Broadband Mid-Infrared Stand-Off Reflection–Absorption Spectroscopy Using a Pulsed External Cavity Quantum Cascade Laser

Abstract

Broadband mid-infrared molecular spectroscopy is essential for detection and identification of many chemicals and materials. In this report, we present stand-off mid-infrared spectra of 1,3,5-trinitro-1,3,5-triazine or cyclotrimethylene trinitramine (RDX) residues on a stainless-steel surface measured by a broadband external cavity quantum cascade laser (QCL) system. The pulsed QCL is continuously scanned over 800 cm in the molecular fingerprint region and the amplitude of the reflection signal is measured by either a boxcar-averager-based scheme or a lock-in-amplifier-based scheme with 1 MHz and 100 kHz quartz crystal oscillators. The main background noise is due to the laser source instability and is around 0.1% of normalized intensity. The direct absorption spectra have linewidth resolution around 0.1 cm and peak height sensitivity around 10 due to baseline interference fringes. Stand-off detection of 5-50 µg/cm of RDX trace adsorbed on a stainless steel surface at the distance of 5 m is presented.

A LabVIEW based virtual instrument system for mid infrared laser spectroscopy measurements

“…Artifacts are noticeable at approximately 1230 cm −1 . These are nonrelated to polystyrene vibrational modes but instead associated with the QCL baseline structure and interference fringes generated by the total internal reflection of the MIR light trapped within the thin polystyrene film [37]. In a recent report by members of this research group, these fringes were used to obtain information from the substrate and the chemicals deposited on them by applying fast Fourier Transform preprocessing of the RAIRS data used as statistical classification tools in multivariate analysis (MVA) [27].…”

Infrared Spectroscopic Imaging Advances as an Analytical Technology for Biomedical Sciences

“…An integrated single device may provide tunability over hundreds of cm –1 , a range that continues to increase . Most commercially available systems today consist of multiple modules, with each tunable over approximately a 200–400 cm –1 range, to cover the full fingerprint region − using an external cavity , and grating to select frequencies.…”

“…An integrated single device may provide tunability over hundreds of cm −1 , a range that continues to increase. 27 Most commercially available systems today consist of multiple modules, with each tunable over approximately a 200−400 cm −1 range, to cover the full fingerprint region 28−32 using an external cavity 33,34 and grating to select frequencies. The single frequency emission from the laser allows a discrete frequency (DF) approach to IR imaging, without a requirement for any interferometer, or additional filters.…”

Mid-Infrared Reflectance Spectroscopy Based on External Cavity Quantum Cascade Lasers for Mineral Characterization

“…4 Published work on ECQCL-based spectroscopy are reported for absorption spectroscopy in gases, 5 in thin liquid layers, 6 in pharmaceutical products, 7 and for chemical agents. 8–10 Applications of ECQCL-based spectroscopy for mineral identification have been reported for quartz, 1 notably in aerosols. 11,12 Recently, mineral measurements were reported for quartz and clays in oil sands.…”