Detection of C 1–C 5 alkyl nitrates by proton transfer reaction time-of-flight mass spectrometry

Abstract

Characteristics of the mass spectra of C 1–C 5 alkyl nitrates (RONO 2) 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 C 1–C 4 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 NO 2 +, RO +, R +, and ROH·H +, the abundances and relative intensities of which depended on the E/ N ratio. The intensity of NO 2 + 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 C 1–C 5 alkyl nitrates, the most predominant ions being the NO 2 + for C 1–C 2 alkyl nitrates and R + ions for C 3–C 5 alkyl nitrates. Although the experimental finding that NO 2 + fragment ions were detected regardless the speciation of alkyl nitrates suggested that the detection of C 1–C 2 alkyl nitrates by this technique is not selective, the abundant fragment ion signals of R + ions could be useful for the identification of C 3–C 5 alkyl nitrates.