Aldehyde and ketone discrimination and quantification using two-stage proton transfer reaction mass spectrometry

A two-stage proton transfer reaction mass spectrometry (PTR-MS) technique has been demonstrated recently in which the normal proton transfer reagent, H3O+, is mixed with a chosen volatile organic compound, designated VOC1, upstream of an analyte gas flow. This process can be used to make protonated VOC1, which in turn can react with volatile organic compounds (VOCs) (designated collectively as VOC2) in an analyte gas stream. Here we show that this approach can be adapted to discriminate between isobaric aldehydes and ketones, which are notoriously difficult to distinguish using conventional PTR-MS with H3O+ as the reagent ion. The approach is shown to yield accurate quantification of the individual components in isobaric aldehyde/ketone pairs. Variation of the compound used as VOC1 also provides a simple and quick means of ‘bracketing’ the proton affinity of a particular organic species and we demonstrate this methodology for hexanal, whose proton affinity has not previously been reported. The proton affinity of hexanal is found to lie within the range 794.4 kJ mol−1 < PA(hexanal) < 797.0 kJ mol−1.