Jet-REMPI for the Detection of Explosives
Molecular Physics Laboratory, SRI International
David R. Crosley, P. I.

A short time after a landmine is buried, explosives (such as trinitrotoluene, TNT) or major explosive contaminants (such as dinitrobenzene, DNB), begin to leak into the surrounding soil. These leach toward the surface, usually undergoing chemical or biological transformation into other explosive related compounds (ERCs). Above the landmine, these can be present as vapors; the important compounds to look for are thought to be DNB, dinitrotoluene, and amino-nitroaromatics. Laser photoionization techniques such as Jet-REMPI are being developed at SRI to detect these ERCs in the gas phase.

Jet-REMPI is a method comprising a supersonic nozzle inlet, a tunable laser, and a time-of-flight mass spectrometer. REMPI stands for Resonantly Enhanced Multiphoton Ionization. The inlet cools the molecules to some 20K, much simplifying their laser absorption spectrum. The laser is tuned to a resonance transition which is excited by one laser photon; absorption of a second photon produces the parent ion. This is then detected by the mass spectrometer.

Jet-REMPI offers a high degree of both sensitivity and selectivity in a relative short period of analysis time. Although it is not a universal detector, only applicable to molecules that absorb in the ultraviolet region of the spectrum, it is particularly suitable for compounds containing an aromatic ring, which is the case for ERCs related to TNT.

Progress to date has concentrated on the use of a noncooling inlet and photofragmentation REMPI to evaluate NO detection as a suitable measurement approach. Many ERCs absorb the photons from a laser with a few ns pulse length in such a manner as to fragment, first to NO2, then to NO. The NO can then be readily detected using REMPI. This has previously been studied elsewhere using both REMPI and laser induced fluorescence of NO, but interferences and limits of detection have not been assessed. We excite a hot band of NO to avoid atmospheric NO, which could be present in concentrations as much as a part per million in a polluted region. However another natural atmospheric constituent, CH3NO3, shows the same absorption spectrum as do the ERCs. This is likely true for peroxyacetylnitrate, present at tens of parts per billion in the ambient atmosphere. These signals will be much stronger than those from the ERCs in a field environment, the latter present at a part per billion at most. Furthermore, detection limits have been established using this method for ERCs at about a part per billion, barely sensitive enough. Therefore, detection via photofragmentation and NO REMPI does not appear a viable method for ERC diagnostics.

Measurements have been made using our versatile but large laboratory Jet-REMPI apparatus, shown in the first photograph. Such a system is crucial to testing diagnostics, but is unsuitable for field work. We have recently constructed a much smaller system on a different grant; it is shown in the second photograph. Although less sensitive and less versatile than the lab instrument, it is what is needed for field work using currently available laser and mass spectrometer components. As time goes on, such a device will become even smaller and more usable in the field./p>

Personnel

David R. Crosley
P. I. Senior Staff Scientist
650-859-2395
david.crosley@sri.com

Harald Oser
Physical Chemist
SRI International
650-859-3311
harald.oser@sri.com

Bethany Pond
Postdoctoral fellow-Stanford Univ. visiting at SRI
650-859-3302
bethany.pond@sri.com