Detecting Hyper-Fast Chromatographic Peaks Using Ion Mobility Spectrometry

University of Hannover researchers created a new type of ion mobility spectrometer (IMS) to capture hyper-fast chromatographic peaks. Their findings were published in the Journal of Chromatography A (1).

Hannover is the capital of the state of Lower Saxony (Germany). | Image Credit: © Joan Vadell - stock.adobe.com

Ion mobility spectrometers (IMS) are used for quickly detecting trace compounds, including explosives, toxic industrial compounds, and chemical warfare agents. When coupled to a gas chromatograph (GC), the range of application of such GC–IMS expands greatly, being usable in industries like medical applications and quality monitoring in the food industry. In many of these applications, simultaneously recording ion species in positive and negative polarities is important for detecting both compounds that just form positive ions and compounds that just form negative ions. This is vital for fast analysis of unknown samples or small sample volumes, which can allow for just one GC–IMS measurement being needed.

Different strategies have been created for dual-polarity IMS, with the simplest method being to connect two individual IMS by splitting the sample gas flow. However, this approach requires two ionization sources and twice the total sample gas flow. Single drift tube IMS with voltage polarity is typically used instead, but this only allows for quasi-simultaneous detection of both ion polarities, which is limited by the polarity switching rate. Besides polarity switching, two other types of configurations are possible: arranging two drift tubes in a parallel configuration or arranging two drift tubes in an axial configuration.

In this study, the scientists created a dual drift tube IMS in axial configuration designed to resolve even hyper-fast GC peaks. The key to this system was a directed sample gas flow through the ionization region, which provides a small yet effective detector volume. The IMS was isothermally heated to 120 °C to avoid sample gas condensation. The drift tube length of only L_D = 41 mm with a drift voltage of U_D = 2.7 kV led to an IMS repetition rate of 100 Hz and a high resolving power of R_p = 60. This even applies to ions with low reduced ion mobility down to 0.43 cm²V^-1s^-1.

By adding GC makeup gas to IMS, the system focuses the gas and a proportion of the drift gas into the reaction region in combination with the new reaction region geometry, sample gas focusing by fluid dynamics. This allows for an effective detector volume of just 40 µL and thus resolves even the shortest chromatographic peaks of just 140 ms FWHM. The hyper-fast GC–IMS with simultaneous recording of both ion polarities can benefit from extreme gains in information at a short measurement time of less than 30 seconds. The hyper-fast GC–IMS system can achieve limits of detection (LODs) in the low ppb_v range, which can make it especially feasible for detecting prohibited or harmful substances in addition to analyzing complex samples. Further, its potential was demonstrated with an exemplary mixture made of ketones, to be detected in the positive polarity, and 1,1,2-trichloroethane and methyl salicylate, to be detected in the negative polarity. Explosive and more complex samples, such as hops, were analyzed in less than 30 seconds, including cooling down time.

The primary aim of this study was to create a new ion source for dual drift tube IMS. While the results’ applicability to real-world scenarios is untested, the hop samples used were simplified and real samples that become very complex in reality due to impurities. Future works will address this aspect, alongside the long-term stability of the device.

Reference

(1) Nitschke, A.; Hitzemann, M.; Winkelholz, J.; Kirk, A. T.; et al. A Dual-Polarity Ion Mobility Spectrometer for Capturing Hyper-Fast Chromatographic Peaks. J. Chromatogr. A 2025, 1750, 465923. DOI: 10.1016/j.chroma.2025.465923