Proton Transfer Reaction Time-of-Flight Mass Spectrometry at Low Drift-Tube Field-Strengths Using an H₂O-Rare Gas Discharge-Based Ion Source

Abstract

A discharge-based proton transfer reaction (PTR) ion source was operated using a mixture of H2O and rare gases such as He, Ne, Ar, and Kr in combination with a custom-built time-of-flight (TOF) mass spectrometer. In contrast to an "H2O-only" discharge, which usually functions above a field strength (E/N) of 100 Td for a drift tube, an "H2O῎rare gas"-based discharge was operated successfully at E/N values between 30 and 50 Td. (E is the electric field strength (V cm ῎1 ), N is the bu#er gas number density (molecule cm ῎3 ), and 1 Td῏10 ῎17 cm 2 V molecule ῎1 .) The intensity of primary ions (H3O ῍ ῌ(H2O)n) generated in the "H2O῎rare gas" discharge was comparable to that in the "H2O-only" discharge. Although detection sensitivities decreased for nonpolar molecules such as isoprene, benzene, toluene, and p-xylene, they increased for polar molecules such as acetone and acetaldehyde. This suggests that the operation of the PTR-TOF mass spectrometer at low drift-tube field-strengths improves both the detection sensitivity and selectivity for the polar molecules. In addition, fragmentation in the drift tube was suppressed significantly for fragile species such as methyl nitrate, in the low E/N operation.

Pressure and Temperature Dependence of Methyl Nitrate Formation in the CH3O2+ NO Reaction

“…According to Aoki et al, NO 2 + is the predominant ion in the PTR detection of C 1 –C 2 alkyl nitrates; however, the observed fragmentation of the protonated nitrates is strongly pressure dependent and in case of CH 3 ONO 2 can give a strong protonated peak m / z 78 under certain conditions . In the present study, the peak assignment was confirmed using detection with the deuteron transfer reaction (DTR) when the H 2 O flow into the IMR was changed to D 2 O.…”

“…Formaldehyde was observed at m/z 31, 49, and 67 corresponding to According to Aoki et al, 26 NO 2 + is the predominant ion in the PTR detection of C 1 −C 2 alkyl nitrates; however, the observed fragmentation of the protonated nitrates is strongly pressure dependent in case of CH 3 ONO 2 can give a strong protonated peak m/z 78 under certain conditions. 27 In the present study, the peak assignment was confirmed using detection with the deuteron transfer reaction (DTR) when the H 2 O flow into the IMR was changed to D 2 O. Figure 2b The calibrated NO 2 signal was used to estimate the trace impurity level of NO 2 as well as to measure NO 2 concentration from reaction 1a.…”

Modified proton transfer reaction mass spectrometry (PTR‐MS) operating conditions for in vitro and in vivo analysis of wine aroma

“…If the method was alleged to be superior to the previous ones, reduction of ethanol effects was obtained still with a certain loss of sensitivity, and the presence of argon in the drift‐tube could even decrease the sensitivity of certain product ions at an extent up to 5 to 10‐fold . Moreover, selective sensitivity detrimental to less polar compounds was observed, confirming previous observations made using an H 2 O‐rare gas discharge‐based ion source at low drift‐tube field strengths …”

“…11 Moreover, selective sensitivity detrimental to less polar compounds was observed, 11 confirming previous observations made using an H 2 O-rare gas discharge-based ion source at low drift-tube field strengths. 15 Non-conventional operating conditions using very large electric field strength to number density ratio E/N values in the drift-tube (where E is the electric field in the drift-tube and N is the gas number density) were also proposed recently. 7,16 With these extreme condi- 7 The main drawback of increasing the E/N value to such an extent was the large increased fragmentation of the analytes resulting in a greater proportion of non-specific low mass fragments to the detriment of protonated molecular ions or more specific ions, 16 which could be a real challenge for complex products in terms of number of volatile organic compounds (VOCs).…”