A dual-polarity ion mobility spectrometer for capturing hyper-fast chromatographic peaks

The goal of this study is to develop a novel dual-polarity ion mobility spectrometer (IMS) optimized for resolving hyper-fast chromatographic peaks in complex mixtures. The system combines a hyper-fast gas chromatograph (GC) with a dual drift tube IMS capable of high repetition rates and resolving power, ensuring sensitive and rapid analysis.

By introducing a directed sample gas flow and maintaining the IMS at 120 °C, the setup minimizes peak broadening and condensation. With a 41 mm drift tube and 100 Hz acquisition rate, the system effectively resolves GC peaks as narrow as 140 ms. This capability is demonstrated through the analysis of hop varieties and explosive mixtures in under 20 seconds.

The original article

A dual-polarity ion mobility spectrometer for capturing hyper-fast chromatographic peaks

Alexander Nitschke, Moritz Hitzemann, Jonas Winkelholz, Ansgar T. Kirk, Christoph Schaefer, Tim Kobelt, Christian Thoben, Martin Lippmann, Jan A. Wittwer, Stefan Zimmermann

Journal of Chromatography A, Volume 1750, 2025, 465923

https://doi.org/10.1016/j.chroma.2025.465923

licensed under CC-BY 4.0

Selected sections from the article follow. Formats and hyperlinks were adapted from the original.

Ion mobility spectrometers (IMS) are typically used for fast detection of trace compounds including explosives [[1], [2], [3], [4], [5]], toxic industrial compounds [6,7], and chemical warfare agents [[8], [9], [10]]. When coupled to a gas chromatograph (GC), the range of application of such GC-IMS significantly expands, e.g., to medical applications for monitoring anesthetic exposure [11,12], respiratory gas analysis [[13], [14], [15]], and quality monitoring in the food industry. Quality monitoring include the processing of olive oil [16,17], tea [18,19], honey [20], e-liquid [21], cheese [22], and beer [22]. In many of these analytical applications, simultaneous recording of ion species in positive and negative polarity is important to detect both compounds that just form positive ions and compounds that just form negative ions. This is particularly important for fast analysis of unknown samples or small sample volumes that allow for just one GC-IMS measurement. One of the reasons for using an upstream GC besides increasing peak capacity and adding a second dimension of separation is separating moister from analytes before entering the IMS reaction region since moisture can significantly affect the ionization efficiency of analytes [23,24].

Different strategies for dual-polarity IMS have been developed. The simplest way for simultaneously recording positive and negative ions is to connect two individual IMS by splitting the sample gas flow. However, this requires two ionization sources and twice the total sample gas flow. Therefore, a single drift tube IMS with voltage polarity switching is usually used, as introduced by numerous studies [[25], [26], [27]]. However, this method just allows for quasi-simultaneous detection of both ion polarities, limited by the polarity switching rate. Even IMS with ultra-fast polarity switching rate [27] cannot resolve the short peaks eluting from the hyper-fast GC in both ion polarities. Besides polarity switching, two other configurations are conceivable: arranging two drift tubes in a) parallel configuration [28,29] or b) two drift tubes in axial configuration [30], both options with one single ionization source and both options permitting simultaneous recording of positive and negative ions.

The use of a short GC column with extremely small diameter, a fast temperature program with a resistively heated GC column, and a temperature gradient across the GC column to focus the GC peaks can be used to significantly reduce GC-IMS measurement time as known from hyper-fast GC [[33], [34], [35], [36]]. However, due to the small diameter and thus the low gas flow through the GC column, coupling IMS to hyper-fast GC requires the development of a new ion source with small internal volume and directed sample gas flow to significantly reduce the effective detector volume. The small effective volume reduces effects such as peak broadening and thus remerging of these GC peaks. For simultaneous recording of both ion polarities, this ion source must be compatible to dual-polarity IMS.

Therefore, the aim of this study is to design a dual drift tube IMS in axial configuration with high sensitivity, high resolving power and a new ion source that prevents merging of peaks eluting from the GC, thus, keeping the GC separation power while adding a new dimension of separation. Preliminary work has already been done using a single drift tube IMS coupled to hyper-fast GC [37], which is extended here to a dual drift tube design in axial arrangement. For demonstration, the hyper-fast dual GC-IMS was tested with two hop varieties and different explosives.

It is important to note, that the primary focus of this manuscript is on the development of a new ion source for dual drift tube IMS that reach high resolving power of R_P = 60 and can simultaneously record spectra in both ion polarities with a repetition rate high enough to resolve the peaks eluting from a hyper-fast GC with peak width at half maximum of just 140 ms. As the IMS repetition rate and resolving power are independent of the application, we just used two exemplary sample types, hops and explosives, to demonstrate IMS performance. Transferring the GC-IMS to real-word scenarios will be the subject of future research.

2. Methods and materials

2.1. Ion source for dual drift tube IMS in axial arrangement with directed flow and low internal volume for coupling to hyper-fast GC

The new ion source for dual drift tube IMS in axial arrangement presented here integrates advancements from prior works based on Kirk et al. [38] and modified by Nitschke et al. [37]. In particular, the new ion source combines low internal volume with a directed sample gas flow and an extended field-switching ion gate [30,39] for coupling two drift tubes in axial arrangement [30]. A decisive point when measuring extremely short GC peaks is a small volume of the reaction region that matches the volume of the GC peaks so that the peaks eluting from the GC in a timely separated fashion do not merge in the reaction region which counteracts GC separation [40,41]. The geometric volume of the reaction region is just 40 µL. Thus, this new ion source combines three key features: compatible with dual drift tube IMS, small internal detector volume and ion transfer from the reaction region to the drift region by extended field-switching as required for highest sensitivity. This results in a reduction of the detectable full width at half maximum (FWHM) by a factor about 10 in comparison to former designs with an internal volume of 360 µL [38,40]. To resolve short GC peaks with an FWHM of approximately 100 ms, e.g. achieved in [35,42], a recording frequency of at least 100 Hz for IMS spectra is mandatory. To reconstruct these short peaks, as already discussed in [37], the frequency of recording IMS spectra needs to be at least 100 Hz [43,44]. In case of GC peaks with FWHM > 100 ms, averaging multiple, individual ion mobility spectra is possible so that the noise decreases. However, an increased frequency of recording IMS spectra decreases the time available for ion-molecule reactions in the reaction region and thus the formation of analyte ions. This in turn reduces sensitivity, especially for dimers and trimers [39]. This can be mitigated by the use of a so-called extended field-switching ion gate, which promotes the formation of dimers and even trimers. The ion formation, within the given experimental setup, requires a collection time of at least 25 ms until the reaction system has reached its thermodynamic equilibrium [45]. As said, a shorter collection time results in a smaller amount of ions collected in the reaction region, leading to a reduced sensitivity thus increasing the limit of detection (LoD). Therefore, the LoD of the IMS suffers at this point, but this is accepted to resolve the short GC peaks eluting from hyper-fast GC.

3. Results and discussion

3.3. Explosives

Due to the limited amount of measuring time available, stand-alone IMS are used in security applications such as the detection of explosives at airports. At this point, we add the second separation dimension of the hyper-fast GC, complemented by the dual polarity IMS, which can reduce false positive alarms while analyzing a complex sample in <30 s. For demonstration, four different explosives were measured in a mixture and plotted in Fig. 7 in form of a topographic plot with the noise level cut off set to 5 pA. The short GC-IMS measurement time including cooling of 30 s would not significantly extend a security check compared to a standard stand-alone IMS, while providing much more information. Furthermore, the necessity of measuring ions in both polarities becomes evident. Figure S9 also shows an exemplary negative spectrum displaying the RIP^- and the monomer peak of NG. The resolving power in negative polarity is R_P = 62. This spectrum was taken from the GC-IMS measurement shown in Fig. 7 at a retention time of 15.75 s. The detection of TNT requires the negative polarity, while the detection of TATP requires the positive polarity. NG and tetryl form positive and negative product ions and can be detected in both polarities. Using the new ion source, the chromatographic resolution can be substantially increased in comparison with couplings between hyper-fast GC and not flow-optimized IMS [58].

Journal of Chromatography A, Volume 1750, 2025, 465923: Fig. 7. Topographic plot of GC-IMS data obtained for a mixture of explosives, with short GC runtime of just 20 s. The mixture contains methanol, acetonitrile and ethanol as solvent, TATP (1) 349 ppmV/ 0.2 µg/mL, NG (2 and 2*) 715 ppmV / 409.45 µg/mL, TNT (3) 159 ppmV / 90.91 µg/mL and tetryl (4 and 4*) 317 ppmV / 181.82 µg/mL. The operating parameters are shown in Table 1 for the IMS and in Table 2 for the GC. The temperature profile (2) and the gradient fan settings of the GC are shown in Figure S3.

4. Conclusion

This work illustrates a new dual-polarity IMS in axial drift tube configuration with a new ion source for coupling a hyper-fast GC. By adding GC makeup gas, IMS focus 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 allows for an effective detector volume of just 40 µL and thus resolving even shortest chromatographic peaks of just 140 ms FWHM. Such a hyper-fast GC-IMS with simultaneous recording of both ion polarities benefits from an extreme gain in information at short measurement time of <30 s. The hyper-fast GC-IMS achieves LoDs in the low ppb_v range, which makes it especially feasible for the detection of prohibited or harmful substances as well as the analysis of complex samples. Here, the potential of the hyper-fast GC-IMS is demonstrated with an exemplary mixture consisting of ketones, to be detected in the positive polarity, and 1,1,2-trichloroethane and methyl salicylate, to be detected in the negative polarity. Furthermore, explosives and more complex samples such as hops were analyzed in <30 s including cooling down time. The primary focus of this work was to develop a new ion source for dual drift tube IMS. The applicability of these results to real-world scenarios remains untested. The hop samples used here are simplified, real samples that become even more complex in reality due to impurities. However, this is an important aspect that will be addressed in a future work. It should be noted that the hyper-fast GC-IMS presented here is still an experimental setup at university level, besides the commercially available hyper-fast GC. Therefore, long-term stability of the device is still an important aspect to be investigated in future studies with a more sophisticated setup that goes beyond the research state.