A hyper-fast gas chromatograph coupled to an ion mobility spectrometer with high repetition rate and flow-optimized ion source to resolve the short chromatographic peaks.

By combining the high selectivity of a gas chromatograph (GC) with the high sensitivity and decent selectivity of an ion mobility spectrometer (IMS), GC-IMS have become increasingly popular in many applications. However, most GC suffer from long analysis times. In contrast, an hyper-fast GC allows for extremely fast analysis in the tens of seconds while reaching comparably high resolution. In turn, coupling such hyper-fast GC with IMS requires sufficiently high repetition rate of recording full IMS spectra to resolve the short GC peaks. Therefore, we present a drift tube IMS with 100 Hz repetition rate. Key is a small effective detector volume combined with short drift length. Therefore, the ion source of the IMS combines a small reaction region with an extended field-switching ion shutter and optimized gas flows. To resolve even the shortest GC peaks with a full width at half maximum of 100 ms, a short drift length of just 41 mm was used, achieving a measurement time of 10 ms per spectrum and hence ten data points across the shortest GC peak. To avoid condensation of the sample, the entire IMS was heated isothermally to 120 °C. Despite short drift times and high temperatures, the IMS still reaches high resolving power of Rp = 60. The hyper-fast GC-IMS reaches low detection limits in the low ppbV range. For demonstration, ketone mixes and three different hop varieties were analyzed in <30 s.

High-Resolution Drift Tube Ion Mobility Spectrometer with Ultra-Fast Polarity Switching.

Besides safety and security applications, ion mobility spectrometry (IMS) is increasingly used in other fields such as medicine, environmental monitoring and food quality analysis. However, some applications require gas chromatographic separation before analysis by IMS. Furthermore, different compounds in the sample may form positive or negative ions during ionization and therefore simultaneous detection of both ion polarities is highly beneficial to avoid two chromatographic runs of the same sample. This can be achieved by ultra-fast polarity switching of a single drift tube IMS, allowing for quasi-simultaneous detection of both ion polarities. By using a ramped aperture voltage during the switching process, we overcome the issue of excessive displacement currents at the detector during polarity switching, which usually lead to overdriving the output signal of the high-gain transimpedance amplifier. Furthermore, mechanical aperture grid oscillations caused by polarity switching were also reduced through the ramped aperture voltage. This enables a polarity switching time of only 7 ms at a drift voltage of 8 kV and a drift length of 103 mm, leading to a high resolving power of RP = 117. Requiring 50 ms to acquire a pair of positive and negative spectrum, the IMS achieves an acquisition rate of 20 Hz. It reaches limits of detection of 20 pptv for dimethyl methylphosphonate and 40 pptv for methyl salicylate. For demonstration, different hop varieties were investigated and could be clearly differentiated by considering both, the positive and negative spectra.

Reliable Detection of Chemical Warfare Agents Using High Kinetic Energy Ion Mobility Spectrometry.

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) ionize and separate ions at reduced pressures of 10-40 mbar and over a wide range of reduced electric field strengths E/N of up to 120 Td. Their reduced operating pressure is distinct from that of conventional drift tube ion mobility spectrometers that operate at ambient pressure for trace compound detection. High E/N can lead to a field-induced fragmentation pattern that provides more specific structural information about the analytes. In addition, operation at high E/N values adds the field dependence of ion mobility as an additional separation dimension to low-field ion mobility, making interfering compounds less likely to cause a false positive alarm. In this work, we study the chemical warfare agents tabun (GA), sarin (GB), soman (GD), cyclosarin (GF) and sulfur mustard (HD) in a HiKE-IMS at variable E/N in both the reaction and the drift region. The results show that varying E/N can lead to specific fragmentation patterns at high E/N values combined with molecular signals at low E/N. Compared to the operation at a single E/N value in the drift region, the variation of E/N in the drift region also provides the analyte-specific field dependence of ion mobility as additional information. The accumulated data establish a unique fingerprint for each analyte that allows for reliable detection of chemical warfare agents even in the presence of interfering compounds with similar low-field ion mobilities, thus reducing false positives.

Ultra-Fast Ion Mobility Spectrometer for High-Throughput Chromatography.

Fast chromatography systems especially developed for high sample throughput applications require sensitive detectors with a high repetition rate. These high throughput techniques, including various chip-based microfluidic designs, often benefit from detectors providing subsequent separation in another dimension, such as mass spectrometry or ion mobility spectrometry (IMS), giving additional information about the analytes or monitoring reaction kinetics. However, subsequent separation is required at a high repetition rate. Here, we therefore present an ultra-fast drift tube IMS operating at ambient pressure. Short drift times while maintaining high resolving power are reached by several key instrumental design features: short length of the drift tube, resistor network of the drift tube, tristate ion shutter, and improved data acquisition electronics. With these design improvements, even slow ions with a reduced mobility of just 0.94 cm2/(V s) have a drift time below 1.6 ms. Such short drift times allow for a significantly increased repetition rate of 600 Hz compared with previously reported values. To further reduce drift times and thus increase the repetition rate, helium can be used as the drift gas, which allows repetition rates of up to 2 kHz. Finally, these significant improvements enable IMS to be used as a detector following ultra-fast separation including chip-based chromatographic systems or droplet microfluidic applications requiring high repetition rates.

Easy to assemble dielectric barrier discharge plasma ionization source based on printed circuit boards.

Here, we present a new and an easy to assemble dielectric barrier discharge plasma ionization source based on printed circuit boards with two parallel isolated electrodes for generating a plasma inside an inert fused silica capillary. For demonstration, this plasma source is coupled to an ion mobility spectrometer. With two different sample gas feeds the analytes can either pass through the plasma or bypass the plasma before entering the reaction region of the ion mobility spectrometer, allowing for different ionization pathways, e.g. electron impact ionization, ionization by excited species, e.g. helium metastables, or chemical ionization via reactant ions generated inside or downstream of the plasma. The plasma source, in particular, the electrode geometry and the capillary diameter were designed with the help of electric field simulations. A rectangular electrode with a height of at least twice the outer diameter of the capillary turned out to be ideal, in both the simulation and the experiment. Furthermore, a simple control electronics has been developed, which can be easily applied to other plasma sources. With the plasma source presented here, detection limits in the mid pptv range have been reached.