A proton transfer reaction-time-of-flight mass spectrometer (PTR-TOFMS) has been developed for real-time measurements of volatile organic compounds in air. The instrument is designed to be operated ...with a hollow cathode discharge ion source and an ion drift tube at relatively high pressures. Each component of the system, an ion source, a drift tube, an ion transfer region, and a time-of-flight mass spectrometer, are in detail characterized by a number of laboratory experiments. The optimized instrumental configuration enables us to gain high intensities of hydronium (H
3O
+) ions, typically ∼7
×
10
5 counts for 1-min integration at a drift tube pressure of ∼5
Torr. It also suppresses background signals, and interferences from sample air (NO
+ and O
2
+), which undergo fast reactions with volatile organic compounds, to ∼0.5% of those of H
3O
+ ions. We find that the use of the custom-built discharge source show higher overall sensitivities than of a commercially available radioactive source. Potentials to detect oxygenated VOCs (aldehydes, ketones, and alcohols), halocarbons, and amines are also suggested. The detection limits for acetaldehyde, acetone, isoprene, benzene, toluene, and
p-xylene were determined to be at the sub-ppbv levels for a 1-min integration time. A good linear response at trace levels is certified, but slight sensitivity dependency on water vapor contents is revealed. We finally demonstrate that the instrument can be used for on-line monitoring to detect large variations from emission sources in real-time.
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.
