Detection of C1–C5 alkyl nitrates by proton transfer reaction time-of-flight mass spectrometry

Characteristics of the mass spectra of C1–C5 alkyl nitrates (RONO2) were systematically examined with a proton transfer reaction time-of-flight mass spectrometer operated at field strengths, E/N, of the drift tube of between 96 and 147 Td. Although protonated alkyl nitrates were detected for C1–C4 alkyl nitrates, their signal intensities were, at most, a few percent of the total ion signals. The major product ions were several fragment ions including NO2+, RO+, R+, and ROH·H+, the abundances and relative intensities of which depended on the E/N ratio. The intensity of NO2+ ions increased with increasing E/N ratio, whereas the intensities of organic fragments such as R+ and RO+ ions, relative to the total product ions, decreased with increasing E/N ratio. Those organic fragment ions partly underwent further fragmentation at high E/N ratios to produce [R − 2H]+, [R − 4H]+, and [RO − 2H]+, particularly for the higher alkyl nitrates. The fragmentation patterns also varied for the C1–C5 alkyl nitrates, the most predominant ions being the NO2+ for C1–C2 alkyl nitrates and R+ ions for C3–C5 alkyl nitrates. Although the experimental finding that NO2+ fragment ions were detected regardless the speciation of alkyl nitrates suggested that the detection of C1–C2 alkyl nitrates by this technique is not selective, the abundant fragment ion signals of R+ ions could be useful for the identification of C3–C5 alkyl nitrates.