Direct absorption spectroscopy of molecules in the gas phase is a very powerful tool for analytical analysis. Among the various direct absorption methods, cavity ring-down spectroscopy (CRDS) is proving to be a valuable approach since it combines high sensitivity with a rather simple experimental arrangement. Paul Dagdigian's laboratory in the Chemistry Department at Johns Hopkins University is pursuing the application of CRDS toward the detection of explosives and explosive-related compounds (ERCs) in the vapor phase within the context of the detection of land mines.

With its high detection sensitivity, CRDS should thus be applicable to the detection of vapors emanating from the soil around a land mine. One can imagine a scenario in which land mines have been buried and exposed to the elements over a period of time. The volatile components of the land mine eventually leak out into the surrounding environment. The vapor pressures of the explosives commonly used in the land mines are very low; however, the ERC degradation products have somewhat larger vapor pressures. For example, TNT is degraded into 2,4-dinitrotoluene, dinitrobenzene, and other species. As discussed below, CRDS is estimated to have sufficient sensitivity for the trace quantities expected of the explosives or ERCs.

In the CRDS technique, the sample is placed inside a high-finesse optical cavity. A laser pulse is injected into the cavity and is reflected back and forth between the mirrors. The absorbance of the sample is determined by measuring the rate of decay since the photon lifetime in the cavity is reduced by molecular absorption. The decay lifetime for photons in the cavity is determined by observing light which passing through the rear mirror. A schematic of a CRDS setup is illustrated in the figure below.

The CRDS technique offers path lengths (several hundred meters) many times the actual sample cavity length (approximately one half meter), and the concentrations are measured in a manner immune to variations of the laser pulse intensity. The minimum concentrations which can be detected by CRDS depend upon the absorption cross section, the reflectivity of the mirrors, and the accuracy of the photon decay lifetime determination. In our approach to the detection of explosives, we will be employing electronic absorption in the ultraviolet. These electronic transitions are very strong, but the transitions are fairly broad. We estimate that we can achieve detection sensitivities of the order of 100 ppt for TNT in air, based on the known absorption cross section for TNT and the reflectivity of mirrors which have been obtained from optics manufacturers.

Within the past few months, our laboratory has assembled an apparatus for the detection of explosives and ERCs using CRDS in the ultraviolet. The frequency-doubled output of an Continuum Panther optical parametric oscillator (OPO) pumped by a Precision Powerlite Nd:YAG laser (10 Hz) is employed as a tunable solid-state laser light source.

A CRDS apparatus for the detection of explosives has been constructed around this OPO laser light source. A resonant optical cavity with high-reflectivity UV mirrors and a trace explosives gas generator has been set up to investigate the UV absorption spectrum of explosives and their breakdown products. Our primary goal is to compile a catalog of ultraviolet spectroscopic signatures and to quantify detection sensitivities. Our secondary goal is to experiment with and evaluate different configurations of this instrument for use in the field.

Postdoc Chris Ramos (Ph. D., Purdue) working at the apparatus

Researchers at Johns Hopkins University:

Dr. Paul Dagdigian
Professor of Chemistry
pjdagdigian@jhu.edu

Dr. Christopher Ramos
Postdoctoral Fellow
chrisram@jhu.edu