Discrimination of isomers and isobars by varying the reduced-field across drift tube in proton-transfer-reaction mass spectrometry (PTR-MS)

Abstract

Proton-transfer-reaction mass spectrometry (PTR-MS) is a powerful technique for the real time trace gas analysis of volatile organic compounds (VOCs). However, quadrupole mass spectrometer (MS) used in PTR-MS has a relatively low mass resolution and is therefore not suitable for differentiating isobars. Furthermore, because of the lack of chemical separation before analysis, isomers can not be identified, either. In the present study, by varying the reduced-field E/N in the reaction chamber with a range of 50-180 Td in PTR-MS, we studied the product ion distribution (PID) of three sets of isobars/isomers, i.e. n-propanol/iso-propanol/acetic acid, propanal/acetone and four structural isomers of butyl alcohol. The profiles of the reduced-field dependence (PFD) of the PID under the chosen E/N-values show obvious differences which can be used to discriminate between these isobars/isomers thus enabling the titled method. Noticeably, we have observed that even the isomers, in the case of four structural isomers of butyl alcohol, which show little difference with each other at high reduced-field, can be discriminated easily at low reduced-field. Finally, two examples for the application of this method are discussed: (1) cyclohexanone was identified to be a major compound in the headspace of medical infusion sets; and (2) the differentiation and quantification of propanal and acetone in three synthetic mixtures with different ratios. This study presents a potential method to distinguish and quantify isobars/isomers conveniently in practical applications of PTR-MS analysis without additional instrumental configurations.

Methods in plant foliar volatile organic compounds research

“…Real-time instruments cannot separate isomers (unless coupled with GC with the time cost), and the best-optimized GC system cannot achieve time resolutions of less than a couple of minutes (Fig. 6) (Blake et al, 2009; Misztal et al, 2012; Shen et al, 2012; Ellis and Mayhew, 2014; Materić et al, 2015). …”

Use of Rapid Reduced Electric Field Switching to Enhance Compound Specificity for Proton Transfer Reaction-Mass Spectrometry

“…This approach was used in the early investigations using PTR-MS, , where changes in the reduced electric field were used to distinguish isomeric compounds. More recent examples exploiting this technique can be found in the literature, − and this same approach was used by González-Méndez et al to discriminate between nitro-glycerine (NG) and the isobaric compound 2,4,6-trinitrotoluene (TNT) . For that study the drift tube voltage was changed manually.…”

Evaluation of a New DC-Ion Funnel Drift Tube for Use in Proton Transfer Reaction Mass Spectrometry

“…Until recently, the design of the drift tube had been relatively unchanged from the first PTR-MS instrument . In most instruments, the drift tube is composed of equidistant ring electrodes with a constant inner diameter, and it generates a homogeneous electric field along the axial direction to enhance the ion kinetic energy and suppress cluster-ion formation. ,, In the transverse direction, ions scatter and diffuse away from the central axis of the drift tube due to ion–gas collisions . Compared to the size of the ion beam, the ion sampling aperture at the end of the drift tube is small, which is necessary to maintain a sufficiently low pressure in the mass analyzer .…”