Tunable ultraviolet visible photoacoustic detection Analysis of the sensitivity and selectivity provided by a xenon flash lamp

A fundamental understanding of the physical and chemical properties that dictate the absorption of pulsed light and subsequent release of heat to generate a transient pressure wave was used to test the concept of a multi-wavelength photoacoustic detector. Analysis of the wave equation predicts that with long-pulse excitation, i.e. microseconds FWHM, absorption by the solvent, and not electrostriction determines the limit of detection. Calculation of the mechanical to electrical conversion efficiencies of piezoelectric transducers shows that absorbed pulse energies of microjoules, typical of pulsed flash lamps, are sufficient to provide measurable photoacoustic signals. A pulsed xenon flash lamp (2µs FWHM) which emits a broad spectrum over the UV and visible region is used as an excitation source to detect trace quantities of metals in aqueous solution using pulsed photoacoustic detection. In this work, dielectric mirrors were used to detect two analytes, CrO42- (monitored at 355nm) and Co2+ (monitored at 532nm), simultaneously. This approach increases both the flexibility and selectivity of pulsed photoacoustic methods. We obtained a detection limit of 2.6×10-4 absorbance units per centimeter in aqueous samples. This work shows that pulsed lasers are not a necessity for ultra-sensitive photoacoustic spectroscopy.