Novel resonance profiling and tracking method for photoacoustic measurements

Resonant photoacoustic (PA) detection is widely used in several atmospheric and industrial monitoring applications due to its high sensitivity and short response time. However, unexpected changes in the acoustic resonance frequency of a PA cell caused by sudden changes either in the composition or the temperature of the sample gas can largely diminish the precision of the PA measurement. This paper describes a novel method for tracking such changes in resonance frequency. Besides improving the measurement precision, the introduced CHIrped modulation for Resonance Profiling (CHIRP) method has the additional advantage of maintaining the fast response time of the PA system without using any additional hardware components. The minimum detectable water vapor concentration, depending on the modulation bandwidth of the CHIRP, was found to be 0.3–0.5 ppm in nitrogen buffer gas. The applicability of the CHIRP method was demonstrated in PA measurements in a buffer gas with varying composition, which are typical in, e.g., industrial monitoring applications.