In the past decade, interest has been renewed in combining ion mobility spectrometry (IMS) with gas chromatography (GC) [1] to measure volatile organic compounds (VOCs) in human breath [2,3], odorants in textiles [4], foods [[5], [6], [7]], and samples with industrial [8], environmental [9] or security [10,11] importance. Recent noteworthy clinical applications of GC/IMS include non-invasive diagnosis of Parkinson Disease [12] and the detection of COVID-19 infection [13]. Introduced in 1970, IMS methods were envisioned early as GC detectors [14] and were refined in 1982 for use with capillary columns [15]. In GC/IMS methods, substances eluting from the chromatographic column are ionized, often using atmospheric pressure chemical ionization (APCI) reactions, and then characterized for ion mobility in electric fields. An early and persistent obstacle for the broad acceptance of GC/IMS methods has been the use of radioactive materials (63Ni, 241Am, or tritium) as the ion sources in IMS drift tubes. Radioactive ion sources are discouraged more so today by costly regulatory requirements for monthly leak tests, inventory reporting, and eventual disposal. Despite these constraints, combinations of GC with IMS have been accepted in some niche applications where radioactive ion sources were approved for specialized uses. These include the Volatile Organic Analyzer (VOA) for monitoring air quality on the International Space Station (ISS, 2000 to 2010) and the Environmental Vapor Monitor [16], a hand-held GC/IMS instrument pioneered by the US Army for in-field measurements [17]. There is today a single commercial producer of GC/IMS instruments with Gesellschaft fuer analytische Sensorsysteme, mBH in Dortmund, Germany.

Another mobility method known as differential mobility spectrometry (DMS) emerged in 2000 with small planar analyzers as a variation on conventional IMS drift tubes [18]. In DMS, ions derived using APCI reactions from a vapor sample, e.g., GC column effluent, are swept between two plates, separated by 0.5 mm, using a flow of air at ambient pressure. An asymmetric waveform often near 1 MHz is applied perpendicular to ion flow creating an electric field which displaces ion off-center based on field dependent mobility coefficients. Ions are moved toward the analyzer plates and are neutralized unless a compensating voltage (DC) creates an opposing electric field and ions are restored, at characteristic DC voltages, to a center-flow between plates and onto the detector. A sweep of a range of compensation voltage provides a measure of all ions in the DMS analyzer and establishes a spectrum as described in several reviews [[19], [20], [21]]. These small, planar analyzers were demonstrated as a GC detector [[22], [23], [24], [25], [26], [27]] and a hand-held GC/DMS, the MicroAnalyzer [28], was used to monitor air quality on the ISS beginning in 2010 [29]. Recently, a tandem DMS analyzer with strong electric field stage to fragment mobility isolated ions was demonstrated using ultra-fast gas chromatography [30]. In both GC/DMS and earlier GC/IMS combinations, radioactive ion sources provided reliable and predictable performance though limited principally to laboratory use only.

Non-radioactive ion sources described for use with ion mobility analyzers and VOCs have included photo-discharge lamps [[31], [32], [33]], soft x-rays [[34], [35], [36]], corona discharges [[37], [38], [39]], helium-based DC plasmas [[40], [41], [42], [43]], and secondary electrospray ionization [44,45]. While embodiments of a few of these ion sources are found today in ion mobility analyzers for security or military applications, their commercial disadvantages can include comparatively short operating lifetime, high initial costs, and differing response compared to the radioactive sources. A corona discharges in negative polarity without special control [49,50], for example, commonly produces NO3− or NO2− rather than O2− as found in radioactive sources [46]. Apart from response with some explosives [47,48], such NOx− ions are largely unreactive for VOCs. Response with a photoionization source is governed by ionization energies. Recently, a micro-plasma ion source (MPIS) with megahertz discharge frequencies has been developed through a series of technical progressions [[51], [52], [53], [54]] with functionality in both positive and negative polarities. In the MPIS, electrons or ions, derived from a microplasma in air at ambient pressure are extracted into a reaction region and mixed with VOCs. A commercially available first-generation design was released in 2021 and replaced later with a second-generation, simplified design. While MPIS could be a compact and inexpensive alternative to radioactive ion sources, response to VOCs has been given in only a single brief description [55].

The specific aim of this work was to complete an assessment of the qualitative and quantitative performance of the MPIS using a DMS as detector for GC with a broad interest replacing radioactive ion sources with suitable non-radioactive alternatives. Such a development may support the commercialization of GC/DMS and GC/IMS technology and their applications. A second interest is MIPS as a stand-alone GC detector with high selectivity based on APCI reactions.