Disagree. These detectors work by detecting molecules and portions of molecules, based on their weight and charge. A and B and C are separated by weight and electronic charge in the analysis chamber, so they are detected individually. The physical mixture of components cannot cause a false positive, because the detection technology functions by separation of different molecules.

The only way what you're saying could be true was if there was a chemical reaction between A and B that produced C, where C was explosive but A and B were not. And I assure you, those reactions are MUCH too difficlut to occur between trace levels of reagents in the open air on a cloth swab. To make an explosive out of ingredients that would not be detected as explosives themselves, you need to convert less energetic molecules into high-energy molecules, and that usually requires high temperature, high pressure, catalysts, appropriate solvents, etc.

Agreed. However, substances not present in sufficient quantity in a single swipe can build up to detectable levels over several swipes.

This.

And, there is an additional concern. I have been told that alarms are generated on quantities of specific chemicals. In addition, there is an alarm for a lower level of two chemicals that may be found in combination in certain explosives. Crude example: Nitrates alone may alarm at a 10. Toluene my alarm alone at 8. However, nitrates and toluene together may alarm at lower levels, 6/4. (All numbers are made up just to create a scenario for example.)

Let's suppose a series of passengers using clean swabs would have the following:

Passenger A: Nitrate 7, Toluene 0
Passenger B: Nitrate 1, Toluene 0
Passenger C: Nitrate 1, Toluene 5

None of these would alarm. However, if consecutive swabs are used:

Passenger A: Nitrate 7, Toluene 0
Passenger B: Nitrate 8, Toluene 0
Passenger C: Nitrate 9, Toluene 5

Absolute levels are under alarm for individual chemicals. Put the two together and an alarm is generated.

I often am around toluene in my work. It is a primary component of TNT, Trinitrotoluene.

I alarm periodically on the ETD and was once told that it was the combination that was the problem. So, I scrub well before going through the CP after being around toluene. It may or may not help. I also noticed a customer using a 35% hydrogen peroxide solution. Oh well. I cleared fine that time.

Finally, I do industrial testing. The multiple use of sterile, calibrated swabs is stupid on top of being prohibited practice for this type of testing. I would not want to be in a line of a sobriety checkpoint in which the blood alcohol level was added from test to test and then I hit the .08. Same thing. Just as stupid.

Heck, all you need is two guys in a row who fertilized their lawns before heading to the airport.

look straight ahead and keep walking
 

Be careful spending too much time watching TSA clerks do their jobs. They get suspicious! :D


http://http://www.flyertalk.com/forum/checkpoints-borders-policy-debate/1405341-tsa-caused-me-miss-my-flight.html

I do a fair amount of Industrial Hygiene sampling as well. The fact that TSA clerks are reusing sampling media proves that there is no scientific point to what they're doing, and it's merely for appearance. When being swabbed, I usually ask what their qualifications are to perform this test (ie, are you a Certified Industrial Hygienist, do you have any educational background in chemistry or toxicology, what chemical are you testing for and what is the PEL/TLV for that substance, when was the machine last calibrated, etc). I just get blank stares.

Note that the liquid bomb plot that the American and British governments fabricated to start the war on liquids involved concentrated hydrogen peroxide as an explosive. The ETDs do not test for this at all.

Add to that two facts: (1) You need lab conditions for a time longer than any flight to manufacture TATP. (2) It is too unstable to transport to the airport -- slam a car door too hard and you disappear into a fine red mist.

If you are doing trace detection, say ppb or ppt levels, whatever method you are using to ionize or vaporize the molecules has to remove virtually everything ionizable or vaporizable from the swab.

Yes, I understand that they look for certain components in combination, but you are assuming that there is anything left on the swab after the ionization step, and I'm not sure that's a valid assumption.
There is a huge difference between measuring precise and accurate concentrations versus simply detecting the presence of a substance. I can't say for certain that you're correct or incorrect, I will do some more research and see if I can intelligently agree or disagree with you.

Just to stir the pot, it looks like TSA claims they do not reuse swabs when they are used to test hands:
http://blog.tsa.gov/2010/02/what-hap...rm-during.html
So we can at least agree that there is a discrepancy there. :D

http://onlinepubs.trb.org/onlinepubs...p_rpt_86v6.pdf

Interesting facts:
--huge numbers of false negatives (e.g. 30%). However, they were testing at or below the advertised detection limit for many of the samples.
--the manufacturer states that swabs can be used up to 10 times unless they are dirty or contaminated. It was unclear whether swabs were re-used in the test protocol.
--warmup time for the machines is 10-15 minutes, but it can take up to 25 or 30 minutes to stop registering false positives for NO3. (a useful factoid if they turn on a machine and immediately run your swab and get a false positive, which happened to me once...)
--inexpensive nitrile gloves are a known contamination source, and are NOT recommended for use with this system
--False positives run about 1.7%. The sites where the tests were conducted included bus depots and other locations expected to have higher levels of environmental contamination that an airport checkpoint

This patent application suggests that the swabs used by the most common manufacturer are baked Nomex fabric:
http://www.google.com/patents/EP1844189A2?cl=en

Based on the manufacturer's guidelines that swabs can be reused 10 times, based on the fact that an extremely heat-resistant fabric (400 C / 750 F) is used to make the swabs, and based on the description of the analysis process, I have to conclude that the swabs are heated far past the vaporization temperature for any explosive material during the testing process. That would mean that virtually all of the vaporizable residues are removed from the swab during the sampling process.

It would also mean that any swab run through the machine is probably sterile... at least the part that got heated.

Thanks for the correction. My error has been noted and I appreciate the accurate info.

You missed the "unless they are dirty or contaminated" part. None of us just walked out of a clean room. We are all dirty and contaminated. Wiping the swab on a person's hands, clothing, luggage, computer, etc exposes them to dirt and contamination. The first time they are used. Even common sense should tell you that you don't reuse sampling media.


"probably sterile" is not a scientific term. It either is sterile (which can be verified) or it isn't. If you don't have the documentation via testing that it's sterile, then it isn't.

Walked through security in DEN yesterday and saw something similar. The clerk who was staffing the line where selectees get pushed ino pre-check was alternating between 2 test strips.

After I went through security, I asked the three-striper at the desk and his answer was as follows: "The manufacturer told us we can keep using the same strips as long as we want to. Typically, we would swap them out when they get dirty."

Sigh. Different applications and different analysis techniques frequently re-use sampling media. "Dirty" generally refers to gross (i.e. large) particulates that are not vaporizable (i.e. visible soil) and 'contaminated' generally refers to a previous positive test result.

These swabs are DESIGNED to be re-used. Period. I'm sorry you've never dealt with an instrument or analytical technique where that is true.
I did not say 'almost sterile', I said 'probably sterile'.

When our lab has done work to design surgical prototypes, and they are being tested on animals, one process we have used on parts that cannot be autoclaved is to soak them in 70% alcohol for 10 minutes. We don't verify sterility of the prototypes, but we assess that they are 'probably sterile' after we use a process that is likely to sterilize them. Would we use the instruments on humans under those conditions without validation? No. Are they 'probably sterile'? Yes.

These swabs undergo brief, intense temperature spikes that are likely to sterilize them. It's analogous to running a needle through a flame before you extract a splinter.

That's all well and good in the case of preventing living things, i.e. bacteria and viruses, from being retransmitted and causing disease outbreak. Heat and alcohol kill dangerous biological organisms, but they don't always destroy or alter or remove the chemicals that the ETD machines are looking for.

There is also the issue of cumulative contamination, and while I don't think it's a terribly pressing issue, I can't completely discount it.

Let's also consider the possibility of cross-contamination through repeated handling by TSOs who fail to change gloves more than once a shift. Swab is used multiple times, swab is left on the table in between uses, swab is handled by multiple TSOs with filthy, unchanged, possibly contaminated gloves...

Then, of course, there is the possibility of a contaminated swab transmitting chemicals to a clean test subject on the first pass, and all subsequent tests using clean swabs picking up the contaminants that were deposited by a tainted swab in the first place.

Cross-contamination is the real issue, and the only way to prevent cross-contamination is to exercise proper testing procedures which include environmental controls around the test equipment, consumables, operators, and test subjects. I.E., make sure that gloves and swabs never come into contact with anything other than one single test subject, ever.

I don't know why this is so hard an issue for TSOs to grasp. They obviously understand it when they scream at cleared pax for touching uncleared pax ("They could have passed something off when the parent hugged the 3yo! We have to grope them both again from scratch! 9/11! 9/11! 9/11!")