Improving particle collection efficiency of sampling wipes used for trace chemical detection

Improvement of the particle collection efficiency of sampling wipes is desirable for optimizing the performance of many wipe-based chemical analysis techniques used for trace chemical screening applications. In this note, commercially available Teflon coated fiberglass and calendered Nomex sampling wipes were modified by mechanically scoring the wipe surface to produce topography that promoted enhanced and localized particle collection. Wipe surface modifications improved particle collection efficiency, relative to unmodified wipes, by factors of 3 to 13 depending on sampling conditions, wipe type, and surface sampled. Improvements were demonstrated for both model polystyrene latex microspheres and inkjet printed explosive particles. The modifications also concentrated particles into pre-defined locations on the wipe which can be engineered to ensure maximum overlap with the thermal desorber of a trace contraband detection system allowing for more effective analysis of collected trace residues.

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.

Discriminative potential of ion mobility spectrometry for the detection of fentanyl and fentanyl analogues relative to confounding environmental interferents

The opioid crisis and emergence of fentanyl, fentanyl analogues, and other synthetic opioids has highlighted the need for sensitive and robust detection for interdiction at screening points, notably vehicles at border crossings and packages at postal facilities. This work investigates the discriminative potential, sensitivity and specificity, of ion mobility spectrometry (IMS) for the detection of fentanyl and fifteen (15) fentanyl-related compounds (analogues, other opioids, and metabolites) relative to confounding environmental interferents. The environmental background interferent levels, frequency and intensity, were derived from over 10 000 screening samples collected from delivery vehicles entering a federal site. A receiver operating characteristic (ROC) curve methodology was employed to quantify the relationship between sensitivity and specificity for these target compounds on two instruments/configurations. These instrument configurations differed in desorption and drift tube temperatures, reactant ion dopant chemistry, and analysis time. This work identified reduced mobility areas of high interference that resulted in increased false positive rates (FPR), effectively reducing sensitivity (true positive rate: TPR) in those regions. Except for a few target compounds on either of the instruments that exhibited elevated FPRs, detection of fentanyl and fentanyl-related species was achieved at single to tens of nanograms with ≥90% TPR and ≤2% FPR. This work established the importance of systematic environmental background characterization at each specific screening setting in evaluating a platform's true performance.

Detection of Nonvolatile Inorganic Oxidizer-Based Explosives from Wipe Collections by Infrared Thermal Desorption-Direct Analysis in Real Time Mass Spectrometry

Infrared thermal desorption (IRTD) was coupled with direct analysis in real time mass spectrometry (DART-MS) for the detection of both inorganic and organic explosives from wipe collected samples. This platform generated discrete and rapid heating rates that allowed volatile and semivolatile organic explosives to thermally desorb at relatively lower temperatures, while still achieving elevated temperatures required to desorb nonvolatile inorganic oxidizer-based explosives. IRTD-DART-MS demonstrated the thermal desorption and detection of refractory potassium chlorate and potassium perchlorate oxidizers, compounds difficult to desorb with traditional moderate-temperature resistance-based thermal desorbers. Nanogram to sub-nanogram sensitivities were established for analysis of a range of organic and inorganic oxidizer-based explosive compounds, with further enhancement limited by the thermal properties of the most common commercial wipe materials. Detailed investigations and high-speed visualization revealed conduction from the heated glass-mica base plate as the dominant process for heating of the wipe and analyte materials, resulting in thermal desorption through boiling, aerosolization, and vaporization of samples. The thermal desorption and ionization characteristics of the IRTD-DART technique resulted in optimal sensitivity for the formation of nitrate adducts with both organic and inorganic species. The IRTD-DART-MS coupling and IRTD in general offer promising explosive detection capabilities to the defense, security, and law enforcement arenas.

Recent advances in ambient mass spectrometry of trace explosives

Ambient mass spectrometry has evolved rapidly over the past decade, yielding a plethora of platforms and demonstrating scientific advancements across a range of fields from biological imaging to rapid quality control. These techniques have enabled real-time detection of target analytes in an open environment with no sample preparation and can be coupled to any mass analyzer with an atmospheric pressure interface; capabilities of clear interest to the defense, customs and border control, transportation security, and forensic science communities. This review aims to showcase and critically discuss advances in ambient mass spectrometry for the trace detection of explosives.

Optical Properties of β-RDX Thin Films Deposited on Gold and Stainless Steel Substrates Calculated from Reflection-Absorption Infrared Spectra

The optical properties for crystalline films of the highly energetic material (HEM) hexahydro-1,3,5-trinitro-s-triazine, which is also known as RDX, deposited on gold (Au) and stainless steel (SS) substrates are presented. RDX has two important stable conformational polymorphs at room temperature: α-RDX and β-RDX. The optical properties obtained in the present work correspond to thin film samples of predominantly β-RDX polymorph. The infrared spectroscopic intensities measured showed significant differences in the β-RDX crystalline films deposited on the two substrates with respect to the calculated real part of refractive index. The β-RDX/Au crystalline films have a high dynamic response, which is characterized by the asymmetric stretching mode of the axial nitro groups, whereas for the β-RDX/SS crystalline films, the dynamic response was mediated by the -N-NO₂ symmetric stretch mode. This result provides an idea of how the electric field vector propagates through the β-RDX crystalline films deposited on the two substrates.

Rapid Analysis of Trace Drugs and Metabolites Using a Thermal Desorption DART-MS Configuration

The need to analyze trace narcotic samples rapidly for screening or confirmatory purposes is of increasing interest to the forensic, homeland security, and criminal justice sectors. This work presents a novel method for the detection and quantification of trace drugs and metabolites off of a swipe material using a thermal desorption direct analysis in real time mass spectrometry (TD-DART-MS) configuration. A variation on traditional DART, this configuration allows for desorption of the sample into a confined tube, completely independent of the DART source, allowing for more efficient and thermally precise analysis of material present on a swipe. Over thirty trace samples of narcotics, metabolites, and cutting agents deposited onto swipes were rapidly differentiated using this methodology. The non-optimized method led to sensitivities ranging from single nanograms to hundreds of picograms. Direct comparison to traditional DART with a subset of the samples highlighted an improvement in sensitivity by a factor of twenty to thirty and an increase in reproducibility sample to sample from approximately 45 % RSD to less than 15 % RSD. Rapid extraction-less quantification was also possible.

Detection and identification of sugar alcohol sweeteners by ion mobility spectrometry

The rapid and sensitive detection of sugar alcohol sweeteners was demonstrated using ion mobility spectrometry (IMS). IMS provides a valuable alternative in sensitivity, cost, and analysis speed between the lengthy gold-standard liquid chromatography-mass spectrometry (LC-MS) technique and rapid point-of-measurement disposable colorimetric sensors, for the Food and Nutrition industry's quality control and other "foodomics" area needs. The IMS response, characteristic signatures, and limits of detection for erythritol, pentaerythritol, xylitol, inositol, sorbitol, mannitol, and maltitol were evaluated using precise inkjet printed samples. IMS system parameters including desorption temperature, scan time, and swipe substrate material were examined and optimized, demonstrating a strong dependence on the physicochemical properties of the respective sugar alcohol. The desorption characteristics of each compound were found to dominate the system response and overall sensitivity. Sugar alcohol components of commercial products - chewing gum and a sweetener packet - were detected and identified using IMS. IMS is demonstrated to be an advantageous field deployable instrument, easily operated by non-technical personnel, and enabling sensitive point-of-measurement quality assurance for sugar alcohols.

Ion mobility spectrometry nuisance alarm threshold analysis for illicit narcotics based on environmental background and a ROC-curve approach

The discriminative potential of an ion mobility spectrometer (IMS) for trace detection of illicit narcotics relative to environmental background was investigated with a receiver operating characteristic (ROC) curve framework. The IMS response of cocaine, heroin, methamphetamine, 3,4-methylenedioxymethamphetamine (MDMA), and Δ(9)-tetrahydro-cannabinol (THC) was evaluated against environmental background levels derived from the screening of incoming delivery vehicles at a federal facility. Over 20 000 samples were collected over a multiyear period under two distinct sets of instrument operating conditions, a baseline mode and an increased desorption/drift tube temperature and sampling time mode. ROC curves provided a quantifiable representation of the interplay between sensitivity (true positive rate, TPR) and specificity (1 - false positive rate, FPR). A TPR of 90% and minimized FPR were targeted as the detection limits of IMS for the selected narcotics. MDMA, THC, and cocaine demonstrated single nanogram sensitivity at 90% TPR and <10% FPR, with improvements to both MDMA and cocaine in the elevated temperature/increased sampling mode. Detection limits in the tens of nanograms with poor specificity (FPR ≈ 20%) were observed for methamphetamine and heroin under baseline conditions. However, elevating the temperature reduced the background in the methamphetamine window, drastically improving its response (90% TPR and 3.8% FPR at 1 ng). On the contrary, the altered mode conditions increased the level of background for THC and heroin, partially offsetting observed enhancements to desorption. The presented framework demonstrated the significant effect environmental background distributions have on sensitivity and specificity.

Particle Fabrication Using Inkjet Printing onto Hydrophobic Surfaces for Optimization and Calibration of Trace Contraband Detection Sensors

A method has been developed to fabricate patterned arrays of micrometer-sized monodisperse solid particles of ammonium nitrate on hydrophobic silicon surfaces using inkjet printing. The method relies on dispensing one or more microdrops of a concentrated aqueous ammonium nitrate solution from a drop-on-demand (DOD) inkjet printer at specific locations on a silicon substrate rendered hydrophobic by a perfluorodecytrichlorosilane monolayer coating. The deposited liquid droplets form into the shape of a spherical shaped cap; during the evaporation process, a deposited liquid droplet maintains this geometry until it forms a solid micrometer sized particle. Arrays of solid particles are obtained by sequential translation of the printer stage. The use of DOD inkjet printing for fabrication of discrete particle arrays allows for precise control of particle characteristics (mass, diameter and height), as well as the particle number and spatial distribution on the substrate. The final mass of an individual particle is precisely determined by using gravimetric measurement of the average mass of solution ejected per microdrop. The primary application of this method is fabrication of test materials for the evaluation of spatially-resolved optical and mass spectrometry based sensors used for detecting particle residues of contraband materials, such as explosives or narcotics.