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Optica Publishing Group
  • Applied Spectroscopy
  • Vol. 47,
  • Issue 3,
  • pp. 265-271
  • (1993)

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 <i>Botryococcus braunii,</i> the unicellular green alga <i>Tasmanites,</i> and n 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<sup>−1</sup> region relative to the C=C stretching band at 1600 cm<sup>−1</sup>. The carboxylic acid C=O stretching band at 1710 cm<sup>−1</sup> 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 CH<sub>2</sub>/CH<sub>2</sub> intensity ratio can be calculated and used to assess the relative average aliphatic chain length and the degree of branching.

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