Negative-mode ion mobility spectrometry-comparison of ion-molecule reactions and electron capture processes

The presented work concerns the impact of ionization mechanisms of analytes on detection sensitivity in negative-mode ion mobility spectrometry. The main part of the work is research conducted for selected organic analytes using DT IMS in the negative mode of operation. In the negative mode of detection, two ionization mechanisms can be used: electron capture and ion-molecule reactions. The type of ionization mechanism depends on the carrier gas. The tests were carried out using two carrier gases: air and nitrogen. This allowed for a comparison of the ionization of analytes in the ion-molecule reaction mode with ionization in electron capture mode. Experiments were carried out for tetrachloromethane, trichloromethane, benzyl chloride, 1-chlorobutane, 1-chlorohexane, 1-chloropentane, tetrachlorethylene, 1-bromobutane, 1-bromopentane, 1-bromohexane, hexafluorobenzene, 2-chloroethyl ethyl sulfide (CEES), and methyl salicylate. Most of the tested substances were ionized with the formation of ionized halogen atoms (Br- or Cl-). It was found that among the tested substances, there are those whose effective ionization is possible with the use of both nitrogen and air as carrier gases, those ionized only in electron capture mode or in ion-molecule reaction mode. The important part of the work was an investigation on the effect of oxygen and water admixtures in carrier gases on the detection efficiency of selected organic compounds.