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

The overall goal of this wipe-sampling methodology is to provide a standardized way to measure collection efficiency. This method can help answer key questions in the surface wipe sampling field, such as how to best detect traces of explosives. The main advantage to this technique is that it minimizes operator variability compared to other techniques that measure sampling efficiency.

The test apparatus is designed with a moveable plane that secures a prepared test wipe. After preparing the apparatus for use, configure the wipe sampling procedure. First, move the plane to the starting position and place a test surface on the testing plane without adhering it.

Then, place a paper template flush to the edges of the test surface and adhere it there with tape. The template marks the start position in the sampling path. Next, move the surface and template back and forth on the plane until the wipe sits on the starting location when the retraining wire is taut.

Then, move the surface and template side to side until the restraining wire is centered down the travel path. The position of the surface on the plane is now correct. Mark this location.

And then, adhere the surface to the plane using double-stick tape. Now, program the instrument software control with the travel distance and travel speed. Then, initiate the movement of the plane.

Check if the wipe follows the sampling path for the entire travel distance, and if it does so smoothly. Sometimes, a test surface will be bumpy, or incur a lot of friction. While smooth motion is desirable, the most critical factor is that the wipe passes through the sample deposit location.

To potentially improve the smoothness of the travel, adjust the angle of the restraining wire. Lastly, measure the travel distance from the location of the sample deposit to the edge of travel. To begin, determine the sampling force needed for the test.

First, clean a test surface with a solvent such as ethanol, and allow it to dry. For fabrics, use pressurized air to blow the surface clean rather than a solvent. Then, put the surface on a scale.

Place a paper template on it, and secure the template at one corner. Next, check a PTFE particle sample for a complete array using glancing illumination, which provides light perpendicular to the particle sample to help visualize the particles. After verifying the array is complete, place the sample deposit-side down on the test surface within the marked sample area.

Now, using at least ten newtons of force, which will read approximately 1, 000 grams on the scale, use one finger to slide the particle sample within the sampling path to dry-transfer the particles. If the test surface has a striated surface, like the brushed steel, move the sample orthogonally to the striations, even if this is perpendicular to the sampling path. Now, using glancing illumination, see that the particle array has been removed.

If any particles remain, make sure that this is within the detection limits, or change the sampling force. Now perform the test. Place the test surface on the plane at the pre-defined location and adhere it there using double-sided tape.

Then, load the selected wipe into the holder and attach the appropriate weights for the selected force. Before proceeding, record the temperate and humidity near the experiment. Then attach the restraining wire to the wipe holder.

Place the holder wipe-side down onto the test surface and immediately initiate the testing process. Once complete, lift the wipe holder off of the test surface and remove the wipe from the holder. To extract and analyze any particle that remains on the PTFE transfer substrate, flow one milliliter of methanol containing an internal standard over the surface and into a two milliliter glass vial.

Here, isotope-tagged RDX is the internal standard. Then, quantify the RDX in the extract using electrospray ionization mass spectrometetry to determine the mass of RDX that was not transferred. Do the same with a sum unused substrates to establish a baseline.

To determine the mass of RDX collected on the wipe, cut the wipe material down to the 30 millimeter diameter circular collection area, and place this inside a two milliliter glass vial. To the vial, add one milliliter of methanol containing the internal standard. Then, cap the vial and vortex it at 10, 000 rotations per minute for 30 seconds.

Within an hour, quantify the RDX levels in the solution before the RDX gets re-absorbed by the wipe. This protocol was performed using commercial EDT wipes made of meta-aramid polymer at 7.5 newtons. The test surface was similar to that of luggage.

Two types of tests were performed. Either the same area was wiped once, or it was wiped three times. The longer travel distance resulted in a lower collection efficiency, likely due to redeposition of the sample particles.

By comparison, using the multiple-pass sampling method, a commercial ETD wipe made of PTFE-coated woven fiberglass had a more consistent collection efficiency than the meta-aramid polymer wipe, but it collected less test sample. After watching this video, you should have a good understanding of how to measure the collection efficiency of your sample testing wipe, using a standardized method.