High sensitivity photoacoustic spectrometer for variable temperature solution studies

Abstract

A single beam, dual cell, photoacoustic spectrometer has been developed for use with solutions from ambient temperature to the boiling point of the liquid. The optical absorbance sensitivity of the apparatus is 3×10−7 absorbance units per cm (base 10 logarithm, 1000 pulse average, 13 mJ/pulse) for solutes in aqueous solution at 90 °C, using an excimer-pumped dye laser as the light source. The cell assembly is designed to thermally isolate the acoustic detector and prevent evaporative loss of the solvent. High accuracy subtraction of solvent optical absorption is achieved using preamplifiers with continuously variable gain, matched electrical bandpass filters, and a high common mode rejection ratio differential amplifier. Design constraints resulting from factors such as the temperature dependence of solute photoacoustic signal amplitude are discussed and results of experimental tests probing some limits of the identified constraints are presented. A data analysis method is described which substantially reduces the effect of transient acoustic interference from suspended particles in sample solutions.