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Spectroscopic techniques are widely used in a variety of applications due to their pre-eminent properties: They allow in situ and online tracing of compounds, require, in most cases, no sample preparation, and can be used to set up rugged and easy-to-use sensor devices. In contrast to chemical sensing methods, the results are available within seconds, making these techniques feasible for the monitoring of distinctive species as well as the accurate determination of their concentration. While the latter is important for industrial applications (e.g., for the identification of concentrations of process gases), monitoring of species and the determination of whether they are present in a concentration larger than a preset threshold value is fundamental for security and safety applications.
Typically, sensing is performed in the gas phase, using gas cells through which the analyte is directed, an open optical path, or some kind of standoff configuration. A sensor containing a gas cell has the advantage that the interaction length can be enlarged using multipass cells, leading to a higher sensitivity and lower detection limits. However, for security applications, the safety of the operator must be taken into account; therefore, at least remote detection is required. This means that the personnel as well as the equipment are at a safe distance from the sample to be analyzed. Far better, yet, is a standoff configuration, where both operator and sensor equipment are at a safe distance. The desired distance depends on the application and ranges from 1 to 20 m as a minimum separation.
For the detection of explosives, another material property needs to be taken into account: The low vapor pressure of nitrogen-based explosives leads to extremely low concentrations within the gas phase.
Furthermore, even these low concentrations might not be reached if only trace amounts of the explosives are present and the equilibrium state is not settled. In this chapter, pulsed laser fragmentation (PLF) and subsequent detection of selected generated molecular fragments within the mid-infrared spectral range is described as a method to circumvent this problem and detect nitrogen-based explosives despite their low concentration in the gas phase.
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