Quantification of C02 concentration in Apatite

Abstract

The infrared (IR) method for determining CO2 concentrations in apatite was calibrated with absolute concentrations obtained through Nuclear Reaction Analysis (NRA). IR data were obtained on double-polished apatite samples of varying thickness using polarized transmission infrared spectroscopy. Due to the various sites and orientations of CO3 2– in apatite, the IR spectra are complicated and do not have the same shape in different apatite samples. Hence, simple peak heights are not used to characterize CO2 concentrations in apatite. The total absorbance (Atotal) was derived using the integrated area under the curves in a given spectral region (for a given vibrational mode but typically include differently sited or oriented CO3 2– subspecies). Then Atotal is calculated as AE//c + 2AE⊥c The calibration has been carried out for two wavenumber regions, one with high sensitivity and the other can be applied to apatite with high CO2 concentrations. One calibration is for the fundamental asymmetric CO3 2– stretching at wavenumbers of 1600-1300 cm–1, and the CO2 concentration in wt% can be obtained as (7.56±0.36)Å~10–6 Atotal/d where d is sample thickness in cm. The fundamental stretching bands are strong and hence sensitive for measuring low CO2 concentrations in apatite, down to ppm level. The second calibration is for the overtone CO3 2– bands at wavenumbers of 2650-2350 cm–1, and the CO2 concentration in wt% is (9.3±0.6)Å~10–4 Atotal/d where d is sample thickness in cm. The overtone bands are weak and hence are useful for measuring high CO2 concentrations in apatite without preparation of super-thin wafers. The anisotropy is significant: difference between AE//c and 2AE⊥c can reach a factor of 2.8. Hence, for high-accuracy, it is best to use polarized IR to determine CO2 concentrations in apatite. For rough estimation, unpolarized IR spectra may be used by estimating Atotal = 3Aunpol, where Aunpol is the average of integrated absorbance from unpolarized spectra.