An important tool we use is a unique analytical instrument called an Open Path Fourier Transform Infrared spectrometer, or OP-FTIR. There are perhaps 100 such instruments available in the USA today. Previous instruments of this type were often large (housed in a small van or truck) and difficult to operate requiring many skilled support staff. We have developed a small, transportable OP-FTIR (sometimes known around the lab as the"baby FTIR"), a small (trunk size), portable instrument based on a unistatic design. We also have a larger commercial instrument that we use for studying trace pollutants in the atmosphere.
The remote sensing FTIR uses infrared light to detect chemicals in the air. Essentially, the instrument measures chemicals by shining an invisible light beam over a distance up to 1000 meters through the air and detecting changes in the intensity and color of the light. Just as visible light can be sorted into a spectrum of different wavelengths corresponding to various colors of light, infrared light can be sorted into a spectrum corresponding to different temperatures or energy levels. The OP-FTIR uses a device called an interferometer that allows us to isolate the wavelengths of light, analogous to a prism that separates white light into colors. The infrared light is gathered by a telescope fitted with a special detector that is cooled by liquid nitrogen to more than 150 degrees below zero. Because infrared light is thermal energy, this type of detector provides extraordinary sensitivity.
A particular chemical will absorb energy only at particular wavelengths in the infrared spectrum, producing a unique pattern or "fingerprint" in the infrared absorbence spectrum for each compound. This absorbence pattern is so unique that it can be used to identify most unknown compounds. We have infrared spectral libraries containing data on more than 6000 compounds and soon plan make a subset of our data avaliable on the web. The amount of energy absorbed (light intensity) tells how many molecules of the compound are in the beam path, a measure of gas concentration. Thus the instrument can qualitatively identify contaminants in the air and quantitatively measure the concentration.
The remote sensing FTIR has many advantages over more conventional sampling technology. First, it is a direct reading instrument that provides rapid on-site information. In a matter of minutes one can identify and quantify contaminants in an area or worksite, and monitor changes in contaminant levels on a continuous basis. In contrast, conventional methods usually require an air sample to be transported back to a laboratory and may require days or weeks of analysis to produce the same information. Second, the instrument can identify complex mixtures of many chemicals, and quantify them at low concentrations. It is possible to identify mixtures containing more than 20 compounds with detection limits down to a few parts per billion (ppb). Third, the remote sensing FTIR is unobtrusive. The instrument can be aimed to measure contaminants at distances several hundred meters away, without requiring people to enter the area or set up sampling devices. This has great practical importance in situations where contaminants may be present in dangerous levels, or where it is necessary to survey a large area with limited access. Some examples of situations like this are chemical or oil spills, hazardous waste sites, or searching for leaks at large worksite like a refinery or chemical factory.