Optoacoustic spectroscopy and its application to molecular and particle absorption

Light absorption in the ocean has been the least studied optical property because of the difficulties in making accurate measurements. With the previously used techniques, large differences have been reported for the specific absorption coefficient of phytoplankton (cultures and natural assemblages). It is difficult to determine if the diversity in these values are methodological or a function of actual variation in absorption. With the renewed interest and activity in optoacoustic spectroscopy (OAS), which accurately measures absorption, some of these discrepancies should be resolved. In this method, as molecules and particles absorb light from a modulated source, they thermally expand and contract, thereby generating acoustic waves, at the modulation frequency, which are detected by a hydrophone. Optoacoustic spectroscopy is ideally suited for measuring dissolved organic material and particle absorptions because of its high sensitivity (10^-5m^-1) and the negligible effect of scattered light. In this paper the instrumental design for an optoacoustic spectrophotometer (OAS), which specifically measures phytoplankton absorption (420-550nm), is described. The spectral absorption of dissolved organic material and a phytoplankton culture is presented. OAS holds promise in being able to measure absorption without use of either filtration or concentration techniques.