title: 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-C6using 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.
authors.affiliation: Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK. P.S.Monks@le.ac.uk.
authors.forename: Robert S
authors.initials: RS
authors.lastname: Blake
authors.affiliation: Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK. P.S.Monks@le.ac.uk.
authors.forename: Saleh A
authors.initials: SA
authors.lastname: Ouheda
authors.affiliation: Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK. P.S.Monks@le.ac.uk.
authors.forename: Corey J
authors.initials: CJ
authors.lastname: Evans
authors.affiliation: Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK. P.S.Monks@le.ac.uk.
authors.forename: Paul S
authors.identifier: http://orcid.org/0000-0001-9984-4390
authors.initials: PS
authors.lastname: Monks