Differential Open-Ended Photoacoustic Cell with a Differential Microphone

Abstract

A differential open-ended photoacoustic cell was developed. The cell consists of two equivalent cell chambers and a differential microphone, adapted from an electret condenser microphone. The performance of the differential open-ended photoacoustic cell was compared to 1) that of a conventional air-tight photoacoustic cell and 2) that of a single open-ended photoacoustic cell (with the same dimensions as each of the differential cells and a conventional electret condenser microphone), using a pulse-excited photoacoustic spectrometer. The rejection ratio of background noise was comparable to that obtained by the conventional photoacoustic cell and superior to that of the single open-ended photoacoustic cell. The signals for carbon black obtained by each side of the differential open-ended photoacoustic cell were of comparable amplitude, though the signal waveforms were mirror images of each other. Using these cells, linear relationships between the amount of Sudan Red spotted on filter papers and photoacoustic signal intensities were obtained in the range of 0 to 300ng. The detection limits of Sudan Red were 1.48, 17.9 and 10.14ng, for a conventional photoacoustic cell, a single open-ended cell and the newly devised cell, respectively. The new cell was applied to a human forearm and photoacoustic signals for melanin and red ink on the skin were observed without significant noise.