Near-infrared spectral imaging as a detection technique for organic materials in porous media

Abstract

Imaging spectroscopy combines the spatial discrimination of imaging techniques with the chemical information of spectroscopy to form a powerful tool for the study of chemically heterogeneous systems. This work describes the in situ qualitative and quantitative analysis of contaminant transport flow cells and of high-performance thin-layer chromatography (HPTLC) plates by near-infrared imaging spectroscopy. A solid-state, near-infrared imaging spectrometer was constructed for these studies. The spectrometer utilized an imaging quality acousto-optic tunable filter for wavelength selection over the 1.3-2.3 μ range and a cryogenically cooled, 240 x 324 pixel platinum silicide camera for detection. Samples were analyzed by either diffuse reflectance or diffuse transmittance using a 250 W quartz-tungsten-halogen lamp for sample illumination. The first series of investigations focused on the analysis of laboratory-scale flow cells, which are used to study the transport of non-aqueous phase liquid (NAPL) contaminants in the soil and groundwater. Current detection systems used for determining NAPL distribution are incapable of distinguishing between chemical components in NAPL mixtures, limiting flow cell experiments to the study of simple systems. This research utilized the near-infrared imaging spectrometer and multivariate calibration techniques to quantitatively determine the concentrations of individual constituents in binary NAPL mixtures within vadose zone and aquifer models. The vadose zone calibration data was used to determine the spatial distribution of each NAPL constituent in situ during a dynamic, multi-component flow cell experiment that modeled the remediation of NAPL contaminated soil. This technique, the first to quantitatively determine the in situ distribution of the individual NAPL phase constituents, represents the state of the art in detection for contaminant transport flow cells. The second series of investigations focused on analysis of samples on HPTLC plates. Conventional systems require visualization techniques to detect compounds lacking a chromophore or fluorophore. This research utilized the near-infrared imaging spectrometer as a non-destructive detection technique to provide qualitative and quantitative information for caffeine samples on HPTLC plates. Both diffuse reflectance and diffuse transmittance measurements provided detection limits of several micrograms. The caffeine spectrum was clearly distinguishable down to 25 μg using a diffuse reflectance geometry with a mirrored backing applied to the HPTLC plate.