We describe a technique for rapid stand-off detection of trace explosives and other analytes of interest. An infrared ͑IR͒ laser is directed to a surface of interest, which is viewed using a thermal imager. Resonant absorption by the analyte at specific IR wavelengths selectively heats the analyte, providing a thermal contrast with the substrate. The concept is demonstrated using trinitrotoluene and cyclotrimethylenetrinitramine on transparent, absorbing, and reflecting substrates. Trace explosives have been detected from particles as small as 10 m.

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Thin films of polyethylene glycol (MW 1500) have been prepared by pulsed-laser deposition (PLD) using both a tunable infrared (λ=2.9 μm, 3.4 μm) and an ultraviolet laser (λ=193 nm). A comparison of the physicochemical properties of the films by means of Fourier transform infrared spectroscopy, electrospray ionization mass spectrometry, and matrix-assisted laser desorption and ionization shows that when the IR laser is tuned to a resonant absorption in the polymer, the IR PLD thin films are identical to the starting material, whereas the UV PLD show significant structural modification. These results are important for several biomedical applications of organic and polymeric thin films.

We present ion mass spectra obtained by matrix-assisted laser desorption/ionization for analytes of approximately 1000 Da across the largest range of wavelengths and pulse durations to date. The matrix used in all cases was 2,5-dihydroxybenzoic acid. Based on the data and fundamentals of laser-material interactions, we evaluated the plausibility of discriminating among such ion formation mechanisms as multiphoton ionization and excited-state ionization from mass spectra alone. Some previously proposed mechanisms appear unlikely for the matrix-analyte systems that we studied, casting doubt on their general applicability. Moreover, although analysis of mass spectra can apparently exclude certain mechanisms, it cannot establish which of several competing mechanisms is actually operative. This is particularly true with respect to variations in pulse duration and wavelength.