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Detecting Explosives by Swiping or Sniffing? Does it matter?


AI Publication - July 2010
By, American Innovations, Inc.

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In our last article to you we asked the question, “How do you buy the right Explosives Detector?” and gave you 8 points to consider when answering this question. This time around we ask the question, “What sampling method is better, Particulate (Swipe) sampling or Vapor (Sniffer) sampling?” Lots of people assume that if you can wave a detection device around a room or car and it doesn’t alarm, there were no explosives present. Others believe you can leave a detector sitting on a desk as people and items walk by, and if it doesn’t alarm, everyone is safe. Quick and simple, right? But is that necessarily true?

Getting to that answer raises a lot of questions: Which of these sampling methods is better for the job I do? Is one ‘better’ than the other? Is one more sensitive than the other? Why would I ever want to touch a bomb? Isn’t it too intrusive to touch a person or their clothing? How much ‘vapor’ does an explosive give off and can it be found by a ‘sniffer’? And, why do most “sniffers” offer swipes with their equipment as well? Isn’t that contradictory?

Once again, this newsletter will not tell you what to buy, but it will hopefully offer enough insight and information so that you can make an informed decision on what to buy.

First, what is the goal? The goal is to collect and analyze microscopic amounts of a material to know whether the surface sampled contains material which is explosive in nature.

Second, let’s define the terms: Vapor – this describes the gas-phase molecules that are emitted from a solid or a liquid. All solids and liquids emit vapor in real world environments, some more readily than others. The concentration of these vapor phase molecules is greatly affected by the ‘vapor pressure’ of that material, meaning how likely that material is to give off particles into the air, which is why you don’t often see solid items ‘evaporate’ over time. Other effects are the amount of time that material has been present in the air around it, its packaging, air temperature, air circulation, humidity, etc. Particulate – these are microscopic particles of the actual explosive material that adhere to surfaces (either directly, such as on hands from handling an explosive; or indirectly, such as doorknobs, clothing, cell phones, IDs, or other items handled by someone who has been in contact with explosives and transferred some of the material to these other objects).

Now, let’s look at the problem of detection. Vapor detection is the process of collecting and detecting air-borne, gas-phase molecules of explosives material without contacting the surface of the sampled item. Most explosives, however, do not evaporate very readily. This is what ‘vapor pressure’ means: It is the MAXIMUM concentration of a material in the air at room temperature. HIGH vapor pressure explosives, such as EGDN or TATP have a vapor pressure of about 1 part per million (ppm). This means that at maximum concentration (or saturation), only one molecule out of every million present will be of the explosives material. MEDIUM vapor pressure explosives, such as Ammonium Nitrate and TNT have a vapor pressure of about 1 part per billion (ppb). LOW vapor pressure explosives, such as RDX, HMX and PETN have a vapor pressure of about 1 part per trillion (ppt). Given that most explosives fall into the medium to low vapor pressure category, and that their vapor pressures are measured at the MAXIMUM concentration in still air at room temperature (which is never found in the field), AND that these measurements are for pure explosives materials -vapor pressures for mixtures of these materials, such as C4 (RDX in a plasticizer, etc.) is even lower-, you can see how difficult it is to capture explosives materials in the air for detection and analysis.

Particulate detection is the process of collecting and detecting microscopic solid explosives material. Since this collection method is of actual material on surfaces, the vapor pressure of the material is no longer relevant. Explosive material in general tends to be sticky, and anyone handling explosives will quickly collect large amounts on his hands and clothing. This material will readily be transferred to every other surface that person touches, spreading the contamination around. Although careful handling (wearing gloves, etc.) can reduce the spread of particulates, reducing it to zero is virtually impossible, and the average bomb maker will have neither the time, patience nor expertise to accomplish this. Therefore, particulate sampling has wide applications. Particulate contamination is usually accomplished by using a sample collection swipe to wipe the surface to be sampled. This method works well with luggage, vehicles, packages, etc., but is often not used to sample skin or clothing, since there are some who would consider this too intrusive.

So, which is better? Consider this: Although it is not possible to make perfect generalizations about how much contamination is contained in a fingerprint, a typical fingerprint exposed to explosives can contain many particles, on the order of about 100 micrograms. When compared to most medium and low vapor pressure explosives, at room temperature, this means there would be present in a fingerprint somewhere between 1,000 and 1,000,000 times more explosives material than would be present in a liter of air saturated with vapor from the same explosive. When Probability of Detection is considered, for most explosives, detection is therefore usually based on particulate detection. This answers the question of why virtually every “sniffer” device available today provides sample collection swipes and trains on how to use them to achieve the highest probability of detection.

Does this mean I have to ‘swipe’ a bomb in order to determine if it’s real? Of course not. Remember, trace explosives detectors are used to indicate the possible presence of explosives materials or a possible explosive device. They are also used to help detect and identify the persons, buildings, tools, vehicles and items used in the storage, manufacture, assembly, transport and emplacement of bombs or IEDs. Once there is any evidence or suspicion that a potential explosive device is present, the detection mission gives way to Render-Safe-Protocols conducted by properly trained and qualified Explosives Experts. In this case, no practical requirement would involve either swiping or getting close enough to “sniff” the device.

Isn’t wiping a person or their clothing too intrusive to be practical? Some would say so, and in some cultures this is more sensitive than others. However, consider that every time you pass through an airport security checkpoint today, you must remove your jackets, sweaters, etc., remove your shoes, take out your laptops, empty your pockets, throw out your water bottles and large toothpaste tubes, and in some cases be scanned by a full body scanner which sees beneath your clothing, only to recollect all your possessions and try to put it all on and away again along with a crush of other travelers doing the same thing……having your hand swiped and analyzed in 30 seconds doesn’t seem all that bad. Certainly it is no more intrusive.

Are there some situations where vapor sampling will be preferred? Yes. Is the same true for Particulate sampling? Yes. The questions need to be carefully considered, the probability of detection factored, and the nature of whether your inspection and sampling methods allows for physical contact determined. Once you’ve done this, you will have taken another great step toward choosing the right explosives detector.

SNEAK PEEK: In our next article, we’ll take a look at the question, “What one simple step can any public venue take to mitigate the threat of Terrorist IEDs?” Stay tuned.

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