Accurate Wavelength Measurements of a Putative Standard for Near-Infrared Diffuse Reflection Spectrometry

The diffuse reflection (DR) spectrum of a sample consisting of a mixture of rare earth oxides and talc was measured at 2 cm−1 resolution, using five different accessories installed on five different Fourier transform near-infrared (FT-NIR) spectrometers from four manufacturers. Peak positions for 37 peaks were determined using two peak-picking algorithms: center-of-mass and polynomial fitting. The wavenumber of the band center reported by either of these techniques was sensitive to the slope of the baseline, and so the baseline of the spectra was corrected using either a polynomial fit or conversion to the second derivative. Significantly different results were obtained with one combination of spectrometer and accessory than the others. Apparently, the beam path through the interferometer and DR accessory was different for this accessory than for any of the other measurements, causing a severe degradation of the resolution. Spectra measured on this instrument were removed as outliers. For measurements made on FT-NIR spectrometers, it is shown that it is important to check the resolution at which the spectrum has been measured using lines in the vibration-rotation spectrum of atmospheric water vapor and to specify the peak-picking and baseline-correction algorithms that are used to process the measured spectra. The variance between the results given by the four different methods of peak-picking and baseline correction was substantially larger than the variance between the remaining five measurements. Certain bands were found to be more suitable than others for use as wavelength standards. A band at 5943.13 cm−1 (1682.62 nm) was found to be the most stable band between the four methods and the six measurements. A band at 5177.04 cm−1 (1931.61 nm) has the highest precision between different measurements when polynomial baseline correction and polynomial peak-picking algorithms are used.

Role of Scattering Coefficients in Extended Near-Infrared Diffuse Reflection Spectrometry

The role of the bulk scattering coefficient in extended near-infrared diffuse reflection (DR) spectrometry of materials with similar or identical absorption coefficients is demonstrated. Spectral differences that arise from intraclass variation in scattering coefficient strongly affect the ability to interpret certain spectral regions. Profound intraclass differences in both band intensities and band shapes in the DR spectra of both polyethylene and polystyrene samples are shown. The effects of saturation and Fresnel reflection are discussed. The enhancement of absorption by the matrix and other analytes in DR spectrometry through the addition of a liquid with an index of refraction which matches that of the matrix is also demonstrated.

Reference Materials in Extended near Infrared Diffuse Reflection Spectrometry—The Misuse of Polytetrafluoroethylene

This work reports that inherent absorption by polytetrafluoroethlyene precludes its use as a near infrared or extended near infrared reflection reference material. The effect of this inherent absorption on certain analyte bands is shown. The effectiveness of a previously published method for removing moisture and hydrocarbons from the surface of polytetrafluoroethylene is shown. It is suggested that diffuse gold should be used as the reflection reference material for all measurements made below 5000 cm−1.

Contribution of Spectrometric Methods to the Study of the Constituents of Chromating Layers

The amorphous constituents of a chromating layer may be defined by the simultaneous use of diffuse reflection spectrometry in the ultraviolet and visible, and of Fourier transform infrared spectrometry. The PO4 group is characterized in the infrared reflection spectrum by bands located near 1070, 1030, and 900 cm−1 for chromium phosphate. The CrO4 group induces bands at 960, 860, and 820 cm−1 for zinc chromate and at about 980, 950, and 860 cm−1 for chromium chromates. The ligandmetal charge transfer bands (LMCT) characteristic of the chromates are situated in the region 3.35 to 4.20 eV. The study of the thermal behavior of chromium phosphate CrPO4 and zinc chromate ZnCrO4 shows that these compounds are stable up to 300°C. Complex chromates of Cr(III) may be observed in the range 150 to 300°C. These analytical data show the contribution of these methods which, used in the reflection mode, are demonstrably the best suited to the analysis of chromating layers on coated steel, of which the primary constituents are chromium phosphate and complex Cr(III) chromates. These results allow the interpretation of the thermal behavior of chromating layers.