Development of photoacoustic spectrometry for measurement of isotope ratio.

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A photoacoustic spectroscopy technique is applied to the isotope ratio measurement of ィイD116ィエD1O and ィイD118ィエD1O in water-vapor samples, using a pulsed tunable dye laser pumped by Nd : YAG laser. The third overtone band (3vィイD2OHィエD2) of water molecules near 950 nm was investigated, for two standard water samples, V-SMOW and SLAP. We identified the absorption lines of HィイD22ィエD2ィイD116ィエD1O and HィイD22ィエD2ィイD118ィエD1O in the photoacoustic spectra that we measured by using the HITRAN database. We measured the difference in the ィイD118ィエD1O/ィイD116ィエD1O isotope ratios for V-SMOW and SLAP, using the photoacoustic method. For each water isotope (HィイD22ィエD2ィイD116ィエD1O and HィイD22ィエD2ィイD118ィエD1O) peak we made 128 times averaging of the photoacoustic wave on the oscilloscope and scanned the laser wavelength within a selected range in 0.002 nm increments. The measurement time was 7000 s for each sample. The value obtained for the difference between the two samples is δィイD118ィエD1OィイD2SMOWィエD2=-55±10‰, whereas the value measured with a conventional isotope mass spectrometer is δィイD118ィエD1O=-55.5‰. That δ-value was certainly an improvement on the previous work. However, the obtained δ-value has a relatively large deviation, because the deviation should be less than 1-2‰ for practical use. Further improvements are necessary to reduce the deviation. The large deviation comes mainly from the low signal-to-noise ratio in the photoacoustic spectra.