Modeling the attenuated total reflectance infrared (ATR-FTIR) spectrum of apatite

Abstract

. Modeling the attenuated total reflectance infrared (ATR-FTIR) spectrum of apatite. Physics and Chemistry of Minerals, Springer Verlag, 2016, 43 (9) 3

Macroscopic electrostatic effects in ATR-FTIR spectra of modern and archeological bones

“…The ATR spectra of these two samples are about four times more intense than those recorded on the collagen-rich bone samples. As previously observed by Aufort et al (2016), significant differences are observed between the spectra recorded using a Ge or a diamond crystal for the whole series of samples. Beside a stronger intensity, the intense n 3 stretching and n 4 bending PO 4 bands of the diamond-ATR spectra are broadened on their low-frequency side, and the n 3 band overlaps with the weaker n 1 PO 4 and n 2 CO 3 bands.…”

“…However, the theoretical spectra of collagen-rich samples overestimate the intensity of their experimental counterparts, which are about four times less intense than those of collagen-poor samples (e.g., for modern ox bone the theoretical intensities require a 0.175 scale factor to match the experimental ones). As discussed in Aufort et al (2016), this theoretical overestimation is most likely related to sample heterogeneities, but further grinding of the collagen-rich samples did not allow recovering the theoretical intensities. Note that a decrease in the mineral fraction modifies the line shape (Supplemental Information) and cannot account for a simple decrease of the whole spectrum intensity.…”

“…We stress that our modeling strategy involves a compromise between a minimal variation of model parameters (Table 1) and a reproduction of relative variations observed among the spectra. The bulk apatite properties (Table 2) are modeled using frequencies and force oscillator strengths close to those previously used to model fluorapatite spectra (Aufort et al 2016). However, significantly higher broadening parameters must be used to account for the lower degree of crystalline order and higher strain of biogenic apatite particles.…”

“…The modeling strategy has been extensively detailed in Aufort et al (2016). Briefly, the theoretical ATR spectrum is obtained by computing the frequencydependent reflection coefficient with a 45° incidence angle at the interface between the ATR crystal, defined as a homogeneous isotropic medium with constant refractive index (2.4 for diamond and 4 for Ge), and the sample.…”

“…The absorption-like ATR spectrum is recorded on the packed sample powder by measuring its reflection coefficient at the interface with a highly refractive material. However, the different geometry and nature of the sample may impede a straightforward comparison of ATR with transmission spectra (Aufort et al 2016). Differences in the crystallinity degree and carbonate-to-phosphate ratio of samples measured in transmission, ATR, and diffuse reflectance geometries have been reported (Beasley et al 2014).…”