Standardized Method for Measuring Collection Efficiency from Wipe-sampling of Trace Explosives

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common ...

National Institute of Standards and Technology

The sizes of explosives particles in fingerprint residues produced from C-4 and Semtex-1A were investigated with respect to a fragmentation model. Particles produced by crushing crystals of RDX and PETN were sized by using scanning electron microscopy, combined with image analysis, and polarized light microscopy was used for imaging and identifying explosive particles in fingerprint residues. Crystals of RDX and PETN fragment in a manner that concentrates mass in the largest particles of the population, which is common for a fragmentation process. Based on the fingerprints studied, the particle size to target for improving mass detection in fingerprint residues by ion mobility spectrometry (IMS) is > or = 10 microm in diameter. Although particles smaller than 10 microm in diameter have a higher frequency, they constitute < 20% of the total mass. Efforts to improve collection efficiency of explosives particles for detection by IMS, or other techniques, must take into consideration that the mass may be concentrated in a relatively few particles that may not be homogeneously distributed over the fingerprint area. These results are based on plastic-bonded explosives such as C-4 that contain relatively large crystals of explosive, where fragmentation is the main process leading to the presence of particles in the fingerprint residues.

Particle characteristics of trace high explosives: RDX and PETN.

We describe gravimetric methods for measuring the mass of droplets generated by a drop-on-demand (DOD) microdispenser. Droplets are deposited, either continuously at a known frequency or as a burst of known number, into a cylinder positioned on a submicrogram balance. Mass measurements are acquired precisely by computer, and results are corrected for evaporation. Capabilities are demonstrated using isobutyl alcohol droplets. For ejection rates greater than 100 Hz, the repeatability of droplet mass measurements was 0.2%, while the combined relative standard uncertainty (u(c)) was 0.9%. When bursts of droplets were dispensed, the limit of quantitation was 72 microg (1490 droplets) with u(c) = 1.0%. Individual droplet size in a burst was evaluated by high-speed videography. Diameters were consistent from the tenth droplet onward, and the mass of an individual droplet was best estimated by the average droplet mass with a combined uncertainty of about 1%. Diameters of the first several droplets were anomalous, but their contribution was accounted for when dispensing bursts. Above the limits of quantitation, the gravimetric methods provided statistically equivalent results and permit detailed study of operational factors that influence droplet mass during dispensing, including the development of reliable microassays and standard materials using DOD technologies.

Inkjet metrology: high-accuracy mass measurements of microdroplets produced by a drop-on-demand dispenser.

A method is described to perform automated mapping of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) particles in C-4 fingerprints. The method employs polarized light microscopy and image analysis to map the entire fingerprint and the distribution of RDX particles. This method can be used to evaluate a large number of fingerprints to aid in the development of threat libraries that can be used to determine performance requirements of explosive trace detectors. A series of 50 C-4 fingerprints were characterized, and results show that the number of particles varies significantly from print to print, and within a print. The particle size distributions can be used to estimate the mass of RDX in the fingerprint. These estimates were found to be within +/-26% relative of the results obtained from dissolution gas chromatography/micro-electron capture detection for four of six prints, which is quite encouraging for a particle counting approach. By evaluating the average mass and frequency of particles with respect to size for this series of fingerprints, we conclude that particles 10-20 microm in diameter could be targeted to improve detection of traces of C-4 explosives.

Automated mapping of explosives particles in composition C-4 fingerprints.

Ion mobility spectrometry (IMS) has been used for trace analysis of illicit drugs, but it can also provide reliable qualitative analysis of bulk forensic drug items, despite the complexity of these samples. The drug/drug and drug/excipient combinations representing over 80% of the samples reported by state and federal forensic laboratories over the past 7 years were compiled from reports of the National Forensic Laboratory Information System (NFLIS). From this set of materials, IMS detection windows were set for eight controlled substances, including methamphetamine, 3,4-methylenedioxymethamphetamine hydrochloride (MDMA), cocaine, heroin, fentanyl, hydrocodone, oxycodone, and alprazolam. The reduced mobilities of the eight controlled substances were measured over an extended period of time to determine variability with respect to the size of the detection windows. Uncertainties in reduced mobilities smaller than 0.001 cm(2)V(-1)s(-1) were obtained, and detection windows were set to between ±0.003 and ±0.005 cm(2)V(-1)s(-1). Reduced mobilities are instrument and operating condition dependent, and must be determined for each instrument. Peak overlaps are observed in the drug/drug combinations, but at least one controlled substance can be detected in each mixture. Excipient concentrations must be quite high (>75 wt%) in binary mixtures to interfere with the detection of the controlled substance. IMS can be used to identify many of the excipients, and can detect multiple (for these samples, as many as 4) substances in complex samples. Over-the-counter (OTC) tablet medications for cold, flu, and allergy relief can be distinguished from tablets containing controlled substances. Bulk materials, including tablets, are sampled simply by using a fine probe to restrict the amount of material transferred to the IMS substrate. IMS represents a distinct advantage over color tests for field analysis of illicit drugs, except in the case of cannabis/THC samples.

Reliability of ion mobility spectrometry for qualitative analysis of complex, multicomponent illicit drug samples.

We report highly reproducible gravimetric and optical measurements of microdroplets that lend insights into the fundamentals of drop-on-demand (DOD) printing. Baseline fluidic pressure within the DOD dispenser was controlled to within 0.02 hPa, enabling long-term stability in dispensed droplet mass with observed variations near 1% (RSD) for isobutanol. The gravimetric measurements were sensitive enough to detect and avoid unwanted effects from air bubbles within the dispenser. The gravimetric and optical velocity measurements enabled consistent determination of droplet kinetic energy that governed baseline behavior across the operational variables. Mass and velocity were influenced in a nonlinear manner by the frequency of droplet ejection, the fluidic pressure within the dispensing device, and the number of droplets dispensed in a burst. Resolved effects were attributable to several possible mechanisms including acoustic resonances, energy partitioning from systematic orifice refill dynamics, pressure wavelets created within the dispenser cavity during "first-drop" formation, and residual ring-down after last-drop emergence.

Inkjet metrology II: resolved effects of ejection frequency, fluidic pressure, and droplet number on reproducible drop-on-demand dispensing.

Atmospheric pressure glow discharge mass spectrometry was used to characterize the active ingredients in pharmaceutical over-the-counter (OTC) drug formulations (Tylenol Allergy, Alka-Seltzer Plus Nighttime, Sudafed, Aleve and Mucinex DM) and drugs of abuse (crack cocaine, methamphetamine, MDMA (ecstasy) and hydrocodone). Material was desorbed and directly ionized under atmospheric conditions by allowing the substance to come in direct contact with the plasma followed by mass spectrometric detection. With this technique, controlled substances and OTC medications were readily distinguished from one another. Characteristic mass spectra were identified for the active ingredients in the OTC and drugs of abuse. Importantly, all drug compounds studied here, both OTC and illicit, demonstrated signals for either molecular ions or protonated molecules as well as fragmentation patterns that are readily identified in the National Institute of Standards and Technology (NIST) electron ionization (EI) mass spectral library. It is believed that this technique holds promise for forensic and law enforcement communities for real-time atmospheric analysis of drugs with database-searchable spectra of controlled substances.

Atmospheric identification of active ingredients in over-the-counter pharmaceuticals and drugs of abuse by atmospheric pressure glow discharge mass spectrometry (APGD-MS).

Ion mobility spectrometry has found widespread use for the detection of explosives and illicit drugs. The technique offers rapid results with high sensitivity and little sample preparation. As such, it is well suited for field deployed screening settings. Here the response of ion mobility spectrometers for three drug-facilitated sexual assault (DFSA) agents - flunitrazepam, ketamine, and MDMA - and related metabolites has been studied in the presence of a simulated sweat. While all three DFSA agents present certain challenges for qualitative identification, IMS can provide useful information to guide the early treatment and investigation of sexual assault cases. Used as a presumptive test, the identification of DFSA agents would later require confirmatory analysis by other techniques.

Measurement of drug facilitated sexual assault agents in simulated sweat by ion mobility spectrometry.

Surface wipe sampling, as performed by human operators, is widely used in environmental monitoring and currently subject to a high degree of variability. Two factors relating to sampling efficiency, applied force and area coverage, have not previously been amenable to measurement during wipe sampling. A force-sensing resistor (FSR) array film is used for the first time in this study to measure these factors and provide feedback for operator training, with the goal of reducing inter-operator variability. A volunteer population of 20 performed wipe sampling within a 10 cm by 10 cm templated area under an instruction to use "firm" force. The applied force for the population ranged from 3 Newtons (N) (306 g-force) to 14 N, with intra-operator variability within each experiment of 20% RSD on average. A statistically significant difference in force among operators was determined by a single-factor analysis of variance (ANOVA) (p-value = 1.6 × 10 (-7)). An average force of 7.4 N (SD = 2.9 N) was determined for the population, and an operator who fell outside the mean was trained with feedback from the FSR, achieving an average force of 6.8 N (SD = 1.8 N). Retention of the training was demonstrated by testing after 6 days resulting in an average force of 6.8 N (SD = 0.8 N). Direct measurements of the sampling path can be made from the FSR using center-of-force (COF) data. Operators were evaluated against a path designed to fill the 10 cm by 10 cm area with 98% coverage. With training, operators achieved coverage of up to 96% using only guidelines marked on the paper template to define the sampling path. Fluorescent polymer microspheres were used to evaluate collection efficiency when performing dry wipe sampling under controlled conditions of force and area coverage. Over the range of forces measured earlier for the population, the collection efficiency varied from 10% to 50%, with an approximate increase of 4% in collection efficiency with each additional 1 N of force. The locations of particles remaining on the surfaces after sampling were compared with the COFs. A high percentage of particles (up to 45%) were redeposited to the surface after collection, indicating that sampling patterns should be designed to minimize the length of the sampling path while still providing maximum area coverage.

Use of force-sensing array films to improve surface wipe sampling.

Errors in the double variation teclinique of refractive index measurement are analyzed using a new approach. The ability to measure matching wavelength is characterized, along with the effect on the calculated refractive index. Refractive index accuracy and precision are very dependent on the specifics of each calibration set, particularly the difference in dispersion between the liquid and solid. Our best precision (±1 or 2×10(-4)) is attained only when the difference in dispersion between liquid and solid is small, and is dependent on an individual operator's ability to perceive changes in relief. This precision is impossible to achieve for the other glass/liquid combinations, where we are limited by a precision of approximately 1 nm in the selection of matching wavelength. A bias in the measurement of matching wavelength exists that affects the accuracy of the calculated refractive indices. The magnitude of the bias appears to be controlled by the bandpass of the graded interference filter. The errors in refractive index using a graded interference filter with a bandpass of 30 nm FWHM (full width at half maximum intensity) are an order of magnitude larger than the errors using a filter with a bandpass of 15 nm FWHM.

Jennifer R. Verkouteren

Materials Measurement Science Division

National Institute of Standards and Technology

Standardized Method for Measuring Collection Efficiency from Wipe-sampling of Trace Explosives

Jennifer R. Verkouteren

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Materials Measurement Science Division

National Institute of Standards and Technology

Standardized Method for Measuring Collection Efficiency from Wipe-sampling of Trace Explosives

Materials Measurement Science Division