Characterization of near-infrared low energy ultra-short laser pulses for portable applications of laser induced breakdown spectroscopy.

We report on the delivery of low energy ultra-short (<1 ps) laser pulses for laser induced breakdown spectroscopy (LIBS). Ultra-short pulses have the advantage of high peak irradiance even at very low pulse energies. This opens the possibility to use compact, rare-earth doped fiber lasers in a portable platform for point detection applications using LIBS for elemental analysis. The use of low energy ultra-short pulses minimizes the generation of a broad continuum background in the emission spectrum, which permits the use of non-gated detection schemes using very simple and compact spectrometers. The pulse energies used to produce high-quality LIBS spectra in this investigation are some of the lowest reported and we investigate the threshold pulse requirements for a number of near IR pulse wavelengths (785-1500 nm) and observe that the pulse wavelength has no effects on the threshold for observation of plasma emission or the quality of the emission spectra obtained.

Off-line direct deposition gas chromatography/surface-enhanced Raman scattering and the ramifications for on-line measurements.

Surface-enhanced Raman spectra (SERS) of molecules separated by gas chromatography (GC) were measured off-line by condensing the analyte on a moving, liquid-nitrogen-cooled ZnSe window on which a 5 nm layer of silver had been formed by physical vapor deposition. After the components that eluted from the chromatograph had been deposited, the substrate was allowed to warm up to room temperature and transferred to the focus of a Raman microspectrometer where the spectrum of each component was measured. Band intensities in the spectrum of 3 ng of caffeine prepared in this way were approximately the same as in the spectrum of bulk caffeine. By making some logical assumptions, it was shown that identifiable GC/SERS spectra of 30 pg of many molecules could be measured over a 300 cm(-1) region in real-time and that if an optimized substrate were used the minimum identifiable quantity could be reduced to 1 pg or less.

Reduction of detection limits of the direct deposition GC/FT-IR interface by surface-enhanced infrared absorption.

Even though the interface of gas chromatography (GC) and mass spectrometry (MS) is by far the most popular way of identifying molecules eluting from a GC in real time, the identification of compositional isomers by MS is equivocal at best. Much better results would be found by infrared spectrometry (IR) if the sensitivity of the GC/IR interface could be improved. In this paper, we show how the smallest quantity of molecules injected into a GC for which an identifiable infrared spectrum can be measured on-line has been reduced by a factor of 10 below the detection limit of the most sensitive current technique. A commercial direct deposition interface between a GC and a Fourier transform infrared spectrometer was modified by vapor-depositing an island film of silver on the surface of the zinc selenide substrate. Band intensities in the spectra of molecules located within approximately 4 nm of the surface of the silver islands were increased by at least 1 order of magnitude through surface-enhanced infrared absorption (SEIRA). The effectiveness of this approach was illustrated by comparing the limits of identification of butylbenzene isomers measured with and without the silver film. Comparison with the spectra of the same molecules measured by mass spectrometry showed the increased sensitivity and specificity of the GC/SEIRA interface.