Developing FTIR microspectroscopy for analysis of plant residues on stone tools

The analysis of residues on stone tools can yield important insights into the tool-using behaviors of Paleolithic hominins. The ambiguity of residue identifications using visible-light microscopy (VLM) has led to the development of additional techniques for their characterization. Reflectance-based Fourier-transform infrared microspectroscopy (FTIRM) is a technique with great potential to aid in non-destructive residue identifications. Recent applications of the technique, however, have been hampered by methodological challenges, causing the infrared signals to be dominated by the stone rather than the residues. We address this problem by systematically testing the limits of FTIRM on five categories of experimental plant residues (wood bark, wood pith, grass leaves, starch, and resin). We demonstrate that it is possible to obtain FTIRM spectra of in situ plant residues wherein the effect of the stone is virtually eliminated. We also generate reflectance FTIRM spectral standards for each plant residue investigated and provide peak assignments for the major peaks in all spectra. The sensitivity of the technique means that slight differences in sample preparation can result in spectral differences as well. This means that archaeological application of the technique will require (1) careful, peak-by-peak analyses of the results, (2) extensive spectral libraries, and (3) research into the effects of decomposition.