Towards a hand-held, fast, and sensitive gas chromatograph-ion mobility spectrometer for detecting volatile compounds

Time-Resolved Ion Mobility Spectrometry with a Stop Flow Confined Volume Reaction Region

An ion source concept is described where the sample flow is stopped in a confined volume of an ion mobility spectrometer creating time-dependent patterns of ion patterns of signal intensities for ions from mixtures of volatile organic compounds and improved signal-to-noise rate compared to conventional unidirectional drift gas flow. Hydrated protons from a corona discharge were introduced continuously into the confined volume with the sample in air at ambient pressure, and product ions were extracted continuously using an electric field for subsequent mobility analysis. 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 a 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. The findings suggest that the concept of stopped flow, introduced here for the first time, may provide a method for the temporal separation of atmospheric pressure ions. This separation relies on ion kinetics and does not require chromatographic technology.

Ratiometric fluoroprobe based on Eu-MOF@Tb3+ for detecting tetracycline hydrochloride in freshwater fish and its application in rapid visual detection

Tetracycline hydrochloride (TCH), a prevalent antibiotic in aquaculture for treating bacterial infections, poses challenges for on-site detection. This study employed the reversed-phase microemulsion method to synthesize a uniform nano metal-organic framework (MOF) material, europium-benzene-p-dicarboxylic acid (Eu-BDC), doped with Tb3+ to form a dual-emission fluorescence probe. By leveraging the combined a-photoinduced electron-transfer (a-PET) and inner filter effect (IFE) mechanisms, high-sensitivity TCH detection in Carassius auratus and Ruditapes philippinarum was achieved. The detection range for TCH is 0.380-75 μM, with a low limit of detection (LOD) at 0.115 μM. Upon TCH binding, Eu-BDC fluorescence rapidly decreased, while Tb3+ fluorescence remained constant, establishing a ratiometric fluorescence change. Investigation into the TCH quenching mechanism on Eu-BDC was conducted using time-dependent density functional theory (TD-DFT) calculations and fluorescence quenching kinetic equations, suggesting a mixed quenching mechanism. Furthermore, a novel photoelectric conversion fluorescence detection device (FL-2) was developed and evaluated in conjunction with high-performance liquid chromatography-diode-array detection (HPLC-DAD). This is the first dedicated fluorescence device for TCH detection, showcasing superior photoelectric conversion performance and stability that reduces experimental errors associated with smartphone photography methods, presenting a promising avenue for on-site rapid TCH detection.

Nontargeted Volatile Metabolite Screening and Microbial Contamination Detection in Fermentation Processes by Headspace GC-IMS

Gas chromatography combined with ion mobility spectrometry (GC-IMS) is a powerful separation and detection technique for volatile organic compounds (VOC). This combination is characterized by exceptionally low detection limits in the low ppbv range, high 2-dimensional selectivity, and robust operation. These qualities make it an ideal tool for nontarget screening approaches. Fermentation broths contain a substantial number of VOC, either from the medium or produced by microbial metabolism, that are currently not regularly measured for process monitoring. In this study, Escherichia coli, Saccharomyces cerevisiae, Levilactobacillus brevis, and Pseudomonas fluorescens were exemplarily used as model organisms and cultivated, and the headspace was analyzed by GC-IMS. Additionally, mixed cultures for every combination of two of the microorganisms were also characterized. Multivariate data analysis of the GC-IMS data revealed that it is possible to differentiate between the microorganisms using PLS-DA with a prediction accuracy of 0.92. The mixed cultures could be separated from the pure cultures with accuracies between 0.87 and 1.00 depending on the organism. GC-IMS data correlate with the optical density and can be used to follow and model growth curves. The root mean squared errors ranged between 10 and 20% of the maximum value, depending on the organism. Peak identification with reference compounds did not reveal specific marker compounds, rather the pattern was found to be responsible for the model performance.

On-Site Detection of Volatile Organic Compounds (VOCs)

Volatile organic compounds (VOCs) are of interest in many different fields. Among them are food and fragrance analysis, environmental and atmospheric research, industrial applications, security or medical and life science. In the past, the characterization of these compounds was mostly performed via sample collection and off-site analysis with gas chromatography coupled to mass spectrometry (GC-MS) as the gold standard. While powerful, this method also has several drawbacks such as being slow, expensive, and demanding on the user. For decades, intense research has been dedicated to find methods for fast VOC analysis on-site with time and spatial resolution. We present the working principles of the most important, utilized, and researched technologies for this purpose and highlight important publications from the last five years. In this overview, non-selective gas sensors, electronic noses, spectroscopic methods, miniaturized gas chromatography, ion mobility spectrometry and direct injection mass spectrometry are covered. The advantages and limitations of the different methods are compared. Finally, we give our outlook into the future progression of this field of research.

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 ppt_v range have been reached.

A comparison between mobile and stationary gas chromatography-mass spectrometry devices for analysis of complex volatile profiles

On-site analysis of volatile organic compounds (VOCs) with miniaturized gas chromatography-mass spectrometry (GC-MS) systems is a very rapidly developing field of application. While, on the one hand, major technological advances are improving the availability of these systems on the market, on the other hand, systematic studies to assess the performance of such instruments are still lacking. To fill this gap, we compared three portable GC-MS devices to a state-of-the-art benchtop (stationary) system for analysis of a standard mixture of 18 VOCs. We systematically compared analytical parameters such as the sensitivity and similarity of the signal response pattern and the quality of the obtained mass spectra. We found that the investigated mobile instruments (i) showed different response profiles with a generally lower number of identified analytes. Also, (ii) mass spectral reproducibility (% relative standard deviation (RSD) of the relative abundance of selective fragments) was generally worse in the mobile devices (mean RSD for all targeted fragments ~9.7% vs. ~3.5% in the stationary system). Furthermore, mobile devices (iii) showed a poorer mass spectral similarity to commercial reference library spectra (>20% deviation of fragment ion relative intensity vs. ~10% in the stationary GC-MS), suggesting a less reliable identification of analytes by library search. Indeed, (iv) the performance was better with higher-mass and/or more abundant fragments, which should be considered to improve the results of library searches for substance identification. Finally, (v) the estimation of the signal-to-noise ratio (S/N) in mobile instruments as a measure of sensitivity revealed a significantly lower performance compared to the benchtop lab equipment (with a ratio among medians of ~8 times lower). Overall, our study reveals not only a poor signal-to-noise ratio and poor reproducibility of the data obtained from mobile instruments, but also unfavorable results with respect to a reliable identification of substances when they are applied for complex mixtures of volatiles.

Ultra-Fast Polarity Switching, Non-Radioactive Drift Tube for the Miniaturization of Drift-Time Ion Mobility Spectrometer

Drift-time ion mobility spectrometer (DT-IMS) is a promising technology for gas detection and analysis in the form of miniaturized instrument. Analytes may exist in the form of positively or negatively charged ions according to their chemical composition and ionization condition, and therefore require both polarity of electric field for the detection. In this work the polarity switching of a drift-time ion mobility spectrometer based on a direct current (DC) corona discharge ionization source was investigated, with novel solutions for both the control of ion shutter and the stabilization of aperture grid. The drift field is established by employing a switchable high voltage power supply and a serial of voltage regulator diode, with optocouplers to drive the ion shutter when the polarity is switched. The potential of aperture grid is stabilized during the polarity switching by the use of four diodes to avoid unnecessary charging cycle of the aperture grid capacitor. Based on the proposed techniques, the developed DT-IMS with 50 mm drift path is able to switch its polarity in 10 ms and acquire mobility spectrum after 10 ms of stabilization. Coupled with a thermal desorption sampler, limit of detection (LoD) of 0.1 ng was achieved for ketamine and TNT. Extra benefits include single calibration substance for both polarities and largely simplified pneumatic design, together with the reduction of second drift tube and its accessories. This work paved the way towards further miniaturization of DT-IMS without compromise of performance.

Coupling Droplet Microfluidics with Ion Mobility Spectrometry for Monitoring Chemical Conversions at Nanoliter Scale

We introduce the coupling of droplet microfluidics and ion mobility spectrometry (IMS) to address the challenges of label-free and chemical-specific detection of compounds in individual droplets. In analogy to the established use of mass spectrometry, droplet-IMS coupling can be also achieved via electrospray ionization but with significantly less instrumental effort. Because IMS instruments do not require high-vacuum systems, they are very compact, cost-effective, and robust, making them an ideal candidate as a chemical-specific end-of-line detector for segmented flow experiments. Herein, we demonstrate the successful coupling of droplet microfluidics with a custom-built high-resolution drift tube IMS system for monitoring chemical reactions in nL-sized droplets in an oil phase. The analytes contained in each droplet were assigned according to their characteristic ion mobility with limit of detections down to 200 nM to 1 μM and droplet frequencies ranging from 0.1 to 0.5 Hz. Using a custom sheath flow electrospray interface, we have further achieved the chemical-specific monitoring of a biochemical transformation catalyzed by a few hundred yeast cells, at single droplet level.