New Method of Ion Mobility Spectrometry Created

Scientists from the University of Helsinki in Helsinki, Finland created a new simplified method of ion mobility spectrometry. Their findings were later published in the journal Analytical Chemistry (1).

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Ion mobility spectrometry (IMS) is the study of how ions move in gases under the influence of an electric field; in other words, it is the study of the electrophoretic mobility of ions in buffer gases (2). spectrometers have been widely accepted in applications where ion formation chemistry is favorable for targeted analytes; such instances include detecting organophosphorus compounds (OPCs) as chemical warfare agents in military applications and explosives determination during handheld luggage screening in airports. In these scenarios, the high selectivity of IMS can be attributed to selectivity of sample ionization using atmospheric pressure chemical ionization (APCI) with hydrated protons in positive polarity or oxygen anions or chloride ions in negative polarity.

In some instances, reagents or dopants can be added to the ion source or reaction region to introduce additional selectivity with alternate reactant ions. When mixtures containing various ionization properties are directly introduced to APCI-based sources in IMS, the subsequent findings may not quantitatively show sample composition. Historically, analysis of mixtures by IMS with general response to volatile organic compounds (VOCs) included preseparation using gas chromatography, with examples being found in fields like air quality monitoring and food quality (1). This process, in principle, may be utilized to separate ions in ionization phase, thus avoiding the need for chromatographic preseparation. Consequently, instrumentation could be simplified, and the detection limits could be improved for compounds otherwise masked with higher concentrations in mixtures.

For this study, the scientists explored time-resolved formation of ions, otherwise known as gas ion distillation (GID), in an IMS drift tube with a modified unidirectional flow (UDF) design, where user-control vapor residence in a reaction region was achieved with a stop flow confined volume (SFCV) control (1). With this design, samples were introduced into a confined volume, with drift gas being extracted before it entered the GID volume or reaction region. The dynamics of ion formation were studied with a ternary mixture and compared to models using a multiphysic finite element method environment. From there, the responses for VOC mixtures containing up to five constituents were recorded semiquantitatively to document time-resolved ion formation with IMS methods.

Ion signal intensities for protonated monomers and proton bound dimers were measured and computationally extracted using mobilities from mobility spectra and exhibited distinct times of appearance over 30 s or more after sample injection. Models, and experimental findings with the ternary mixture, suggest that the separation of vapors as ions over time was consistent with differences in the reaction rate for reactions between primary ions from hydrated protons and constituents and from cross-reactions that follow the initial step of ionization. With these findings, the scientists concluded that the concept of stopped flow, which was first introduced here, can provide a method for the temporal separation of atmospheric pressure ions. This type of separation, rather than relying on chromatographic technology, instead relies on ion kinetics (1).

References

(1) Anttalainen, O.; Karjalainen, M.; Lattouf, E.; Hecht, O.; et al. Time-Resolved Ion Mobility Sprectrometry with a Stop Flow Confined Volume Reaction Region. Anal. Chem. 2024, 96 (25), 10182–10192. DOI: 10.1021/acs.analchem.4c00434

(2) Dodds, J. N.; Baker, E. S. Ion Mobility Spectrometry: Fundamental Concepts, Instrumentation, Applications, and the Road Ahead. J. Am. Soc. Mass. Spectrom. 2019, 30 (11), 2185–2195. DOI: 10.1007/2Fs13361-019-02288-2