Pulsed photoacoustic detection of isotopic mixtures of chloroform

Abstract
Photoacoustic spectroscopy is applied to the detection of small concentrations of CDCl₃ in CHCl₃-N2 using a TEA CO₂ laser tuned at the 10P(48) line. At this laser line both species H/D are excited but the ratio between absorbances is such that a relatively higher excitation of the deuterated species is expected compared to other available laser wavelengths. The photoacoustic signal is digitized by an oscilloscope and, at the same time, acquired through the sound port of a PC, pulse by pulse, for comparison of both acquisition methods. The signal processing is performed through the calculation of the standard deviation of each transient. For a fixed mixture the processed signal shows a very good linear dependence on laser energy around 1 mJ. Different mixtures of CDCl₃ in a fixed proportion of CHCl₃ / N₂ are prepared for studying the capability of the photoacoustic technique of distinguishing the deuterated component from the hydrogenated background. The results show that the sensitivity of this setup allows to measure smaller quantities of CDCl₃ in CHCl₃ than those corresponding to natural abundance (150 ppmV).