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

1. Apparatus

1. Select or fabricate a device with a moveable plane (see schematic in Figure 1).
   NOTE: Here, use a TL-slip/peel tester but this device has features, such as the measurement of frictional forces, that are not necessary to this method and may increase the cost over a simpler device.
   1. Select plane dimensions with a minimum length of 15 cm by a minimum width of 3 cm. The length controls the maximum travel distance for a single sampling path (Figure 1).
   2. Choose a plane that moves at defined speeds from 50-400 mm/s with a repeatability at the chosen speed of ± 10%. The range is based on data from a volunteer population performing wiping experiments.¹⁰
2. Fabricate a wipe holder (Figure 2). CAD drawings available in supplementary information.
   1. Include a clamping mechanism to hold the wipe and expose a circular collection area 30 mm in diameter. The collection area is based on typical ETDs where the desorber area in the instrument defines the allowable collection area.
   2. Include a removable soft backing behind the collection area to provide an even distribution of force. It is removable in case of contamination that cannot otherwise be removed by cleaning. The backing can be made of sponge rubber foam, as described in ASTM D1894¹⁴, or another soft material, such as felt, cut to size.
      NOTE: The properties of the sponge rubber described in ASTM D1894 include a required softness measured as the ability to compress the foam 25% when using a pressure of 85 ± 15 kPa (12.5 ± 2.5 psi). We evaluate the effectiveness of any backing material to evenly distribute force by mapping the pressure using a force-sensitive film^8,10. The pressure over the entire collection area (30 mm diameter circle) can be calculated based on the total force only for uniform distributions of force.
   3. Include attachable weights to provide total forces (combined weight of holder and weight) on the wipe ranging from approximately 1 to 15 N (approximately 100 to 1,500 gram-force). Set the minimum force by the weight of the wipe holder. The force range is based on data from a volunteer population performing wiping experiments, where the average force exerted was 7 N.¹⁰ The maximum force will be limited by the ability to ensure smooth movement over the surface during travel.
   4. Include an eye-hook or similar device for attaching a restraining wire. The wire restrains the wipe holder from moving during the motion of the plane. The wire should be parallel to the surface or be at a slight positive angle during movement of the plane.

2. Material Selection and Instrumental Configuration

1. Select test surfaces based on similarity to the screening environment. Choices may include synthetic leather, metal, plastic, cardboard, fabric, etc. Use surfaces that are flat and fit on the plane of the testing device. Very flexible surfaces may need to be backed by a rigid surface to prevent movement during wipe-sampling.
   1. Cut surfaces to size if necessary and clean with solvent (ethanol or methanol are generally suitable) and/or by blowing off particles with pressurized air. Clean surfaces immediately prior to the conduct of wipe-sampling.
2. Use wipes made from any material considered for use in wipe-sampling. They must have minimum dimensions to cover the 30 mm diameter circular collection area on the wipe holder and be clamped in place.
   1. Cut wipes to size if necessary to fit into wipe holder.
   2. Test a subset of wipes prior to use following procedures described in section 4 to determine extraction efficiency and blank contamination levels with respect to RDX or other contaminants that could interfere with the analysis.
3. Prepare RDX particle standards by inkjet printing arrays onto polytetrafluoroethylene (PTFE) substrates. Their fabrication and use is described in detail in the publication under review. 200 ng of RDX is a minimum amount, given typical analytical detection limits of the quantification technique of approximately 5 ng/mL, and the maximum amount, based on reported amounts of RDX in fingerprints, should be a few micrograms. The samples can be held under refrigeration for up to 30 days after printing.
   NOTE: The particles derived from these standards range in size from 1 μm to 40 μm in diameter, simulating well the particles in fingerprints made after handling plastic explosives¹². The area distribution of the transferred sample is dependent on the printed array size, but will typically be within a 5 mm by 5 mm area; well within the 30 mm diameter circular sampling area. This protocol uses RDX particle standards produced by inkjet printing that have a known particle size distribution and a known area distribution when transferred onto the test surface. Other dry-transfer samples¹³ can be used if the same parameters are known. Samples produced by direct solution deposition onto the test surfaces are not recommended.
4. Configure and test device for wipe sampling.
   1. Move the plane to the starting position (Figure 1).
   2. Referring to Figure 3, place a test surface, without adhering it, on the device plane.
      1. Prepare a template from paper, as shown in the schematic in Figure 1, and place it flush to the edges of the test surface, as shown in Figure 3. The template marks the location of the wipe starting position and the location and length of the sampling path.
      2. Adhere the template to the surface using tape. Move the surface, with the template, back and forth on the plane until the wipe sits on the starting location when the restraining wire is taut. Move the surface, with the template, side-to-side on the plane, until the restraining wire is centered down the travel path.
   3. Mark the location on the plane where the substrate will be adhered, as determined above. Adhere surface, with the template, to the plane using double-stick tape.
   4. Use software controls for the instrument to input travel distance and travel speed.
   5. Initiate movement of the plane to test that the wipe follows the sampling path for the entire travel distance, and to ensure smooth travel.
      NOTE: Some combinations of wipe and test surface may result in a high level of friction during motion. Skipping and lifting of the wipe during motion is undesirable. The wipe may deviate from the sampling path for long travel distances or for some combinations of wipe and test surface. The most critical factor is to ensure that the wipe passes through the sample deposit location. Adjusting the angle of the restraining wire may help alleviate the problem.
   6. Measure the travel distance from the location of the sample deposit to the end of travel.
      NOTE: If the sample is placed near the start of the sampling path, as in Figure 1, the travel distance will be at its maximum for the test surface length. Smaller travel distances can be selected by limiting the total length of travel, or by moving the location of the sample.

3. Wipe-sampling

1. Clean test surface and allow to dry.
2. Place the surface on a top loading balance and place a paper template on top (see 2.4.2), holding it in place at a corner.
3. Take a particle sample in hand and use glancing illumination to check that the array is complete.
4. Place a finger behind the deposit and put the PTFE substrate deposit-side down on the test surface, with the deposit inside the marked sample area. Translate the PTFE substrate along the test surface within the sampling path using a minimum of 10 N (observe the weight on the balance to equal or exceed 1,000 gram-force) to dry-transfer the particles.
   1. For test surfaces with a striated texture, translate the PTFE substrate along the surface orthogonal to the striations, even if this is orthogonal to the sampling path.
5. Use glancing illumination to inspect the PTFE substrate after dry-transfer to ensure the removal of the array. If array elements remain, choose whether to continue or discard the experiment and start again. The choice will depend on the detection limits from extraction and analysis, and the minimum mass needed on the surface.
6. Reserve the PTFE substrate for extraction and determination of transfer efficiency.
7. Place the test surface on the plane in the previously defined location and adhere it to the plane using double stick tape or equivalent.
8. Load the selected wipe into the holder and attach the appropriate weights for the selected force.
9. Record the temperature and humidity near the experiment to within ± 2 °C and ± 5% RH.
10. Attach the restraining wire to the wipe holder and place the holder wipe-side down on the test surface. Immediately initiate movement of the plane. Lift the wipe holder off of the test surface after movement ceases and remove the wipe from the holder.

4. Extraction and Analysis

1. Extract and analyze any RDX remaining on PTFE transfer substrate.
   1. Flow 1 mL of methanol containing an internal standard over the surface and into a 2 mL glass vial. Use an isotopically tagged RDX as an internal standard. A suitable analogue with similar chemical structure and physical properties can be used if an isotopically tagged standard is not obtainable. For RDX, an additional acceptable internal standard would be cyclotetramethylenetetranitramine (HMX). The method of preparing the PTFE transfer substrate suggests wrapping the PTFE around the paper to minimize solvent loss to the paper.
   2. Quantify solutions using previously developed analytical protocol. The protocol used in this study is based on electrospray ionization mass spectrometry (ESI-MS).
2. Extract and analyze RDX collected on the wipe.
   1. Cut the wipe material down to the 30 mm diameter circular collection area and place the cut portion inside a 2 mL glass vial. Add 1 mL of methanol containing the internal standard.
   2. Cap the vial and vortex at 10,000 rpm for 30 s.
   3. Quantify solutions as rapidly as possible to prevent re-adsorption of the analyte and/or internal standard onto the wipe material. Complete analyses within an hour of extracting whenever possible.
3. Extract and analyze a subset of unused RDX particle standards on PTFE to obtain a baseline starting mass in the same manner as 4.1.
4. Calculate transfer efficiency (TE) from the PTFE substrate to determine the mass of RDX deposited on the surface.
   
   where RDX_Initial is the average deposited mass of the extracted baseline samples (step 4.3) and RDX_Remain. is the mass of RDX remaining on the PTFE substrate after dry-transfer (step 4.1).
5. Calculate collection efficiency (CE) of the wipe relative to the deposited mass on the surface.
   
   where RDX_Wipe is the mass of RDX extracted from the wipe (step 4.2).

5. Quality Control

1. Perform a minimum of 3 replicates. Variability in CE can be relatively high and 10 or more replicates may be needed to determine the significance of various sampling factors.
   1. Clean and reuse test surfaces for replicates if blank testing indicates the efficacy of the cleaning procedure. Solvents may affect the surface texture, and any procedure requiring their use must apply to all replicates.
   2. Use fresh wipes for each replicate.
2. Measure process blanks by following the same procedure but with blank PTFE substrates.

6. Reporting

1. Calculate and report the average and standard deviation of TE and CE for (n) replicates.
2. Report 1) type of wipe, 2) test surface, 3) force, 4) speed, 5) travel distance, 6) temperature, and 7) humidity.
3. Report the type and details of sample used. If samples have been prepared other than by inkjet printing, report estimated particle size and reproducibility.
4. Report any other factors, controlled or observed.