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Wednesday, October 26, 2011

Forensics Q&A: Explosives Detection in Airports

By Kristy Lahoda | @KristyLahoda

Disclaimer: The information provided in this post should not be used for malicious intent unless it is in the form of crime writing. The author is an explosives expert, not an expert in airport security. While every attempt was made to ensure the accuracy of this information, for security purposes, some details may have been withheld. 
QUESTION: I am writing a thriller in which my hero has been carrying explosives in his bag, but no longer is. Would airport security (in the US) know that he had been carrying explosives? How would they know this?

ANSWER: As a result of the explosion of Pan Am Flight 803 over Lockerbie, Scotland in December of 1988 as well as airline bombings over Africa and Columbia the following year, the United States passed the Aviation Security Improvement Act of 1990 which started the U.S. on the track to finding an explosives detection technology to prevent similar disasters.

Shortly after the September 11, 2001 attacks, President Bush signed the Aviation and Transportation Security Act into law. This placed the TSA under the U.S. Department of Transportation. One hundred percent of all luggage was to be screened for explosives. There are multiple ways that explosives screening is performed on luggage across the United States: Explosives Detection Systems (EDS) and Explosives Trace Detection (ETD) also called Trace Explosives Detection (TED).

EDS uses Computer-Aided Tomography technology, i.e. CAT scans. The density and mass of objects are measured within luggage. Explosives have a fairly unique range of densities. X-rays produced from the CAT scans are translated into cross-sectional images by computer software whose densities are then compared to densities of known explosives by software algorithms. The operator can inspect the item if something suspect is indicated.

Explosives Trace Detection (ETD)

ETD is something more visual at the airport and is used in a few different ways (that I know of). One way is to use an “electronic nose” which is sampling the ambient air for explosive vapors. This technique requires high vapor pressure explosives such as organic peroxides mainly used in Improvised Explosive Devices (IEDs).  

Another ETD is a portal. When a person walks through this, air is blown over them and then sucked into the instrument and collected at a filter where the sample is pre-concentrated. The filter is heated and the vaporized particles are carried to the sensor where an instrument called an Ion Mobility Spectrometer (IMS) is used to determine if the sample contains an explosive(s). An ion is a positively or negatively charged particle. The ion particles can be identified as to whether or not they are from explosives based on the length of time it takes them to travel to the detector.  

A third way is when the luggage screener pulls you aside because something suspicious was seen in your belongings or maybe because you were selected for a random sampling. I have been screened this way several times. You are directed to an area behind the screening conveyor belt. A different screener swipes your personal belongings or the items inside and possibly your hands with a paper disk. The swipe is placed into an IMS and within 10-15 seconds the swipe has been analyzed. If explosive particulates were present on the swipe, the detector will sound and the type of explosive present will be indicated.

Ion Mobility Spectrometer (IMS)

Let’s take a closer look at how IMS is used to determine whether a sample contains an explosive(s).  

When the swipe is placed in the oven of the instrument, the sample is vaporized and some of the vapor is turned into ions, or ionized, usually by a radioactive source. A common source is the beta emitter, 63Ni. Ions of the sample are carried through a drift tube by what is called the drift gas. Different ions will have different drift speeds, called mobilities, and as a result will separate and arrive at the detector at different times. Typically smaller ions have a higher mobility than larger ions.  

The IMS can operate in two different modes: positive ion mode and negative ion mode. For explosives detection, the IMS will operate mostly in the negative ion mode. Explosives such as those with —nitro groups (—NO2), for example trinitrotoluene (TNT), are very electronegative. This means that they attract electrons very easily. As a result, when ionized, these types of explosives will form negatively charged ions, called anions. If the IMS is in the negative ion mode and TNT is detected on the swipe, an alarm will signal on the IMS and indicate that TNT has been detected.

Triacetone triperoxide, TATP, is a high explosive that is a favorite of terrorists because of how easily it can be made. It forms a positive ion (cation) when an adduct is formed, i.e. when it combines with ammonium (NH4+). If TATP is present when the IMS is operated in the positive ion mode, then an alarm will sound to indicate that TATP was detected. This screening process only takes seconds!

Quality Assurance Testing to Provide Accurate Results

These instruments must go through quality assurance testing to ensure that they will provide accurate results.  The National Institute of Standards and Technology (NIST) develops Standard Reference Material (SRM) that scientists use for validation of instruments and procedures. A new SRM was developed in the spring of this year to meet the American Society for Testing and Materials (ASTM) requirements for acceptable minimum performance of the Explosives Trace Detectors (ETDs).

This SRM contains calibrated solutions of the explosives RDX (an ingredient in C-4), PETN, and TNT. The test protocol involves adding one drop of RDX along with a solvent blank to the swipe. After the solvent is allowed to evaporate, the swipe is tested and the alarm should indicate the presence of an explosive. This is repeated with the other two calibrated explosives solutions mentioned. These calibration solutions were developed to be close to the detection limit (DL) of the instruments meaning they are developed, tested, and sold at the lowest concentration that the instrument could reliably detect. If they were lower than the DL, then the instrument would not be able to determine that an explosive was present when these calibration solutions were used. This new SRM will allow for the ETDs performance and reliability to be routinely tested in order to help ensure airport security.

Kristy Lahoda, Ph.D., is an explosives analyst contractor in a crime lab as well as a science content editor for a major educational publishing company.  She writes Christian forensic suspense and discusses forensics on her blog called Explosive Faith.  You can follow her on Facebook and Twitter.

If you have a forensics question for Dr. Lahoda that you'd like to see answered on the QueryTracker Blog, send your question via Carolyn Kaufman using the email link under Contact Us in the right-hand column of the main QTB page.


She Wrote said...

Excellent column - valuable information.

Mary Lindsey said...

Great post! Thank you for being a QT guest. I love the name of your blog, Explosive Faith!

Stina Lindenblatt said...

Thanks for the awesome post, Kristy. :D

kristylahoda said...

Thanks all!!!