a particular analytical challenge to commonly used chemical ionisation methods such as proton-transfer from water owing to their basicity. It is demonstrated that the fluorocarbon ions CF₃⁺ and CF₂H⁺, generated from CF₄, as reagents provide an effective means of detecting light n-alkanes in the range C₂–C₆ using direct chemical ionisation mass spectrometry. The present work assesses the applicability of the reagents in Chemical Ionisation Mass Spectrometric (CI-TOF-MS) environments with factors such as high moisture content, operating pressures of 1–10 Torr, accelerating electric fields (E/N) and long-lived intermediate complex formation. Of the commonly used chemical ionisation reagents, H₃O⁺ and NO⁺ only react with hexane and higher while O₂⁺ reacts with all the target samples, but creates significant fragmentation. By contrast, CF₃⁺ and CF₂H⁺ acting together were found to produce little or no fragmentation. In dry conditions with E/N = 100 Td or higher the relative intensity of CF₂H⁺ to CF₃⁺ was mostly less than 1% but always less than 3%, making CF₃⁺ the main reagent ion. Using O₂⁺ in a parallel series of experiments, a substantially greater degree of fragmentation was observed. The detection sensitivities of the alkanes with CF₃⁺ and CF₂H⁺, while relatively low, were found to be better than those observed with O₂⁺. Experiments using alkane mixtures in the ppm range have shown the ionisation technique based on CF₃⁺ and CF₂H⁺ to be particularly useful for measurements of alkane/air mixtures found in polluted environments. As a demonstration of the technique's effectiveness in complex mixtures, the detection of n-alkanes in a smoker's breath is demonstrated.