An optimized GC-IMS system with significantly increased sensitivity

Abstract

Ion mobility spectrometers are extremely fast and sensitive trace gas detectors, but when analyzing complex mixtures, they suffer from disadvantages associated with atmospheric pressure chemical ionization (APCI). This ionization is however necessary for achieving outstanding sensitivity. Thus, for applications requiring the analyses of complex mixtures, a pre-separation e.g. by gas chromatography is required. Here, we present important design considerations for GC-IMS systems. On the one hand, when optimizing the gas chromatograph it is necessary to consider the volume inside the ionization region of the ion mobility spectrometer. This can be accurately represented by adding an additional term to the Golay equation, allowing choosing correct column dimensions and gas flows. On the other hand, when pre-separation is applied, the ion mobility spectrometer just needs to separate co-eluting species and product ions from reactant ions. Thus, a modest resolving power may be sufficient. However, as the eluted analytes are only present for a few seconds, achieving low limits of detection during extremely short averaging times becomes crucial. In fact, extremely high sensitivities that are not practical with a direct inlet due to the very limited dynamic range in IMS become feasible with GC pre-separation. Thus, for our optimized GC-IMS, we optimized our standard IMS with Rp = 100 for significantly increased sensitivity, being able to achieve limits of detections for a measuring time of just one second in the single digit pptv-range and even sub-pptv-range for some compounds.