Abstract
Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) is a versatile technique for real-time detection and quantification of volatile organic compounds [Sovova et al., 2011; Dryahina et al., 2011]. Using a known rate coefficient for the ion-molecular reaction between selected reagent ions and sampled molecules it is possible to calculate the compound concentration in the media.
However, there are many organic compounds that have the same molecular weight. This leads to the overlap of peaks in a mass spectrum.
In order to remove this limitation a new Selected Ion Flow Drift Tube (SIFDT) apparatus was built. The inherent feature of SIFDT is the possibility to apply uniform electric field to the flow tube and hence use it as a flow-drift tube.
The analyte compounds are usually present in air containing water vapor (for example breath or headspace). Presence of water vapor leads to the formation of the reagent and product ion hydrates in the flow-drift tube.
Results of the first studies carried out using a newly constructed SIFDT instrument provide original data for the kinetics of association reaction of the H_3O^+ reagent ions with water molecules which is the first critical step in formation of hydrates of protonated molecules. It is observed that the association rate coefficient is approximately inversely proportional to the center of mass energy of the collisions of H_3O^+ with He carrier gas atoms, Ec.