Air Plume and Chemical Analysis
█ BRIAN HOYLE
An air plume is a layer of warm air that immediately surrounds a person's body. It has also been referred to as a human thermal plume.
The skin's surface temperature is typically 33° Celsius, which is approximately nine degrees warmer than the surrounding air at a typical room temperature. The temperature difference causes heat to be lost from the entire surface of the skin to the surrounding air.
Because warm air rises, the plume rises up the body and flows off the top of the head and shoulders, instead of radiating outward to the surrounding air from all parts of the body. As the air moves up and away from a person, tiny bits of the skin and chemicals that were present on the skin's surface can also be carried upward. The presence of clothing has no effect on the upward movement of the air.
The presence of clothing also does not block the migration of chemicals from items being carried in the clothing. Particles of an explosive in a pocket, for example, will be able to pass through the pores of the fabric to the immediate vicinity of the skin. There, they will encounter the air plume and migrate upward with the airflow.
The chemicals that are carried in the air plume can be detected using sophisticated detection equipment. The chemical analysis of an air plume can detect explosives and even the aromas emitted by microorganisms.
The analysis of an air plume has grown out of studies that relied on the use of what is termed a schlieren system. The word schlieren is German for streaks, and describes the appearance of air in a special optical system. Schlieren optics measure air flow based on the scattering of light due to differences in density at the interface between moving air and relatively motionless air.
Scientists interested in imaging the schlieren patterns produced by people modified the small optical system so that it could be accommodated in a larger device. The device is similar in appearance to the walk through X-ray machines that are now commonplace in airport security areas.
When a subject walks through the portal, the air plume is drawn into an analysis chamber positioned in the portal's archway. Any particles present are collected in a trap. As well, the vapors in the air plume can be condensed onto the trap. Chemical analysis is performed using a machine called an ion trap mobility spectrometer.
The trapping of particles and condensation of the vaporous air plume concentrates any compounds that are present. The trapped sample is delivered to a chamber that converts the sample molecules to ions. Typically, bombarding the sample atoms with electrons accomplishes this conversion. When an electron collides with a sample ion, an electron is dislodged from the sample atom, producing a positively charged ion. As voltage is applied along the length of the chamber, the positively charged sample ions move toward the negatively charged cathode. Separation of the ions occurs based upon their different sizes and masses. For example, smaller ions move down the chamber faster than larger ions. As ions arrive at the cathode, a current is produced. The current can be amplified to produce a detectable signal. The different signals can be plotted to produce a spectrum. The different peaks in the spectrum can be related to known ions to determine the ionic composition of the sample.
The pattern of the spectrum produced by the nitrate (NO) groups in an explosive such as 2,4,6–dinitrotoluene (TNT) is characteristic of the arrangement of the NO groups within the chemical structure, and is different from the pattern produced by other NO-containing explosives like nitroglycerine, ethylene glycol dinitrate nitroglycerin, cyclotrimethylenetrinitramine, and pentaerythritoltetranitrate.
The spectrometer is extremely sensitive and fast. Chemicals that are present in only a few parts per billion will be detected in about 10 seconds. Thus, even a very small amount of explosive carried in a pocket would register in the spectrometer.
Currently, the chemical analysis of the air plume is geared towards the detection of explosives. The incorporation of other sensors, such as the "electronic nose" that can detect and identify some bacteria based on the unique chemical vapors given off by the cells will enable biological analysis of air plumes in addition to chemical analysis. Incorporating a metal detector into the device could enable one device to be used to screen for conventional, chemical, and biological weapons.
█ FURTHER READING:
Settles, Gary S. Schlieren and Shadowgraph Techniques. Heidelberg: Springer-Verlag, 2001.
Crabb, C. "Biosensors Enliven the Science of Detection." Chemical Engineering August (1998): 35–39.
Settles, G.S., and W.J. McCann. "Potential for Portal Detection of Human Chemical and Biological Contamination." SPIE Aerosense no. 4378 (2001): paper 01.