Method to derive the infrared complex refractive indices n(λ) and k(λ) for organic solids from KBr pellet absorption measurements.

Obtaining the complex refractive index vectors n(ν~) and k(ν~) allows calculation of the (infrared) reflectance spectrum that is obtained from a solid in any of its many morphological forms. We report an adaptation to the KBr pellet technique using two gravimetric dilutions to derive quantitative n(ν~)/k(ν~) for dozens of powders with greater repeatability. The optical constants of bisphenol A and sucrose are compared to those derived by other methods, particularly for powdered materials. The variability of the k values for bisphenol A was examined by 10 individual measurements, showing an average coefficient of variation for k peak heights of 5.6%. Though no established standards exist, the pellet-derived k peak values of bisphenol A differ by 11% and 31% from their single-angle- and ellipsometry-derived values, respectively. These values provide an initial estimate of the precision and accuracy of complex refractive indices that can be derived using this method. Limitations and advantages of the method are discussed, the salient advantage being a more rapid method to derive n/k for those species that do not readily form crystals or specular pellets.

Hydrolysis of methylphosphonic anhydride solid to methylphosphonic acid probed by Raman and infrared reflectance spectroscopies.

Much is still unknown about the mechanisms and rates of environmental degradation of organophosphorous pesticides and agents. In this study we focus on the degradation of one organophosphorous compound, namely solid methylphosphonic anhydride [CH3P(O)OHOP(O)OHCH3, MPAN] and its rate of conversion to methylphosphonic acid (MPA) via heterogeneous hydrolysis. Pure MPAN was synthesized and loaded in open sample cups placed inside exposure chambers containing saturated salt solutions to control the relative humidity (RH). The reaction was monitored in the sample cup at various times using both infrared hemispherical reflectance (HRF) spectroscopy and Raman spectroscopy. Calibrated HRF and Raman spectra of both pure reagents as well as gravimetrically prepared mixtures were used to quantify the concentrations of MPAN and MPA throughout the reaction. Results show both HRF and Raman spectroscopies are convenient non-invasive methods for detection of solid chemicals as long as a large area is sampled to average out any spatial inhomogeneities that occur on the sample surface and minimal phase changes occur during the course of the reaction. The samples for the 54 and 75% RH studies showed significant deliquescence, and the liquid water had to be removed prior to measurement; this effect led to differences in the sample form, such that the calibration spectra were no longer valid for quantitative analysis using HRF spectroscopy. Raman spectroscopy, on the other hand, proved to be less sensitive to these effects and provided better estimation of the MPAN and MPA concentrations. The MPAN degradation rate displayed a very strong dependence on relative humidity: at room temperature the reaction showed 50% conversion of the MPAN in 761 ± 54 h at 33% RH, 33 ± 4 h at 43% RH, 17 ± 2 h at 54% RH and just 7 ± 1 h at 75% RH.