Photothermal detection of trace optical absorption in water by use of visible-light-emitting diodes

Visible light emitting diodes of three different colors have been used to detect an absorbing compound (potassiumpermanganate) in trace quantities in aqueous solution. Photothermal absorption in a closed cell caused deflection of a water meniscus held at a small pinhole. The displacement was monitored using optical fiber interferometry. The technique was limited by LED emission intensities and environmental acoustic noise, giving minimum detectable absorption coefficients of 2x10-4 cm-1 at 478 nm and 658 nm, and 3x10-4 cm-1 at 524 nm. The magnitude and form of meniscus deflection signals were shown to be in good agreement with theory.