Ion spectrometric detection technologies for ultra-traces of explosives: a review

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.

Enhanced TNT vapor sensing through a PMMA-mediated AIPE-active monocyclometalated iridium(III) complex: a leap towards real-time monitoring

Based on the explosive nature and harmful effects of nitro-based explosive materials on living beings and the environment, it is extremely important to develop luminescence-based probe molecules for their detection with excellent selectivity and sensitivity. Two AIPE (aggregation-induced phosphorescence emission)-active iridium(III) complexes (M1 and M2) were developed for the sensitive detection of TNT in both contact and non-contact modes. The aggregate solutions of both complexes (M1 and M2 in THF/H2O, 1/9 by volume) detected TNT at the pico-molar (pM) level. These complexes showed greatly enhanced emission intensity while embedded in a PMMA(polymethyl methacrylate) matrix film. The amplified quantum efficiency, improved phosphorescence lifetime, and enhanced porous network of M2-PMMA composite helps to improve the sesitivity of TNT vapor detection. Interestingly, the sensitivity of the detection of TNT by the M2 complex was significantly improved (5-fold) in a PMMA-incorporated complex (CP) with an observed limit of detection (LOD) of 12.8 ppb. From the BET analysis of CP, it was observed that the mesoporous network of CP has an average pore diameter of 8.52 nm and a surface area of 2.03 m2 g-1. The porous network of CP assists in trapping TNT vapor in a polymeric network containing an electron-rich probe (iridium(III) complex, M2), which helps to effectively trap TNT, thus enhancing electronic communication. As a result, significant emission quenching was observed.

Accumulation of Large Ion Populations with High Ion Densities and Effects Due to Space Charge in Traveling Wave-Based Structures for Lossless Ion Manipulations (SLIM) IMS-MS

The accumulation of very large ion populations in traveling wave (TW)-based Structures for Lossless ion Manipulations (SLIM) has been studied to better understand aspects of "in-SLIM" ion accumulation, and particularly its use in conjunction with ion mobility spectrometry (IMS). A linear SLIM ion path was implemented that had a "gate" for blocking and accumulating ions for arbitrary time periods. Removing the gate potential caused ions to exit, and the spatial distributions of accumulated ions examined. The ion populations for a set of peptides increased approximately linearly with increased accumulation times until space change effects became significant, after which the peptide precursor ion populations decreased due to growing space charge-related ion activation, reactions, and losses. Ion activation increased with added storage times and the TW amplitude. Lower amplitude TWs in the accumulation/storage region prevented or minimized ion losses or ion heating effects that can also lead to fragmentation. Our results supported the use of an accumulation region close to the SLIM entrance for speeding accumulation, minimizing ion heating, and avoiding ion population profiles that result in IMS peak tailing. Importantly, space charge-driven separations were observed for large populations of accumulated species and attributed to the opposing effects of space charge and the TW. In these separations, ion species form distributions or peaks, sometimes moving against the TW, and are ordered in the SLIM based on their mobilities. Only the highest mobility ions located closest to the gate in the trapped ion population (and where the highest ion densities were achieved) were significantly activated. The observed separations may offer utility for ion prefractionation of ions and increasing the dynamic range measurements, increasing the resolving power of IMS separations by decreasing peak widths for accumulated ion populations, and other purposes benefiting from separations of extremely large ion populations.

Influence of ionization volume and sample gas flow rate on separation power in gas chromatography-ion mobility spectrometry

In this work, the influence of the sample gas flow rate and the ionization region volume of an ion mobility spectrometer (IMS) used as a detector in gas chromatography (GC) on GC-IMS peak shape has been investigated. Therefore, a drift tube IMS with a field-switching ion shutter, a defined ionization region volume and an ultra-violet radiation source was used. To identify the influence of the sample gas flow rate entering the ionization region (equals the GC carrier gas flow rate if no further make-up gas is used) and the ionization region volume on peak broadening and signal intensity, different sample volumes as they would elute from a GC were tested at a variety of sample gas flow rates at a given ionization region volume. The results clearly show that for low sample gas flow rates a depletion of sample molecules in the ionization region leads to a significant decrease in effective detector volume but also to reduced signal intensities. Therefore, for optimal performance of a GC-IMS, the optimal operating point of the GC should match the flow range, where the IMS provides the best compromise between signal-to-noise ratio and peak broadening.

Role of microbiota and its ecological succession on flavor formation in traditional dry-cured ham: a review

Traditional dry cured ham (DCH) is favored by consumers for its distinctive flavor, derived from an array of volatile organic compounds (VOCs). Microbiota play a pivotal role in the formation of VOCs. To fully comprehend the pathway by which the microbiota enhance the flavor quality of DCH, it is imperative to elucidate the flavor profile of DCH, the structural and metabolic activities of the microbiota, and the intricate relationship between microbial and VOCs. Thus far, the impact of microbiota on the flavor profile of DCH has not been comprehensively discussed or reviewed, and the succession of bacteria, especially at distinct phases of processing, has not been adequately summarized. This article aims to encapsulate the considerable potential of ferments in shaping the flavor characteristics of DCH, while elucidating the underlying mechanisms through which VOCs are generated in hams via microbial metabolism. Throughout the various stages of DCH processing, the composition of microbiota undergoes dynamic changes. Furthermore, they directly participate in the formation of VOCs in DCH through the catabolism of amino acids, metabolism of fatty acids, and the breakdown of carbohydrates. Several microorganisms, including Lactobacillus, Penicillium, Debaryomyces, Pediococcus, and Staphylococcus, exhibit considerable potential as fermenters in ham production.

Recent development and trends in the detection of peroxide-based explosives

Peroxide-based explosives (PBEs) are increasingly common in criminal and terrorist activity due to their easy synthesis and high explosive power. The rise in terrorist attacks involving PBEs has heightened the importance of detecting trace amounts of explosive residue or vapors. This paper aims to provide a review on the developments of techniques and instruments for detecting PBEs over the past ten years, specifically discussing advancements in ion mobility spectrometry, ambient mass spectrometry, fluorescence techniques, colorimetric methods, and electrochemical methods. We provide examples to illustrate their evolution and focus on new strategies for improving detection performance, specifically in terms of sensitivity, selectivity, high-throughput, and wide explosives coverage. Finally, we discuss future prospects for PBE detection. It is hoped this treatment will serve as a guide to the novitiate and as aid memoire to the researchers.

Development of a Flavor Fingerprint Using HS-GC-IMS for Volatile Compounds from Steamed Potatoes of Different Varieties

The variations in flavor substances across different varieties of steamed potatoes were determined by headspace-gas chromatography ion mobility spectrometry (HS-GC-IMS) combined with sensory evaluation. Results showed that 63 representative compounds, including 27 aldehydes, 14 alcohols, 12 ketones, 4 esters, 2 furans, 1 acid and others, together acted as contributors to the flavors in steamed potatoes. Analysis found that species and concentrations of aldehydes, alcohols and ketones in six varieties were the most abundant. In addition, esters, furans and acid were also responsible for flavor. PCA results showed that volatile compounds in Atlantic, Longshu No. 23, Longshu No. 7 and Longshu No. 14 were similar, while Russet Burbank and Longshu No. 16 had distinct characteristic volatiles, which was consistent with sensory evaluation. The combination of sensory evaluation and HS-GC-IMS provided useful knowledge for charactering volatile compounds of steamed potatoes from different varieties, and also demonstrated the promising application of HS-GC-IMS in the detection of potato flavor with various cooking methods.

Ion storage biases in the ion funnel trap of a Hybrid ion mobility spectrometer/time of flight mass spectrometer

A detailed experimental characterization on the ion storage biases in an ion funnel trap, related to ion structure, charge state and RF voltage applied to the ion funnel trap, is reported by using both cytochrome C and ubiquitin samples. It was first observed experimentally that an unavoidable ion overflow would occur when the incoming ions exceeded the capacity of ion funnel trap. The conformers with extended structures would lose preferentially in the ion overflow process. Accordingly, a significant structural bias in the ion mobility spectrometry/time of flight mass spectrometry (IMS-TOF MS) spectrum was created, as the peak intensities for conformers with compact structures and extended structures would continuously increase and decrease, respectively, when the ion overflow time of the ion funnel trap was increased. Furthermore, the experimental results also showed that the effect of this ion structural bias was more significant when the RF voltage applied to the ion funnel trap was increased. In addition, an ion charge state bias in the ion funnel trap was also observed. The effect of the ion structural bias depends significantly on the specific charge state of the ions. For a given analyte, its lower charge state ions show a greater sensitivity to the ion structural bias than the higher charge state ones under the same ion funnel trap operating conditions. Therefore, it is extremely important to set a reasonable operation condition for the ion funnel trap to avoid ion storage biases in IMS-TOF MS.

A High Kinetic Energy Ion Mobility Spectrometer for Operation at Higher Pressures of up to 60 mbar

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are usually operated at absolute pressures around 20 mbar in order to reach high reduced electric field strengths of up to 120 Td for influencing reaction kinetics in the reaction region. Such operating points significantly increase the linear range and limit chemical cross sensitivities. Furthermore, HiKE-IMS enables ionization of compounds normally not detectable in ambient pressure IMS, such as benzene, due to additional reaction pathways and fewer clustering reactions. However, operation at higher pressures promises increased sensitivity and smaller instrument size. In this work, we therefore study the theoretical requirements to prevent dielectric breakdown while maintaining high reduced electric field strengths at higher pressures. Furthermore, we experimentally investigate influences of the pressure, discharge currents and applied voltages on the corona ionization source. Based on these results, we present a HiKE-IMS that operates at a pressure of 60 mbar and reduced electric field strengths of up to 105 Td. The corona experiments show shark fin shaped curves for the total charge at the detector with a distinct optimum operating point in the glow discharge region at a corona discharge current of 5 μA. Here, the available charge is maximized while the generation of less-reactive ion species like NO_x⁺ is minimized. With these settings, the reactant ion population, H₃O⁺ and O₂⁺, for ionizing and detecting nonpolar substances like n-hexane is still available even at 60 mbar, achieving a limit of detection of just 5 ppb_V for n-hexane.

Neural network classification of mobility spectra for volatile organic compounds using tandem differential mobility spectrometry with field induced fragmentation

A spectral library of field induced fragmentation (FIF) spectra for 45 oxygen-containing volatile organic compounds from 5 chemical classes was obtained using tandem differential mobility spectrometry (DMS). Protonated monomers were mobility isolated in a first DMS stage, fragmented with electric fields >10,000 V/cm in a middle (or reactive) stage, and mobility characterized in a second DMS stage. Other spectral libraries were obtained for protonated monomers and for complete mobility spectra from a single DMS stage. Neural networks from Python/Tensorflow software, prepared in-house, and from commercial NeuralWorks Professional II/PLUS were trained to assign spectra into a chemical class. The success at classification was determined for familiar and unfamiliar spectra from these three libraries. Classification test scores were best with FIF spectra with >0.99 for familiar compounds and 0.52 for unfamiliar compounds and were consistent with neural network learning of structural information from fragment ions when compared to other spectral libraries. Radar charts are introduced as measures of classification and as a tool to explore mis-classification. This work shows that ion fragmentation with multi-stage tandem DMS portends molecular identification with the portability and robustness of ambient pressure ion mobility analyzers.

Changes of volatile substance composition during processing of nine-processed tangerine peel (Jiuzhi Chenpi) determined by gas chromatography-ion mobility spectrometry

Nine-processed tangerine peel (Jiuzhi Chenpi in Chinese) is a famous Chinese traditional snack. The composition and contents of volatile substances during its processing is unclear. Gas chromatography combined with ion mobility spectrometry (GC-IMS) was applied to determine the characteristic changes of volatile components throughout the production process. Four stages such as untreated dry tangerine peel (raw material), debittered tangerine peel, pickled tangerine peel, and final product were examined. A total of 110 flavor compounds including terpenes, alcohols, aldehydes, ketones, esters, acids, and two others were successfully detected in tangerine peel samples across the various production stages. There were abundant amounts of terpenes contributing to the flavor, including limonene, gamma-terpinene, alpha-pinene, myrcene, beta-pinene, and alpha-thujene which were reduced at the later stage of production. Large amounts of esters and alcohols such as methyl acetate, furfuryl acetate, ethyl acetate, benzyl propionate, 2-hexanol, linalool, and isopulegol, were diminished at the early stage of processing, i.e., soaking for debittering. One the other hand, the final product contained increased amount of aldehydes and ketones including pentanal, hexanal, 2-hexenal, 2-heptenal (E), 2-pentenal (E), 1-penten-3-one, 6-methyl-5-hepten-2-one, 2-methyl-2-propenal, and 2-cyclohexen-1-one, and very high level of acetic acid. Present findings help to understand the formation of the unique flavor of nine-processed tangerine peel and provide a scientific basis for the optimization of processing methods and quality control.

Effect of Traveling Waveform Profiles on Collision Cross Section Measurements in Structures for Lossless Ion Manipulations

We evaluated the effect of four different waveform profiles (Square, Sine, Triangle, and asymmetric Sawtooth) on the accuracy of collision cross section (CCS) measurements using traveling wave ion mobility spectrometry (TWIMS) separations in structures for lossless ion manipulations (SLIM). The effects of the waveform profiles on the accuracy of the CCS measurements were evaluated for four classes of compounds (lipids, peptides, steroids, and nucleosides) at different TW speeds (126-206 m/s) and amplitudes (15-89 V). For the lipids and peptides, the TWIMS-based CCS (TWCCS) deviations from the corresponding drift-tube-based CCS (DTCCS) measurements were significantly lower in experiments conducted using the Sawtooth waveform compared to the square waveform. This observation can be rationalized by the lower maximum electric field experienced by ions with a Sawtooth waveform, as compared to the other waveforms, resulting in a lower probability for significant ion heating. We also observed that given approximately comparable resolution for all four waveforms, the Sawtooth waveform resulted in lower TWCCS error and a better agreement with DTCCS values than the Square waveform. In addition, for the steroids and nucleosides, an opposite TWCCS trend was observed, with higher errors with the Sawtooth waveform and lower with the Square waveform, suggesting that these molecules tend to become slightly more compact under ion heating conditions. Under optimum conditions, all TWCCS measurements on the SLIM platform were within 0.5% of those measured in the drift tube ion mobility spectrometry.

Detection of explosives in vapor phase by field asymmetric ion mobility spectrometry with dopant-assisted laser ionization

Detection of low-volatile explosives in concentrations below 10^-14 g/cm³ is a great challenge for portable ion mobility spectrometers (IMS) and field asymmetric IMS (FAIMS). We study the capabilities of FAIMS detector with ultraviolet laser ionization combined with organic additives (dopants) toluene and 1-methylnaphtalene to sense nitro-explosives: trinitrotoluene (TNT) and low-volatile cyclonite (RDX) and nitropentaerythritol (PETN). Differential mobility coefficients were measured for target ion peaks of TNT, RDX and PETN. Presence of dopants in the sample results in multiple growth of ion yield at laser intensities lower than 2 × 10⁷ W/cm². Limits of detection with dopant-assisted laser ionization were determined: 4.7 × 10^-16 g/cm³ for RDX and 9.8 × 10^-15 g/cm³ for PETN. Obtained results propose a way to further improve sensitivity of detectors along with improvement of portability of current laser-based FAIMS prototypes by using less powerful and smaller lasers.

Arc-Induced Nitrate Reagent Ion for Analysis of Trace Explosives on Surfaces Using Atmospheric Pressure Arc Desorption/Ionization Mass Spectrometry

This study presents the rapid surface detection of explosives by employing atmospheric pressure arc desorption/ionization mass spectrometry (APADI-MS) using point-to-plane arc discharge. In APADI, neutral explosives readily bind to the gas-phase nitrate ion, NO₃^-, induced by arc discharge to form anionic adducts [M+NO₃]^-. This avoids the need for inorganic anionic additives such as NO₃^-, NO₂^-, Cl^-, acetate, and trifluoroacetate for unique explosive ionization pathways and simplifies mass spectra. The analytical performance of APADI was thoroughly evaluated for the rapid detection of 10 explosives at levels in the range of 800 fg-1 μg. Arc-induced nitrogen oxide anions promoted the formation of characteristic adducts, such as [M+NO₃]^-, and improved the instrument response for all the explosives tested. APADI showed considerable sensitivity in the negative ion mode with limits of detection in the low picogram range, particularly when explosives were analyzed on a copper or aluminum foil substrate. APADI coupled with an Orbitrap mass spectrometer displayed a good linear response for the studied explosives. The linearity and intraday and interday precisions were evaluated, demonstrating the feasibility and robustness of APADI-MS for the detection of trace explosives on solid surfaces. The mechanisms of APADI for explosive ionization and desorption were examined and verified by performing density functional theory calculations.

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.

Analysis of flavor formation during production of Dezhou braised chicken using headspace-gas chromatography-ion mobility spec-trometry (HS-GC-IMS)

In order to help the poultry industry to generate higher quality products, the headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS) technique was used to identify volatile substances formed during the processing of Dezhou braised chicken (DBC). A total of 37 volatile substances including aldehydes, alcohols, ketones, esters, terpenoids, furans and pyrazines were identified during DBC processing across seven sampling stages. The analyses identified 2-ethylhexanol as a key flavor chemical within the chicken carcasses, and found that ethyl acetate, 1-hexanol, 4-methyl-2-pentanone and 1-pentanol were mainly produced during the deep-frying stage of processing. Stewing with herbs and spices was found to be an important stage in the flavor impartation process. 2-Butanone, n-nonanal, heptanal and ethanol were positively related to processing stage 3, whereas processing stage 4 was characterized by ethyl propanoate, benzaldehyde, butyl acetate, 2-pentyl furan and 2-heptanone. The processing stages 5, 6 and 7 were not significantly different (P > 0.05) from each other.

Open port sampling interface mass spectrometry of wipe-based explosives, oxidizers, and narcotics for trace contraband detection

Rapid screening for chemical traces of explosives and narcotics is widely used to support homeland security and law enforcement. These target compounds span a range of physicochemical properties from organic to inorganic, with preferential ionization pathways in both negative and positive mode operation. Nonvolatile inorganic oxidizers present in homemade fuel-oxidizer mixtures, pyrotechnics, and propellants create a unique challenge to traditional thermal desorption-based technologies. Developments in solid-liquid extraction techniques, specifically, open port sampling interface mass spectrometry (OPSI-MS) provide compelling capabilities to address these hurdles. In this proof of concept study, we investigated the trace detection of wipe-based (i.e., common swipe sampling collection method) explosives, oxidizers, and narcotics using an OPSI source and compact single quadrupole mass analyzer. The liquid dissolution and extraction capabilities of OPSI enabled detection of both traditional military-grade explosives and homemade explosive oxidizers. OPSI-MS sensitivities to a series of seven target compounds from polytetrafluoroethylene (PTFE) coated fiberglass sampling wipes were on the order of several nanograms to sub-nanogram levels. Comparisons with direct solution-based sample analysis enabled quantification of wipe-based sample extraction effects. The system demonstrated quick temporal responses, polarity switching capabilities, and rapid signal decay with minimal carryover, all critical to high throughput screening applications. Coupling traditional swipe sampling with OPSI-MS offers a promising tool for contraband screening applications.

Selective dual detection of Hg2+ and TATP based on amphiphilic conjugated polythiophene-quantum dot hybrid materials

The design of multifunctional sensors based on biocompatible hybrid materials consisting of conjugated polythiophene-quantum dots for multiple environmental pollutants is a promising strategy for the development of new monitoring technologies. Herein, we present a new approach for the "on-off-on" sensing of Hg²⁺ and triacetone triperoxide (TATP) based on amphiphilic polythiophene-coated CdTe QDs (PQDs, PLQY ∼78%). The emission of the PQDs is quenched by Hg²⁺ ions via electron transfer interactions. Based on the strong interaction between TATP and Hg²⁺ ions, the addition of TATP to the PQD-Hg²⁺ complex results in a remarkable recovery of the PQD emission. Under the optimized conditions, the PQD sensor shows a good linear response to Hg²⁺ and TATP with detection limits of 7.4 nM and 0.055 mg L^-1, respectively. Furthermore, the "on-off-on" sensor demonstrates good biocompatibility, high stability, and excellent selectivity in the presence of other metal ions and common explosives. Importantly, the proposed method can be used to determine the level of Hg²⁺ and TATP in environmental water samples.

Resolution enhancement of overlapping peaks of ion mobility spectrometry based on improved particle swarm optimization algorithm

RATIONALE

Ion mobility spectrometry (IMS) is a powerful analytical tool that has been widely applied in many fields. However, the limited structural resolution of IMS results in peak overlapping in the analysis of samples with similar structures. We propose a novel method, improved particle swarm optimization (IPSO), for the separation of IMS overlapping peaks.

METHODS

This method, which prevents local optimization, is used to identify the peak model coefficients of IMS. Moreover, we use the half-peak width characteristics of IMS to determine the particle position range, which eliminates impossible combinations of single peaks and reduces the difficulty of identification of coefficients.

RESULTS

During a comparison in performance between IPSO and the genetic algorithm (GA), the results show that the maximum separation error of IPSO is only 1.45%, while the error of the GA is up to 17.43%. Moreover, the time consumed by IPSO is 95% less than that of the GA, and IPSO has a greater robustness under the same separation error conditions.

CONCLUSIONS

The method proposed provides accurate analytical results in separating overlapping IMS peaks even in cases of severe overlaps, which greatly enhances the structural resolution of IMS.

Machine Learning Improves Trace Explosive Selectivity: Application to Nitrate-Based Explosives

Ion mobility spectrometry (IMS) is the method of choice to detect trace amounts of explosives in most airports and border crossing settings. For most explosives, the IMS detection limits are suitably low enough to meet security requirements. However, for some explosive families, the selectivity is not sufficient. One such family is nitrate-based explosives, where discrimination between various nitrate threats and ambient nitrates is challenging. Using a small database, machine learning methods were utilized to examine the extent of improvement in IMS selectivity for detection of nitrate-based explosives. Five classes were considered in this preliminary study: ammonium nitrate (AN), an ∼95:5 mixture of AN and fuel oil (ANFO), urea nitrate (UN), nitrate due to environmental pollution, and samples that did not contain any explosive (blanks). The preliminary results clearly show that the incorporation of machine learning methods can lead to a significant improvement in IMS selectivity.

Aggregation-Induced Enhanced Emission (AIEE)-Active Conjugated Mesoporous Oligomers (CMOs) with Improved Quantum Yield and Low-Cost Detection of a Trace Amount of Nitroaromatic Explosives

The article reports a straightforward strategy for the design and synthesis of highly luminescent conjugated mesoporous oligomers (CMOs) with an "aggregation-induced enhanced emission" (AIEE) feature through Wittig polymerization of a molecular rotor. Typical molecular rotors such as triphenylamine (TPA) and tetraphenylethene (TPE) as B2-, and A4- and A3-type nodes have been used to construct AIEE-active CMOs, namely, CMO1 and CMO2. The quick dissipation of the excited photons is successfully controlled by the restriction of rotation of the phenyl units through the formation of a mesoporous network scaffold in a solid/thin film, which provides high quantum yields for the interlocked CMO system. Both the CMOs are sensitive and selective to the various nitroaromatic explosives, whereas CMO1 is more sensitive (K_sv = 2.6 × 10⁶ M^-1) toward picric acid. The increased quenching constant for CMO1 is due to its increased quantum yield and high energy-transfer efficiency. The mechanism for sensing has been studied in detail. The larger pore size and pore density in the mesoporous network of CMO1 are found to be responsible for the greater extent of energy transfer from CMO1 to picric acid. Furthermore, CMO1 has been employed for low-cost filter-paper-based detection of a trace amount of nitroaromatic explosive materials.

High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS) at 40 mbar

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are usually operated at an absolute pressure of 20 mbar reaching high reduced electric field strengths of up to 125 Td for controlled reaction kinetics. This significantly increases the linear range and limits chemical cross sensitivities. Furthermore, HiKE-IMS enables the ionization of compounds normally not detectable in ambient pressure IMS, such as benzene, due to new reaction pathways and the inhibition of clustering reactions. In addition, HiKE-IMS allows the observation of additional orthogonal parameters related to an increased ion temperature such as fragmentation and field-dependent ion mobility, which may help to separate compounds that have similar ion mobility under low field conditions. Aiming for a hand-held HiKE-IMS to carry its benefits into field applications, reducing size and power consumption of the vacuum system is necessary. In this work, we present a novel HiKE-IMS design entirely manufactured from standard printed circuit boards (PCB) and experimentally investigate the analytical performance in dependence of the operating pressure between 20 mbar and 40 mbar. Hereby, the limit of detection (LoD) for benzene in purified, dry air (1.4 ppm_V water) improved from 7 ppb_V at 20 mbar down to 1.8 ppb_V at 40 mbar. Furthermore, adding 0.9 ppm_V toluene, the signal of the benzene B⁺ peak decreased by only 2% at 40 mbar. Even in the presence of high relative humidity in the sample gas above 90% or toluene concentrations of up to 20 ppm_V, the LoD for benzene just increased to 9 ppb_V at 40 mbar.

Chemical Classification of Explosives

This work comprehensively reviews some fundamental concepts about explosives and their two commonly used classifications based on either their velocity of detonation or their application. These classifications are highly useful in the military/legal field, but completely useless for the chemical determination of explosives. Because of this reason, a classification of explosives based on their chemical composition is comprehensively revised, discussed and updated. This classification seeks to merge those dispersed chemical classifications of explosives found in literature into a unique general classification, which might be useful for every researcher dealing with the analytical chemical identification of explosives. In the knowledge of the chemical composition of explosives, the most adequate analytical techniques to determine them are finally discussed.

Emerging techniques for the detection of pyrotechnic residues from seized postal packages containing fireworks

High volume screening of parcels with the aim to trace the illegal distribution and selling of fireworks using postal services is challenging. Inspection services have limited manpower and means to perform extensive visual inspection. In this study, the presence of solid pyrotechnic residues collected from cardboard shipping parcels containing fireworks was investigated for direct in-field chemical detection. Two emerging trace detection techniques, i.e., capillary electrophoresis (CE)-based inorganic oxidizer detector and infrared thermal desorption (IRTD) coupled with direct analysis in real time mass spectrometry (DART-MS), were investigated for their potential as screening tools. Detection of non-visible pyrotechnic trace residues from real-case seized parcels was demonstrated using both screening techniques. However, the high nitrate background in the commercial CE system complicated its screening for black powder traces. IRTD-DART-MS allowed differentiation between flash and black powder by identification of the molecular inorganic ions. Compared to the portable CE instrument, rapid screening using IRTD-DART-MS is currently limited to laboratory settings. The capabilities of these emerging techniques established solid particle and trace residue chemical detection as interesting options for parcel screening in a logistic setting.

Qualitative Detection Toward Military and Improvised Explosive Vapors by a Facile TiO2 Nanosheet-Based Chemiresistive Sensor Array

A facile TiO₂ nanosheets-based chemiresistive gas sensor array was prepared to identify 11 kinds of military and improvised explosive vapors at room temperature. The morphology of TiO₂ nanosheets was well-controlled by adjusting the concentration of HF applied during the preparation. Owing to the morphology difference, the TiO₂ nanosheet-based sensors show different response values toward 11 kinds of explosives, which is the basis of the successful discriminative identification. This method owes lots of advantages over other detection techniques, such as the facile preparation procedure, high response value (115.6% for TNT and 830% for PNT) at room temperature, rapid identifying properties (within 30 s for 9 explosives), simple operation, high anti-interference property, and low probability of misinforming, and consequently has a huge potential application in the qualitative detection of explosives.

Traveling-Wave-Based Electrodynamic Switch for Concurrent Dual-Polarity Ion Manipulations in Structures for Lossless Ion Manipulations

We describe the development of a dual-polarity traveling-wave (TW) structures for lossless ion manipulations (SLIM) ion mobility spectrometry (IMS) device capable of switching both positive and negative ions that are traveling simultaneously along the same path to different regions of the SLIM. Through simulations, the routing efficiency of the SLIM TW switch was compared to a SLIM direct-current-based (DC) switch developed previously for IMS-MS. We also report on the initial experimental evaluation of a dual-polarity SLIM platform, which uses the TW-based ion switch to achieve higher resolution multipass serpentine ultralong path with extended routing (SUPER) IMS separations. Overall, these results show that the dual-polarity TW switch is not only as effective as DC switching in terms of routing efficiency but also is agnostic to the polarity of the ions being routed.

Increased ion throughput using tristate ion-gate multiplexing

For time dispersive ion mobility experiments detail control over the mechanism of ion beam modulation is necessary to establish optimum performance as this parameter greatly influences the temporal width of the ion beam arriving at the detector. When sampling continuous ion sources the temporal sampling or the incoming ion beam is often achieved by the electronic modulation of a grid or electric field. Not surprisingly, the rate at which a given ion population traverses this gating region is directly proportional to an ion's population and the applied electric field. This scenario establishes conditions where discrimination of the incoming ion beam may occur when the ion gate modulation rate is minimized. Recent developments in the mechanical construction of ion gates and their subsequent operation suggest that the mobility discrimination during ion gating may be minimized, however, it is remains unclear how this behavior will translate to ion beam multiplexing approaches. In this present work, we compare the performance levels of the tri-state ion shutter (3S-IS) to the two-state ion shutter (2S-IS) using a series of Fourier transform ion mobility mass spectrometry (FT-IMMS) experiments. The performance of the two different shutter operating principles were evaluated using ion multiplexing using tetraalkylammonium salts (TXA ions; T5-T8, T10, T12) bradykinin, and a set of reversed sequence isomeric pentapeptides using a variety of different ion gate frequency sweeps. Noticeable increases in ion throughput were observed for the 3S-IS with 95% and 45% increases in ion counts for the T5 and T12 ions respectively compared to the 2S-IS. Similarly, a 27% and 55% increase in ion counts was observed for the [M + 2H]2+ and [M + H]+ ions of bradykinin, respectively. In addition, a 10% increase in resolving power was also observed for the 3S-IS compared to the 2S-IS. Overall, utilization of the 3S-IS effectively minimizes both discrimination of slower ions and the impact of gate depletion effect common to traditional ion gating techniques.

Nitrogen-Activated Oxidation in Nitrogen Direct Analysis in Real Time Mass Spectrometry (DART-MS) and Rapid Detection of Explosives Using Thermal Desorption DART-MS

Direct analysis in real time mass spectrometry (DART-MS) was used to analyze an array of explosives including nitro-based explosives, peroxide explosives, and energetic heterocyclic compounds with different DART discharge gases (helium, argon, and nitrogen). Profound analyte oxidation was observed for particular compounds (TNT (9) and 2, 4-DNT (10)), whose mass spectra were completely dominated by the oxidation products when nitrogen was substituted for helium in DART analysis. This interesting phenomenon suggested that a highly oxidative environment provided by N₂ DART ion source. A possible mechanism involved in nitrogen DART was proposed which may help further understanding the different chemistry involved in the ionization process. This work also presents a thermal desorption DART (TD-DART) configuration that can enable rapid, specific analysis of explosives from swipes. The screening of swipes with three different compositions (fiberglass, Hybond N⁺ membrane, and filter paper) showed that fiberglass swipe has the best performance which was then used for the subsequent TD-DART analysis. A direct comparison of TD-DART with traditional DART demonstrated that TD-DART indeed gives better response than traditional DART (provided that the distance between the DART source and mass spectrometer is the same) and will have wider applications than traditional DART.

Quantification of nitroaromatic explosives in contaminated soil using MALDI-TOF mass spectrometry

Contamination from various sources is a global environmental and health threat, with mining and military activities in particular having spread nitroaromatic compounds, such as 2,4,6-trinitrotoluene and its degradation products and by-products, to the soil. The investigation and monitoring of large contaminated areas requires new detection methods since the established ones are expensive and time-consuming. Hence, we established a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) method using 1,5-diaminonaphthalene as the matrix substance and an internal standard for quantification. Analyzing standard substances, we found specific signals for radical and fragment ions of different nitrotoluenes and nitrobenzenes with good reproducibility and detection limits down to 0.25 ng/μL. The analysis of soil sample extracts from a former production site showed clear signals for 2,4,6-trinitrotoluene and the primary degradation products aminodinitrotoluenes. Furthermore, quantification gave results comparable to those obtained by conventional liquid chromatography-tandem mass spectrometry analysis. The MALDI-TOF MS method has a comparatively lower reproducibility, with relative standard deviations of 6% to 20% for multiple measurements of standard solutions and soil sample extracts. Nevertheless, a comparison of both methods revealed the advantages of MALDI-TOF MS analysis of explosive-contaminated areas with regard to costs, time, and handling. Finally, our MALDI-TOF MS method fulfills all the needs for high sample throughput and can therefore be a valuable screening tool for explosive-contaminated areas. Graphical abstract.

Fingerprinting of Nitroaromatic Explosives Realized by Aphen-functionalized Titanium Dioxide

Developing sensing materials for military explosives and improvised explosive precursors is of great significance to maintaining homeland security. 5-Nitro-1,10-phenanthroline (Aphen)-modified TiO₂ nanospheres are prepared though coordination interactions, which broaden the absorption band edge of TiO₂ and shift it to the visible region. A sensor array based on an individual TiO₂/Aphen sensor is constructed by regulating the excitation wavelength (365 nm, 450 nm, 550 nm). TiO₂/Aphen shows significant response to nitroaromatic explosives since the Aphen capped on the surface of TiO₂ can chemically recognize and absorb nitroaromatic explosives by the formation of the corresponding Meisenheimer complex. The photocatalytic mechanism is proved to be the primary sensing mechanism after anchoring nitroaromatic explosives to TiO₂. The fingerprint patterns obtained by combining kinetics and thermodynamics validated that the single TiO₂/Aphen sensor can identify at least six nitroaromatic explosives and improvised explosives within 8 s and the biggest response reaches 80%. Furthermore, the TiO₂/Aphen may allow the contactless detection of various explosives, which is of great significance to maintaining homeland security.

Ultra-high-resolution ion mobility spectrometry-current instrumentation, limitations, and future developments

With recent advances in ionization sources and instrumentation, ion mobility spectrometers (IMS) have transformed from a detector for chemical warfare agents and explosives to a widely used tool in analytical and bioanalytical applications. This increasing measurement task complexity requires higher and higher analytical performance and especially ultra-high resolution. In this review, we will discuss the currently used ion mobility spectrometers able to reach such ultra-high resolution, defined here as a resolving power greater than 200. These instruments are drift tube IMS, traveling wave IMS, trapped IMS, and field asymmetric or differential IMS. The basic operating principles and the resulting effects of experimental parameters on resolving power are explained and compared between the different instruments. This allows understanding the current limitations of resolving power and how ion mobility spectrometers may progress in the future. Graphical abstract.

In-depth structural analysis of glycans in the gas phase

Although there have been substantial improvements in glycan analysis over the past decade, the lack of both high-resolution and high-throughput methods hampers progress in glycomics. This perspective article highlights the current developments of liquid chromatography, mass spectrometry, ion-mobility spectrometry and cryogenic IR spectroscopy for glycan analysis and gives a critical insight to their individual strengths and limitations. Moreover, we discuss a novel concept in which ion mobility-mass spectrometry and cryogenic IR spectroscopy is combined in a single instrument such that datasets consisting of m/z, collision cross sections and IR fingerprints can be obtained. This multidimensional data will then be compared to a comprehensive reference library of intact glycans and their fragments to accurately identify unknown glycans on a high-throughput scale with minimal sample requirements. Due to the complementarity of the obtained information, this novel approach is highly diagnostic and also suitable for the identification of larger glycans; however, the workflow and instrumentation is straightforward enough to be implemented into a user-friendly setup.

Visual detection of peroxide-based explosives using novel mimetic Ag nanoparticle/ZnMOF nanocomposite

A simple and selective colorimetric method for the detection of perilous peroxide explosives was developed using the peroxidase mimetic activity of silver nanoparticles/flake-like zinc metal-organic framework nanocomposite (Ag@ZnMOF). The synthesis of Ag@ZnMOF contained the formation of silver nanoparticles (AgNPs) inside the fine pores of Zn metal-organic framework (ZnMOF). High reactive AgNPs as well as great surface area of MOFs provided a synergetic and high improved catalytic activity for the composite which was studied as a peroxidase mimic in hydrogen peroxide (H₂O₂)-based oxidations. The achieved system was used for detection of Triacetone triperoxide (TATP) as one of the most hazardous peroxide explosives. TATP was decomposed in an acidic condition to generate H₂O₂, which was then applied to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of the Ag@ZnMOF as a catalyst. This reaction led to the production of well-known blue colored charge transfer complex (oxTMB), which was recognized by the colorimetric technique. A linear relationship was obtained between the absorption intensity of the produced blue solution and the TATP concentration in the range of 0.4-15 mg L^-1, with a detection limit of 0.1 mg L^-1. A portable test kit was prepared using the same reagents for TATP measurement in real samples.

Recent Developments in Spectroscopic Techniques for the Detection of Explosives

Trace detection of explosives has been an ongoing challenge for decades and has become one of several critical problems in defense science; public safety; and global counter-terrorism. As a result, there is a growing interest in employing a wide variety of approaches to detect trace explosive residues. Spectroscopy-based techniques play an irreplaceable role for the detection of energetic substances due to the advantages of rapid, automatic, and non-contact. The present work provides a comprehensive review of the advances made over the past few years in the fields of the applications of terahertz (THz) spectroscopy; laser-induced breakdown spectroscopy (LIBS), Raman spectroscopy; and ion mobility spectrometry (IMS) for trace explosives detection. Furthermore, the advantages and limitations of various spectroscopy-based detection techniques are summarized. Finally, the future development for the detection of explosives is discussed.

A Comparison of ACQ, AIE and AEE-Based Polymers Loaded on Polyurethane Foams as Sensors for Explosives Detection

An aggregation-caused quenching (ACQ)-active polymer (PF), an aggregation-induced emission (AIE)-active polymer (PFTPE) and an aggregation-enhanced emission (AEE)-active polymer (PTTPE) were synthesized by tetraphenylethane (TPE), fluorene and thiophene moieties. Polyurethane (PU) foams modified by PF, PFTPE and PTTPE, namely PU-PF, PU-PFTPE and PU-PTTPE, using ultrasonication-assisted method have been prepared. A comparative study of PU-PF, PU-PFTPE and PU-PTTPE for detection explosives had been performed, and significant fluorescence quenching was observed with the introduction of PA solutions. The as-prepared PU-PF, PU-PFTPE and PU-PTTPE sensors exhibited a superior sensitivity for PA solutions with different concentrations. Remarkably, PU-PF gave a quenching efficiency of 96.2%, higher than 93.5% for PU-PFTPE and 86.7% for PU-PTTPE at a PA concentration of 180 µg·mL⁻¹ in methanol, which was attributed to the effective energy transfer from the fluorophore (PF) to the nitro explosive (PA). This suggested that some ACQ polymers, applied to detect explosives, could afford better performances than AIE or AEE polymers through modification of structures and selection of adequate carriers. At the same time, these chemical sensors can be recycled many times.

Structural characterization of small molecular ions by ion mobility mass spectrometry in nitrogen drift gas: improving the accuracy of trajectory method calculations

An accurate theoretical collision cross section calculation method in nitrogen was developed for reliable structural ion mobility mass spectrometry.

Fast Orthogonal Separation by Superposition of Time of Flight and Field Asymmetric Ion Mobility Spectrometry

Ion mobility spectrometry is a powerful and low-cost technique for the identification of chemical warfare agents, toxic chemicals, or explosives in air. Drift tube ion mobility spectrometers (DT-IMS) separate ions by the absolute value of their low field ion mobility, while field asymmetric ion mobility spectrometers (FAIMS) separate them by the change of their ion mobility at high fields. However, using one of these devices alone, some common and harmless substances show the same response as the hazardous target substances. In order to increase the selectivity, orthogonal data are required. Thus, in this work, we present for the first time an ambient pressure ion mobility spectrometer which is able to separate ions both by their differential and low field mobility, providing additional information for selectivity enhancement. This novel field asymmetric time of flight ion mobility spectrometer (FAT-IMS) allows high repetition rates and reaches limits of detection in the low ppb range common for DT-IMS. The device consists of a compact 44 mm drift tube with a tritium ionization source and a resolving power of 70. An increased separation of four substances with similar low field ion mobility is shown: phosgene (K₀ = 2.33 cm²/(V s)), 1,1,2-trichlorethane (K₀ = 2.31 cm²/(V s)), chlorine (K₀ = 2.24 cm²/(V s)), and nitrogen dioxide (K₀ = 2.25 cm²/(V s)). Furthermore, the behavior and limits of detection for acetonitrile, dimethyl methylphosphonate, diisopropyl methyl phosphonate in positive polarity and carbon dioxide, sulfur dioxide, hydrochloric acid, cyanogen chloride, and hydrogen cyanide in negative polarity are investigated.

Ion Mobility Spectrometry - High Resolution LTQ-Orbitrap Mass Spectrometry for Analysis of Homemade Explosives

The detailed chemical characterization of homemade explosives (HMEs) and other chemicals that can mimic or mask the presence of explosives is important for understanding and improving the performance of commercial instrumentation used for explosive detection. To that end, an atmospheric-pressure drift tube ion mobility spectrometry (IMS) instrument has been successfully coupled to a commercial tandem mass spectrometry (MS) system. The tandem MS system is comprised of a linear ion trap and a high resolution Orbitrap analyzer. This IMS-MS combination allows extensive characterization of threat chemical compounds, including HMEs, and complex real-world background chemicals that can interfere with detection. Here, the composition of ion species originating from a specific HME, erythritol tetranitrate, has been elucidated using accurate mass measurements, isotopic ratios, and tandem MS. Gated IMS-MS and high-resolution MS have been used to identify minor impurities that can be indicative of the HME source and/or synthesis route. Comparison between data obtained on the IMS/MS system and on commercial stand-alone IMS instruments used as explosive trace detectors (ETDs) has also been performed. Such analysis allows better signature assignments of threat compounds, modified detection algorithms, and improved overall ETD performance. Graphical Abstract ᅟ.