White powder identification using broadband coherent light in the molecular fingerprint region

We show that a variety of white powder samples, including painkillers, amino acids, stimulants and sugars are readily discriminated by diffuse reflectance infrared spectroscopy involving no preparation of the sample and no physical contact with it. Eleven powders were investigated by illuminating each sample with broadband coherent light in the 8–9-μm band from an OPGaP femtosecond optical parametric oscillator. The spectra of the scattered light were obtained using Fourier-transform spectroscopy. Similarities between different spectra were quantified using Pearson’s correlation coefficient, confirming that spectral features in the 8–9-μm wavelength region were sufficient to discriminate between all eleven powders evaluated in the study, offering a route to simple and automated non-contact chemical detection. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.


Introduction
Various agendas motivate the identification of white powders, such as detecting counterfeit pharmaceuticals [1,2] and foodstuff analysis [3].Raman and Fourier-transform spectroscopy (FTS) can discriminate between materials according to their chemistry and crystallography, expressed by characteristic absorption signatures in the 6-12 µm region (833-1667 cm −1 ).Typically, FTS of powders proceeds via the attenuated total internal reflection (ATR) method [4], in which intimate contact between a solid sample and the ATR cell is ensured by applying high pressure.ATR is an established field technique [5] but encounters certain limitations [6], and crucially requires contact between the sample and the ATR interface, risking sample contamination or exposing a user to hazardous materials.By contrast, diffuse reflectance FTS needs no sample preparation and is a non-contact method, but is difficult to implement with the spatially incoherent thermal sources utilized in ATR embodiments.Raman spectroscopies face other constraints due to the weak cross-sections of spontaneous Raman scattering, or the complexity of more sensitive methods like coherent anti-Stokes Raman spectroscopy [7].
Spatially coherent broadband mid-infrared (mid-IR) sources have seen significant recent development, particularly in the molecular fingerprint region.The narrow spectra from external cavity quantum cascade lasers (EC-QCLs) can be rapidly and widely tuned (e.g.7.7-10.1 µm in [8]), and QCL arrays can cover from 6.5 to 11 µm in a small package for standoff detection [9].An EC-QCL system has been used to identify bulk powders like paracetamol by collecting diffuse reflectance spectra at a range of 1 m, and identifying trace amounts of powdered explosive residue [10].New nonlinear materials like orientation-patterned gallium phosphide [11] (OPGaP) avoid the multi-phonon IR absorptions associated with oxide materials such as LiNbO 3 , allowing the generation of wavelengths 5 µm.Notably, the femtosecond OPGaP optical parametric oscillator (OPO) we reported previously [12] produces spec novel spectro [14,15] and OPGaP OPO, identification the sample su information d features, so c to capture inf due to their b of white powd

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Summary
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