Swabbing for security

The TSA started swabbing passengers' hands for explosives residue. This is the first sensible thing they have done is a very long time. The procedure has many pitfalls but also may be effective if used correctly.
Explosives, if handled with bare hands, will leave a residue that can be detected. The problem is deciding which chemicals to look for and how to use the swab test's results. First the chemicals: since most explosives, especially those easily accessible to potential terrorists, are based on nitrates it makes sense to try detecting those first. The next in line would be components of black powder - sulfur being one that is easy to detect.
The pitfalls are obvious: does the positive result we just got from the 80 year old grandma indicate that she made bombs just before leaving for the airport or is it an indication that she took a nitroglycerin pill for her heart condition? Here is where common sense should be used. It would be best if some profiling was implemented so that this grandmother wouldn't be bothered in the first place - but our Government is not very good at this.
Then there is the danger of relying on the test for most of our security. The fact that the nice young man from Nigeria who just came from training in Yemen and has no return ticket or passport did not test positive can mean that he didn't touch explosives recently. It can also mean that he handled them with rubber gloves which he neglected to submit for swabbing or that he handled an explosive we are not testing for. Letting him on a plane just because he seemed to be clean may lead to serious consequences.
I am not sure that the TSA employees have the training to determine what to do but this is a step in the right direction.