Amount-ratio determinations of water isotopologues by dual-laser absorption spectrometry

We propose a method for the measurements of the O-17/O-16 isotope amount ratio in water, based upon the use of a pair of offset-frequency locked extended-cavity diode lasers at 1.39 mu m. This method enables one to acquire absorption spectra with an extremely high fidelity, exploiting the highly accurate, absolute, and repeatable frequency axis. One of the two lasers, namely the so-called slave laser, is continuously scanned across a pair of (H2O)-O-16 and (H2O)-O-17 lines at 7183.5 cm(-1) and it interacts with a water vapor sample inside a multiple reflections cell, thus producing absorption spectra with a signal-to-noise ratio of the order of 4000 for a detection bandwidth of 1 kHz. The determination of the isotope amount ratio is performed through a careful analysis of the acquired spectra, by using semiclassical line profiles. In this respect, the influence of the choice of the line shape model is investigated. The experimental reproducibility of the spectrometer has been carefully assessed by means of an Allan variance analysis. Finally, the application of the Kalman filtering technique has shown that a precision of 0.6% can be achieved, from repeated spectral acquisitions over a time span of 6000 s.