Simultaneous photoacoustic detection of multiple compounds based on orthogonal functions stimulation

The paper presents a concept of photoacoustic measurements based on use of two different light wavelengths which intensities are modulated with sine waves of the same frequency but with phase difference of 90 degrees. Resultant photoacoustic signal is of the same frequency, but its amplitude and phase depends on the absorption at both wavelengths. Taking into consideration that sine and cosine are orthogonal functions, and having measured amplitude of the photoacoustic signal and its phase referred to the phase of the stimulating light modulation, it is possible to retrieve both components corresponding to sine and cosine modulation. As a result, the method can be applied to simultaneous detection of two compounds. An important advantage of the method is that it can be comfortably used with high Q-factor cells.