Applications of laser photoacoustic spectroscopy using an optical parametric oscillator to the study of complexation equilibria in dilute aqueous solutions

A 30 Hz YAG laser pumped, narrow-line, optical parametric oscillator (OPO) has been used to investigate the chemical states of selected compounds in aqueous solution. The OPO system has the advantage that it can scan wavelengths over large ranges without significant loss of output power. The visible spectra of rare earth complexes in dilute aqueous solutions (as chemical analogs for the actinides) have been studied and preliminary qualitative results for erbium acetate complexes at the 100 micromolar level are in good agreement with literature data. Quantitative measurements of the protonation constant for phenol red at the 100 nanomolar level were measured. The particular implementation of the OPO system used in this work introduced a number of spectral artifacts due to optical signal overlap with near-infrared water absorption bands. In addition, it was demonstrated that the concentration limiting factor for photoacoustic spectroscopy is the intrisic absorption of water in the visible region of the spectrum.