Listening to Colloidal Silica Samples: Simultaneous Measurement of Absorbed and Scattered Light Using Pulsed-Laser Photoacoustics

Abstract

Laser photoacoustic spectroscopy (LPAS) has been used to simultaneously measure scattered light and absorbed light with the use of a single piezoelectric transducer detector. Samples of Ludox™ colloidal silica, with and without added potassium chromate, were illuminated with pulses of 308 or 355 nm light. Signals were measured with a 1 MHz ultrasonic transducer clamped to the side of the cuvette. The resulting oscilloscope tracing shows a pattern of signals originating from light scatter and light absorption that is consistent with experimental geometry and the speeds of light and sound, and linear with incident laser pulse energy. The absorbed light signals are independent of the distance of the laser beam from the transducer, while the scattered light signals are strongly distance-dependent. The absorbed light signals are more indicative of true sample absorbance than are readings from a standard spectrophotometer. A spectrophotometer sums contributions from scattering and absorbance to give an optical density reading, while LPAS separates scattering from absorbance to give quantitative information on each function. Scattered light and fluorescent light behave very similarly in the LPAS technique. Our experiments were done with well-characterized colloidal silica samples, but the technique is readily extended to larger, more heterogeneous, and photochemically complex biological and environmental samples.