PNNL

Abstract

A benchtop Fourier transform infrared spectrometer and specular reflection accessory has been used to record reflection spectra of several chemical compounds coating a loamy, psamment soil. Unpolarized reflection spectra were recorded between 4600 and 500 cm-1 with a resolution of 4 cm-1 for the compounds dimethyl methylphosphonate, trimethyl phosphate, methylphosphonic acid, 2,2'-thiodiethanol, diazinon, diesel fuel, and ammonium nitrate mixed separately into soil samples with concentrations of 10, 5, and 1 mg of analyte per gram of soil. The soil reflection spectra are compared to liquid or solid transmission spectra of the pure compound and frequency shifts and relative intensity changes for the absorption features are noted. As an example of detection sensitivity, we have estimated that we can detect one of the C - H stretch modes of dimethyl methylphosphonate with a signal-to-noise (peak-to-peak) of 3 for 300 nanograms of analyte on the surface layer of soil in the focal spot of the reflectance accessory. We also estimate that the flux of infrared photons reaching the soil is 1018/sec. We have recorded polarization resolved reflection spectra of the uncoated soil and coarse and fine grained sands as a function of angle of incidence and reflection and have determined the degree of polarization for light reflected off of these materials at frequencies associated with volume scattering and surface scattering features. As might be expected, the volume scattering features show a significant depolarization of the light - degree of polarization after reflection is 75%. We have also recorded polarization resolved spectra of tributyl phosphate on the soil and found significant differences between the s- and p-polarized spectra. This fact could be used to employ polarization modulation detection to improve detection sensitivity.