Reflectance FT-IR Microspectroscopy of Fossil Algae Contained in Organic-Rich Shales

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Abstract
A microscope sampling accessory interfaced to a Fourier transform infrared (FT-IR) spectrometer has been employed to characterize the remains of individual microscopic fossil algae and algal colonies contained in organic-rich shales. The microspectrometer is able to measure reflectance IR spectra of samples with cross-sectional areas as small as 20 × 20 microns. The fossil algae studied include the colonial green alga Botryococcus braunii, the unicellular green alga Tasmanites, and an unidentified filamentous alga. It was found that IR spectra of the fossil algae, in common, contain intense aliphatic C-H stretching bands in the 2900-cm−1 region relative to the C=C stretching band at 1600 cm−1. The carboxylic acid C=O stretching band at 1710 cm−1 is moderately intense. The relative intensities of these bands vary among the three different fossil algae. Maximum-likelihood spectral restoration and subsequent curve fitting of the stretching vibrations of the aliphatic C-H bands provide greater insight into the aliphatic structures of fossil algae. The CH2/CH3 intensity ratio can be calculated and used to assess the relative average aliphatic chain length and the degree of branching.