Open-ended photoacoustic cells:  application to two-layer samples using pulse laser-induced photoacoustics

To measure the difference between samples in situ, two types of open-ended photoacoustic cells were constructed. One type was a differential open-ended photoacoustic cell with a differential microphone, which connected two equivalent open-ended cell chambers through two waveguides. The other design used single open-ended photoacoustic cell chambers, each with an electric condenser microphone. A pair of these was used to suppress background signal by subtraction of the signals. The performance of these cells was compared to that of a conventional photoacoustic apparatus with a chopper. Detection limits of Sudan red for those cells were about 1.29 and 2.35 ng for a differential open-ended cell and a single open-ended cell, respectively. The photoacoustic cells were then applied to the pulse mode operation, using a nitrogen laser as a pulse light source. The laser beams were led to the photoacoustic cells through quartz optical fibers. Using this instrument setup, depth profiling analysis was carried out for two-layer samples made of poly(methyl methacrylate) (PMMA) or polystyrene (PS) films. A linear relationship was obtained between the signal delay time and the film thickness. The delay time of the signal (τ) was expressed in terms of the thermal diffusivity (κ/cm(2) s(-)(1)) of the film as τ (ms) ≅ (2.25 × 10(-3)) x (μm) κ(-1/2). The method was successful in measuring the thickness of transparent surface layers. We applied this technique to observe the generation of a certain chromophore, thought to be melanin, under the surface of human skin tanned by exposure to long-wavelength UV light.