ADVANCES IN OPTICAL WATER ISOTOPE RATIO MEASUREMENTS
7. DISCUSSION
Optical isotope ratio measurements have clearly matured to the point where the deuterium analysis of water has become competitive with IRMS in terms of accuracy. Here, fractionation effects during sample handling constitute the dominant source of analytical errors. Although, it appears unlikely that an optical method will be able to reach the very high accuracy (~0.03‰) possible with IRMS for δ18O determinations on natural samples, current optical instruments are coming close, and are certainly able to satisfy the needs of many users. For highly enriched samples the precision and accuracy of laser spectrometry have already been shown to exceed those of IRMS. The possibility of using laser spectrometry to measure δ17O easily may be used in relation with the mass-dependent fractionation relation [28] for independent verifi cation of δ18O measurements on the same sample [15], or to verify the applicability of this relation. Or in altogether more exotic applications, such as the determination of the 17O concentration in the heavy water used in a solar neutrino experiment [29].
A growing number of spectrometers use CRDS, or one of its derivatives.
This is a logical choice for an ultra-sensitive spectrometer for the isotope analysis of water at the extremely low water concentration encountered in the upper troposphere and lower stratosphere, but may not be so for an instrument designed to measure water at more elevated concentrations or liquid samples.
One reason is that such an instrument is almost inevitably built with just one gas cell. Sample and reference are therefore measured sequentially, increasing the likelihood of a temperature difference between the two measurements producing an apparent isotope shift (typically several ‰ per degree K). For this reason we have implemented the possibility to measure a second H16OH feature with a very different line strength temperature coeffi cient, thus providing a build-in gas thermometer by which to correct the measurements.
Of course, precision and accuracy are not the only factor to be considered when comparing laser spectrometry to IRMS. There are many applications for which other aspects, like robustness, portability, remote operation in a hostile environment, compactness, and the real-time measurement capability, are more important. Other advantages of optical methods that may be considered are the high selectivity, thus avoiding the need of chemical sample pretreatment, the non-destructive nature of the measurement, making it possible to recover the sample if needed, the absence of consumables (like an ionization source
fi lament), and, low cost. Last but not least, methods based on direct absorption are conceptually simple, which helps reduce the required scale correction and normalization with respect to IRMS.
The arrival of a number of commercial suppliers of optical spectrometers signals the acceptance of end users of the technique. The near future will surely see even more precise and accurate, as well as more sensitive, devices. Also, more attention will be paid to sample handling and calibration strategies.
ACKNOWLEDGEMENTS
We are indebted to past and present students and post-doctoral fellows who work(ed) in our laser spectrometry laboratory, and our colleagues Harro Meijer and Henk Visser. We also like to thank all our collaborators, and in particular Sigfus Johnsen, Gianluca Gagliardi, Livio Gianfrani, Daniele Romanini, Marc Chenevier, Samir Kassi, and, last but not least, Hans-Jürg Jost. Substantial fi nancial support was obtained from the Dutch Foundation for Research on Matter (FOM), the Royal Academy of Arts and Sciences (KNAW), and the University of Groningen.
This paper is dedicated to the memory of Henk Visser, who passed away much too early on June 3, 2007, after a courageous and inspirational battle with cancer. His great enthusiasm and love for science will continue to guide us.
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