Because different substances flow through the tube at different speeds, the explosive becomes separated from the other components of the sample. The concentration of the explosive can then be determined by bathing this part of the sample in ultraviolet light and measuring how much light is absorbed. Organic compounds in soil, for example, can also absorb ultraviolet light, which can mask traces of explosive. So McGuffin and her colleague John Goodpaster turned to a technique called fluorescence quenching. After separating the different fractions with chromatography, they add a fluorescent chemical called pyrene to the sample and illuminate it with a laser.
Nitrates interact with pyrene, which stops it fluorescing.
Fluorescence quenching responds only to the nitrated explosives. This increases the contrast between explosives and innocent substances. The degree of quenching is a measure of the concentration of the explosive. The researchers say in a forthcoming issue of Analytical Chemistry that this technique can detect nitrate concentrations of between 10 and 100 parts per billion. The detection limit for the standard method is measured in parts per million, says McGuffin.