Improving ion mobility spectrometer sensitivity through an optimized sample gas flow

Abstract

The gas flow transporting the sample molecules into and through the reaction region of an ion mobility spectrometer (IMS) has a profound influence on its response, determining both sensitivity and response time. In the past, several key improvements relating to the sample gas flow have been reported, for example the use of a drift gas counter-flow by Baim et al., on-axis sample introduction by St. Louis et al. or side-flow sample introduction by Lee et al. Here, we also aim to optimize the sample gas flow inside an IMS equipped with an extended field switching shutter. By introducing a laminar flow curtain orthogonal to the drift direction through the small field-free reaction region, a significant increase in sensitivity and decrease in response time could be achieved for an already extremely sensitive IMS. The constructed ion source is equipped with both a 10.6 eV UV lamp to initiate direct photo ionization (APPI) and a low-energy X-ray source to initiate chemical ionization (APCI). For an averaging time of just one second during continuous sample introduction, limits of detection in the pptv-range using APPI and in the ppqv-range using chemical ionization (APCI) are reached. For pulsed sample introduction, as in GC-IMS, limits of detection are currently under investigation. However, rise and fall times of less than a second have already been observed for volatile organic compounds at a system temperature of 45 °C. When using APCI, the combination of extremely high sensitivity with fast response times is especially suitable as an ultra-sensitive, orthogonal GC detector for analyzing complex mixtures. When using APPI, the combination of improved linearity with still high sensitivity is especially useful for directly analyzing moderately complex samples.