Blind phase projection as an effective means of recovering pure component spectra from phase modulated photoacoustic spectra

Photoacoustic spectroscopy can be used to non-destructively retrieve depth related information from heterogeneous samples consisting of multiple layers. Phase modulation step scan spectroscopy with digital signal processing allows simultaneous collection of spectra at orthogonal phase angles. Due to different sampling depths, probed by the in-phase and quadrature spectra, these spectra contain depth related information. Phase projection, calculating spectra at specific phase angles derived from the in-phase and quadrature data, is typically used to try to isolate the spectra corresponding to the different physical layers of the sample. This report presents an alternative approach. Independent component analysis is applied to the in-phase and quadrature photoacoustic data acquired from a sample consisting of two layers. Comparison with the phase-projection results suggests that ICA can unscramble mixed photoacoustic spectra into their pure component spectra without user intervention. This may be of value for analysis of complex heterogeneous samples that lack information about constituents or their distribution within the sample.