Drift tube ion mobility spectrometers (IMS) are widely used for fast trace gas detection in air, but portable compact systems are typically very limited in their resolving power. Decreasing the initial ion packet width improves the resolution, but is generally associated with a reduced signal-to-noise-ratio (SNR) due to the lower number of ions injected into the drift region. In this paper, we present a refined theory of IMS operation which employs a combined approach for the analysis of the ion drift and the subsequent amplification to predict both the resolution and the SNR of the measured ion current peak. This theoretical analysis shows that the SNR is not a function of the initial ion packet width, meaning that compact drift tube IMS with both very high resolution and extremely low limits of detection can be designed. Based on these implications, an optimized combination of a compact drift tube with a length of just 10 cm and a transimpedance amplifier has been constructed with a resolution of 183 measured for the positive reactant ion peak (RIP(+)), which is sufficient to e.g. separate the RIP(+) from the protonated acetone monomer, even though their drift times only differ by a factor of 1.007. Furthermore, the limits of detection (LODs) for acetone are 180 pptv within 1 s of averaging time and 580 pptv within only 100 ms.

With recent advances in ionization sources and instrumentation, ion mobility spectrometers (IMS) have transformed from a detector for chemical warfare agents and explosives to a widely used tool in analytical and bioanalytical applications. This increasing measurement task complexity requires higher and higher analytical performance and especially ultra-high resolution. In this review, we will discuss the currently used ion mobility spectrometers able to reach such ultra-high resolution, defined here as a resolving power greater than 200. These instruments are drift tube IMS, travelling wave IMS, trapped IMS and field asymmetric or differential IMS. The basic operating principles and the resulting effects of experimental parameters on resolving power are explained and compared between the different instruments. This allows understanding the current limitations of resolving power and how ion mobility spectrometers may progress in the future.

We present a high kinetic energy ion mobility spectrometer (HiKE-IMS) for quantitative gas analysis. Drift tube and reaction tube can be operated at reduced fields up to 110 Td. At such conditions the distribution of reactant ion water clusters is shifted toward smaller clusters. Due to the resulting presence of bare reactant ions (e.g., H3O(+)) and the kinetic control of the ionization process with decreasing reaction time, unlike conventional IMS, a quantitative detection with ppbv detection limits of low proton affine analytes even in humid gas mixtures containing high proton affine compounds is possible using a direct sample gas inlet. A significantly improved dynamic range compared to conventional IMS is achieved. An incremental change in reduced fields enables the observation of parameters like field dependent ion mobilites or analyte fragmentation. Also, the characteristic of the analyte signal with respect to the reduced reaction field gives insight into the ionization process of the analyte. Thus, HiKE-IMS enables substance identification by ion mobility and additional analytical information that are not observed with conventional IMS. The instrumental effort is similar to conventional desktop IMS with overall dimensions of the drift and reaction tube of 4 cm × 4 cm × 28.5 cm. However, the mobility resolution is limited and between 30 and 40. Because of the moisture independent ionization and the decrease in competing ion-molecule reactions, no preseparation or membrane inlet is necessary when the compounds of interest are distinguishable either by a significant difference in ion mobility or the additional analytical information.

There is increasing evidence that breath volatile organic compounds (VOC) have the potential to support the diagnosis and management of inflammatory diseases such as COPD. In this study we used a novel breath sampling device to search for COPD related VOCs. We included a large number of healthy controls and patients with mild to moderate COPD, recruited subjects at two different sites and carefully controlled for smoking. 222 subjects were recruited in Hannover and Marburg, and inhaled cleaned room air before exhaling into a stainless steel reservoir under exhalation flow control. Breath samples (2.5 l) were continuously drawn onto two Tenax(®) TA adsorption tubes and analyzed in Hannover using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS). Data of 134 identified VOCs from 190 subjects (52 healthy non-smokers, 52 COPD ex-smokers, 49 healthy smokers, 37 smokers with COPD) were included into the analysis. Active smokers could be clearly discriminated by higher values for combustion products and smoking related VOCs correlated with exhaled carbon monoxide (CO), indicating the validity of our data. Subjects from the study sites could be discriminated even after exclusion of cleaning related VOCs. Linear discriminant analysis correctly classified 89.4% of COPD patients in the non/ex-smoking group (cross validation (CV): 85.6%), and 82.6% of COPD patients in the actively smoking group (CV: 77.9%). We extensively characterized 134 breath VOCs and provide evidence for 14 COPD related VOCs of which 10 have not been reported before. Our results show that, for the utilization of breath VOCs for diagnosis and disease management of COPD, not only the known effects of smoking but also site specific differences need to be considered. We detected novel COPD related breath VOCs that now need to be tested in longitudinal studies for reproducibility, response to treatment and changes in disease severity.

In contrast to classical ion mobility spectrometers (IMS) operating at ambient pressure, the high kinetic energy ion mobility spectrometer (HiKE-IMS) is operated at reduced pressures between 10–40 mbar. In HiKE-IMS, ions are generated in a reaction region before they are separated in a drift region. Due to the operation at reduced pressure, it is possible to reach high reduced electric field strengths up to 120 Td in both the reaction as well as drift region, resulting in a pronounced decrease in chemical cross sensitivities and a significant enhancement of the dynamic range. Until now though, only limited knowledge about the ionization pathways in HiKE-IMS is available. Typically, proton bound water clusters, H+(H2O) n , are the most abundant positive reactant ion species in classical IMS with atmospheric chemical ionization sources. However, at reduced pressure and increased effective ion temperature, the reactant ion population significantly changes. As the ionization efficiency of analyte molecules in HiKE-IMS strongly depends on the reactant ion population, a detailed knowledge of the reactant ion population generated in HiKE-IMS is essential. Here, we present a coupling stage of the HiKE-IMS to a mass spectrometer enabling the identification of ion species and the investigation of ion molecule reactions prevailing in HiKE-IMS. In the present study, the HiKE-IMS–MS is used to identify positive reactant ion populations in both, purified air and nitrogen, respectively. The experimental data suggest the generation of systems of clustered primary ions (H+(H2O) n , NO+(H2O) m , and O2 +(H2O) p ), which most probably serve as reactant ions. Their relative abundances highly depend on the reduced electric field strength in the reaction region. Furthermore, their effective mobilities are studied as a function of the reduced electric field strength in the drift region.