Optimized sampling protocols and the development of new wipe materials can be facilitated by standardized measurements of collection efficiency from wipe-sampling. Our approach for sampling trace explosives uses an automated device to control speed, force, and distance during wipe-sampling followed by extraction of collected 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 procedure for collecting traces of explosives, and standardized measurements of collection efficiency are needed to evaluate and optimize sampling protocols. The approach described here is designed to provide this measurement infrastructure, and controls most of the factors known to be relevant to wipe-sampling. Three critical factors (the applied force, travel distance, and travel speed) are controlled using an automated device. Test surfaces are chosen based on similarity to the screening environment, and the wipes can be made from any material considered for use in wipe-sampling. Particle samples of the explosive 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) are applied in a fixed location on the surface using a dry-transfer technique. The particle samples, recently developed to simulate residues made after handling explosives, are produced by inkjet printing of RDX solutions onto polytetrafluoroethylene (PTFE) substrates. Collection efficiency is measured by extracting collected explosive from the wipe, and then related to critical sampling factors and the selection of wipe material and test surface. These measurements are meant to guide the development of sampling protocols at screening venues, where speed and throughput are primary considerations.
Screening for traces of explosives at airports and other venues is a crucial step in the protection of the public against the threat of terrorism. Current practices are heavily focused on wipe-sampling of surface contamination from items handled by people, the people themselves, and items destined for cargo holds. Collection wipes are analyzed immediately in the field using commercial explosive trace detectors (ETDs) that are typically based on thermal desorption of collected solid material, with detection by ion mobility spectrometry1 or, more recently, mass spectrometry. The total amount of time available for sample collection and analysis is limited by the need to minimize the impact on passenger and cargo throughput. Sampling protocols must be optimized to collect the most sample in the shortest time, which requires standardized measurements that can weigh factors important to wipe collection.
Wipe-sampling is a general practice used for sampling surface contamination in health, environmental, and regulatory arenas2,3,4,5,6,7. Typical practices include holding the wipe by hand and sampling within a fixed area using a general coverage pattern. To increase control over wiping factors, including force and speed, we developed an instrumental approach to simulate wipe-sampling8, which has also been used to evaluate efficiencies in biological wipe-sampling9. A commercial device intended for adhesion measurements was adapted to the purpose; it includes a planar surface that moves at a fixed speed and distance under a stationary wipe. The force during sampling is controlled by a weight placed on top of the wipe holder. Surfaces of interest (fabrics, plastics, metals, etc.) are placed on the planar surface and a particle sample is placed in a fixed area on that surface. Our earlier work used polystyrene latex microspheres as the test particles, and particle size was shown to have an effect on particle collection, with larger (42 μm) spheres collected more efficiently than smaller (9 μm) spheres. We also found some improvement in collection efficiency with an increase in applied force during sampling, and observed differences in collection from different surfaces and for different wipes.
In subsequent work, we found that polystyrene particles could be redeposited by continuing to wipe the surface after collection, reducing the apparent collection efficiency10. This is an important consideration in trace explosives detection, as items sampled in screening scenarios, such as suitcases, can be large relative to the wipe collection area, requiring extensive travel distances to cover even a small percentage of the area of the item. Therefore, the travel distance on the surface after collection of the sample is an important factor, and field protocols typically define a maximum allowable distance covered prior to each analysis.
The shapes of microspheres are unlike real explosive particles11,12 and their chemical and physical properties may make them an inadequate simulant for explosives in wipe collection experiments. To address this limitation, we developed a test material containing the explosive 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) with a known particle size. The test material is made by inkjet printing nanoliter volumes of an RDX solution in arrays on Teflon substrates, with micrometer-sized solid deposits formed by evaporation at each point in the array. The deposits are transferred to the test surfaces by rubbing onto the surface, and the resultant particle sizes are defined by the starting deposit size. The desired particle diameters, as determined by analysis of fingerprints containing trace explosives, is 10 to 20 μm. Deposits can also be formed by pipetting microliter volumes of solution onto Teflon substrates13, but they will dry into a single large deposit, generally much larger that the desired range of particle sizes (for RDX masses relevant to this work). The inkjet RDX particle standard is used in this work along with quantitative extraction and analysis procedures to demonstrate the method for determining wipe collection efficiency. These measurements are designed to promote the development of new sampling wipes with better collection efficiencies, and support best practices in field sampling, including targeting surfaces that yield more sample, the appropriate force to use during collection, and the area to cover prior to analysis.
1. Apparatus
2. Material Selection and Instrumental Configuration
3. Wipe-sampling
4. Extraction and Analysis
5. Quality Control
6. Reporting
The ability of this protocol to accurately measure collection efficiency from a wide variety of possible test surfaces is dependent on the physical characteristics of the sample and its confinement to a specific area on the surface. If the sample is outside the defined area, it may not be fully encountered during wipe-sampling, and the collection efficiency will be artificially reduced. In addition, if the particles are significantly different from real particles expected in trace explosives residues, the collection efficiency measurements may not be representative. For these reasons, we recommend the use of a specific type of sample which has been demonstrated to generate appropriate particle size characteristics and to transfer to test surfaces within a confined area consistent with the protocol. Direct solution deposition to form particles is dependent upon the texture and composition of the surface and may not result in representative samples.
Results are given in Table 1 for a commercial ETD wipe 1 (meta-aramid polymer) given a 7.5 N force and a test surface representative of luggage (ballistic nylon woven fabric), for two different travel distances. The travel speed for all experiments is 50 mm/s, and the temperature and relative humidity during collection were 20 ± 2 °C and 40 ± 4% RH, respectively. The results show that a longer path length results in a reduced collection efficiency, which is expected due to redeposition of particles10. The 36 cm travel distance was achieved by using three separate passes on the surface, lifting the wipe at the end of each path and translating the surface to expose a fresh sampling path. This method of extending the travel distance requires that the wipe is lifted and placed down multiple times, and may produce different results compared to a continuous sample path. In screening scenarios, it is likely that the wipe is lifted and replaced many times on the item, so that this approach for extending the travel distance is appropriate.
The TEs of the RDX deposits from the PTFE substrate are high, as expected for this surface. Because the TEs are close to 100%, and there is quality assurance provided by visual inspection of the substrate (step 3.2.3), the measurement of TE could be eliminated without significantly affecting the CE results for this test surface. Other test surfaces may have lower or more variable TEs. The uncertainties in CE are within the range expected for this technique based on our experience to date. A second commercial ETD wipe (PTFE-coated woven fiberglass) generally has lower uncertainties than the meta-aramid polymer wipe, although it also has lower CEs in general (Figure 4). Our previous work with polystyrene microspheres8 is consistent with the lower collection efficiencies observed for ETD wipe 2 compared with wipe 1.
Figure 1. Schematic for wipe sampling apparatus (left and middle) with template for sample placement on the test surface (right). The footprint of the wipe collection area, a 30 mm diameter circle, is shown at the start and end of the sampling path. The wipe is placed on the test surface, travels directly through the sample location (typically 5 mm by 5 mm or smaller), and ends on the surface. The travel distance is from C, the location of the sample, to the end. Please click here to view a larger version of this figure.
Figure 2. Example wipe holder. The component parts for the custom holder are shown in the top left, and include two plastic components produced by 3D printing. These two components serve to clamp the wipe in place and are held together by two thumb screws. The attachable stainless steel weight is a solid rod with a threaded stud at one end for attachment to the holder. The eye bolt is for attachment of the restraining line.
Figure 3. Configuration of device. A yellow paper template is made to fit a 10 cm by 10 cm square steel test surface, with a cutout for the sampling path. The surface with template is placed on the moveable plane and adjusted until the restraining line is taut and centered over the sampling path. The template is used to configure the device and when transferring the test sample, but is not in place during wipe sampling. Please click here to view a larger version of this figure.
Figure 4. Results for synthetic leather test surface and a 36 cm travel distance, achieved by using 3 passes of 12 cm each, for two different wipes. Uncertainties in CE are given as 1 standard deviation.
Travel distance (cm) | Force (N) | TE (%) | RSD (%) | CE (%) | RSD (%) | n |
36* | 7.5 | 97.4 ± 2.1 | 2.2 | 11.7 ± 4.0 | 34.0 | 9 |
12 | 7.5 | 98.5 ± 1.3 | 1.3 | 22.6 ± 3.4 | 15.2 | 4 |
*3 passes of 12 cm each. |
Table 1. Results for commercial ETD wipe 1 and woven nylon fabric test surface for two different travel distances. Uncertainties in TE and CE are given as 1 standard deviation.
Sample collection is currently seen as the limiting step to improving detection capabilities in screening environments. Wipe-sampling is in need of measurement and standardization in order to evaluate current capabilities and support the development of new sampling materials and protocols. The approach described here is designed to provide this measurement infrastructure, and controls most of the factors known to be relevant to wipe-sampling. Previous work has shown that particle size, applied force during collection, test surface, sampling wipe, and travel distance are all important factors to control. The instrumental approach allows for control over the applied force, speed of wiping, and travel distance, and the values selected for these parameters should fall within the range expected in real situations. The force is applied by using a backing weight over the collection area, and care should be taken to achieve an even distribution of force in order to calculate the pressure.
Test surfaces are selected by the user and should relate to real screening environments to replicate the expected range of sampling challenges. Sampling wipes are selected in order to evaluate current practices and/or measure the efficacy of newly designed materials. In order to compare results among laboratories, the same test surfaces and wipes must be used, which can be done by specifying critical parameters or by sharing materials purchased from a single source. The ETD wipes are commercially available, but they are continually under production and different lots may have different properties. These are issues that can be addressed in the future by coordinated interlaboratory efforts.
The samples used to evaluate collection efficiency should match the physical characteristics expected in real situations. In the case of explosives, we have developed an approach for inkjet printing solutions of RDX to produce micrometer sized deposits which transfer efficiently to a range of substrates and produce particle deposits ranging in size from 1 to 40 μm. Alternatively, fixed-size polystyrene microspheres could be used. Pipetting RDX solutions onto Teflon substrates usually results in a single deposit that may be quite large, and the particle sizes after transfer to surface are unknown. This approach can be used for sampling studies if the particle sizes are characterized and shown to be reproducible.
This method was described for evaluating sampling efficiency for explosives, but can also be applied to environmental, nuclear, or forensic science applications. The samples, again, should be developed to match the real applications, and in the case of particle residues, the same type of dry transfer from Teflon would be appropriate. For surface contamination arising from sources other than particle transfer, such as condensation from vapor, different types of samples might be more appropriate.
A current limitation of the technique is the inability to change directions in sampling. The current configuration allows for movement in a single direction only, and therefore cannot control for directional changes that typically occur in field sampling of objects. We are currently addressing this need by incorporating x – y movement and allowing for specific sampling patterns to fill an area.
The authors have nothing to disclose.
Dr. Jayne Morrow and Dr. Sandra Da Silva, both from NIST, contributed to an earlier version of the method. The Science and Technology Directorate of the U.S. Department of Homeland Security sponsored the production of a portion of this material under Interagency Agreement HSHQPM-15-T-00050 with the National Institute of Standards and Technology (NIST).
Slip/Peel Tester | Imass | TL-2300 | replaces TL-2200 used in protocol |
3D printer | Stratasys | Connex500 | VeroWhite resin as printing material |
steel rod with thread | McMaster-Carr | 7786T14 | cut to size for desired weight, multiple online vendors available |
felt or rubber | backing material in wipe holder, multiple online vendors available | ||
PTFE substrate | SPI Supplies | 01426-AB | 1" wide Bytac Bench and Shelf protector, Al-backed, cut to size |
RDX solution | Cerilliant Analytical Reference Standards | ERR-001S | 1000 mg/mL in acetonitrile |
Inkjet printer | MicroFab Technologies, Inc. | jetlab4 xl-B | |
Isotopically tagged RDX | Cambridge Isotope Laboratories | CLM-3846-S | For internal analytical standard |
2 mL glass vial | Restek | 21140 /24670 | |
Methanol | Sigma Aldrich | 14262 | Chromasolv grade |
ETD wipe 1 | DSA Detection | DSW8055P | Ionscan 500 DT wipe |
ETD wipe 2 | DSA Detection | ST1318P | Itemiser DX wipe |
Ballistic nylon fabric | Seattle Fabrics | 1050 Denier Ballistics | |
Synthetic leather fabric | contact authors for sample |