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Use of Liquid Reagents for Positive Chemical Ionization on the 5973 MSD

Abstract: Two powerful features of the technique of positive chemical ionization (PCI) mass spectrometry are selectivity and control over the extent of fragmentation of the analytes of interest. This is made possible by selecting reagent gases that have different proton affinities (PAs). The potential advantages in using liquids as PCI reagents are their availability and purity, ease of handling and introduction into the mass spectrometer, low consumption, and low cost. For example, the common organic solvents—methanol, acetone, and acetonitrile—have high vapor pressures and a range of PAs that make them attractive as potential PCI reagents (Table 1). Intense adducts of the molecular ion (M+) are possible for additional confidence in molecular identity. This is helpful in situations in which dehydration or abstraction mechanisms may convolute establishing molecular identity. Also these additional intense confirming adduct ions can improve quantitative confidence. For example, methane, the most common PCI reagent, usually produces a protonated molecular ion (M+H)+ as the most abundant ion for an analyte in PCI spectra with the (M+C2H5)+ and (M+C3H5)+ adducts being less than approximately 10% and 5%, respectively. These adduct ratios are very stable and are reproducible on the 5973 MSD and, by adjusting the source parameters, can be easily controlled in such a way as to be favorable for confirmation work.

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