Quantum cascade laser back‐reflection spectroscopy at grazing‐angle incidence using the fast Fourier transform as a data preprocessing algorithm

Abstract

A simple optical layout for a grazing-angle probe (GAP) mount for coupling to a midinfrared (MIR) quantum cascade laser (QCL) spectrometer is described.This assembly enables reflectance measurements at high incident angles. In the case of optically thin films and deposits on MIR reflective substrates, a double-pass effect occurs, which is accompanied by the absorption of deposited samples in a reflection-absorption infrared spectroscopy modality. The optical system allows MIR light to pass through the sample twice.Applications to cleaning validation and detection of traces of explosives using the QCL-GAP is reported. Principal component analysis and partial least squares multivariate chemometrics methods were employed to analyze MIR spectra to evaluate an analytical methodology for confirming the presence of residues of pharmaceutically active ingredients (irbesartan) and of traces of explosives (cyclotrimethylenetrinitramine [RDX]) that have been deposited on metallic substrates. The performance of spectral preprocessing via fast Fourier transform (FFT) analysis was evaluated for the ability to extract more powerful and accurate information from the obtained reflectance spectra.According to the figures of merit of this new technique, FFT with chemometric routines can obtain sensitivity and specificity values of 1.000. The limits of detection that were obtained for irbesartan and RDX were 26 and 8 ng/cm 2 , respectively. The experimental results demonstrate that the proposed system, when used together with proper chemometrics routines, constitutes a powerful tool for the development of methodologies that have lower detection limits for a range of applications that involve detecting traces of analytes that reside on substrates as contaminants. KEYWORDS fast Fourier transform (FFT), grazing angle MIR laser spectroscopy, irbesartan/RDX, partial least squares (PLS), principal component analysis (PCA)

Mid-Infrared Laser Spectroscopy Detection and Quantification of Explosives in Soils Using Multivariate Analysis and Artificial Intelligence

“…Compared to the existing sources in the MIR range, QCL sources have a higher output power that allows remote sensing and analysis. The rapid detection of HEs on highly reflective substrates using this method was demonstrated and reported by other investigators [34][35][36][37][38]. This spectroscopic analysis can be achieved without contact and in a non-destructive way.…”

Surface Persistence of Trace Level Deposits of Highly Energetic Materials

“…The value obtained for the LOD with the PLS model was 22 ng/cm2; this value is very low but it is in the order of published in the literature for RDX from grazing angle [62]. A second model using classical least squares (CLS) regression (or linear regression) was used for comparison.…”

Chemical Sensing of Common Microorganisms Found in Biopharmaceutical Industries Using Mid-Infrared Laser Spectroscopy and Multivariate Analysis

“…Bacterial samples Sa, Se, andMl were used to create the following mixtures: Sa/Se ,Sa/Ml , and Se/Ml . A sample smearing technique using a micropipette tip (˜1 mm diameter) was used to deposit the sample onto the substrate [20]. The smearing procedure accounts for minimal sample loss during handling.…”

“…The solutions deposited on the substrate were left air-dry for 20 minutes until the solvent evaporated. Each bacterial sample acquired twenty (20) replicas by moving the optical stage in the QCL-GAP setup.…”