Calibrations for the determination of water species in bulk and hydrated obsidian by infrared photoacoustic spectroscopy

The obsidian hydration dating method requires a measurement of the amount of diffused water (%H2OD) within the surface hydration layer and an assessment of the structural water (%OH) contained within the underlying glass matrix. Infrared photoacoustic spectroscopy (IR-PAS) has the ability to sample deeply within the glass and capture both regions simultaneously. The calibrated infrared water band at 3570 cm− 1 may be used to measure total water (%H2Ot = H2Om + OH) content and the concentration of diffused molecular water (%H2OD) is assessed with the band at 1630 cm− 1. Subtraction of the band specific concentrations provides an estimate of the obsidian structural hydroxyl (%OH) content. Calibrations of the IR-PAS water bands, to permit inferring water concentrations from IR-PAS measurements, were developed using obsidians of known water content established by standard infrared transmission. Three calibration equations are presented, for H2Ot, H2Om, and OH, along with computed estimates of accuracy.