Detecting explosives with CO2 laser

Published in The Hindu on October 19, 2006

For C. Kumar N. Patel, the man who invented the carbon dioxide laser way back in 1964, the quest of finding newer applications using his invention continues unabated.

With the heightened threat of chemical warfare and terrorists attempting to use bombs to blow up planes, Dr. Patel has turned his attention to using his invention to detect chemicals used in making bombs and chemical warfare agents even when present in extremely low concentrations. Dr. Patel who is the Chief Executive Officer and Chairman of the California based Pranalytica, Inc, was in Chennai recently.

The use of sarin, a chemical warfare agent in a Tokyo subway in 1995, and the anthrax mailings following 9/11 have made a case for reliable, unambiguous and early detection of even traces of such deadly chemicals.

The limitations

Till date, the weight of molecules is studied for detecting explosives and chemical warfare agents. But this method has its imitations.

For instance, TNT and nitroglycerine have identical molecular weight. “So if you are a heart specialist, you will be stopped,” Dr. Patel noted. “Nitroglycerine is the active component of the medicine (in pill form) prescribed for angina problems. Handling these pills leaves traces of nitroglycerine on the fingers.”

While nitroglycerine is an explosive, it is very unstable and hence seldom used by terrorists.

Novel approach

Dr. Patel seems to have found a way to overcome this limitation – looking for chemical structure. Unlike in the case of the molecular weight, TNT and nitroglycerine have different chemical structures. Hence detection becomes more accurate.

The optical sensors are built around the principle that all molecules absorb light. “And the light absorption characteristic reveals the chemical structure and not molecular weight,” he noted.

According to a paper published by him and others in the Journal of Applied Physics last year, a vast majority of chemical warfare agents absorb light in the 3-14 micrometre wavelength.

Carbon dioxide lasers have wavelength in the 8-12 micrometre range, thus matching the wavelengths of the chemicals of interest. The sensor works by allowing a sample to come into, say, a box and the amount of laser radiation absorbed by the sample is measured. T

he chemicals present in the sample absorb the laser radiation whenever their wavelengths match the laser’s wavelength. When absorption takes place, the temperature of the sample increases and this in turn increases the pressure. And when pressure increases sound is produced due to pressure fluctuation. “So we just have to put a sensitive sensor to detect sound,” he explained. Hence the technique is called laser photoacoustic technique.

Specificity test

The ability of the sensor to detect only gases of interest and distinguish different chemicals, called specificity, should be very high. According to Dr. Patel, the laser photoacoustic technique has high sensitivity and can distinguish all chemical warfare agents. This is quite a difficult task since the sample may contain many gases whose wavelengths match the laser radiation’s wavelength.

So controlling the wavelength of the laser to match the wavelengths of the target gases is important; carbon dioxide has a wavelength of 8-12 micrometres. The next requirement is to have a databank of wavelengths of chemicals used as warfare agents and explosives as well as harmless chemicals encountered every day.

Dr. Patel already has a databank numbering 400 gases. The number of wavelengths to be measured is a sum total of the target gases plus other chemicals.

“In an indoor environment we have to be bothered about just 45 gases that can cause interference with our target gases [chemical warfare agents],” he said.

So more the number of wavelengths known and measured, the better will be the possibility of targeting only the gases of interest. “Bettering the rate of rejection of interference [of non-target gases] is the name of the game,” he underlined.

Apart from the ability to detect only the target gases, the need to detect them even when present in very low concentrations – sensitivity – cannot be overemphasised.

Achieving high sensitivity levels is all the more difficult, as the detection has to be made in the presence of so many other gases. “We have achieved a sensitivity of one ppb (parts per billion),” he noted. The molecular weight technique has lesser sensitivity (>10 ppb).

Higher sensitivity

“So it is nearly impossible to wash off these chemicals at ppb levels,” he said, “unless severe methods are used.” Higher sensitivity and specificity alone are not sufficient. The ability to never miss even one case is mandatory. But the number of false positives – where the technique detects the presence of warfare chemicals even when none is present – has to be very low too.

“The number of false positives is one in every 100 million,” he said. However, a paper authored by him and others in the journal Applied Physics Letters this year reports the probability of coming across false positives being less than one in a million.

According to him, the lower sensitivity mentioned in the paper was based on highly contaminated air created artificially and is not characteristic of urban environments such as airports.

With very high sensitivity and specificity and less probability of false positives, Dr. Patel is confident that his technique may soon find application in routine screening for chemical warfare agents in airports and other places.