Anal. Chem. 1980, 52,2445-2448
2445
Determination of Deuterium/Hydrogen Ratios in Natural Waters by Fourier Transform Nuclear Magnetic Resonance Spectrometry R. S. Tse,' S. C. Wong, and C. P. Yuen Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong
T h e knowledge of deuterium and oxygen-18 isotope abundance ratios in natural waters is of interest in hydrology, geology, glaciology, oceanography, meteorology, soil science, a n d mineral prospecting. These ratios are traditionally determined in specially constructed mass spectrometers of fairly high resolution (1). T h e oxygen-18/oxygen-16 ratio is commonly determined mass spectrometrically by directly measuring the H21sO+/H2160+ ratio. A commercial low-resolution instrument is sufficient. For the determination of the deuterium/ hydrogen ratio, the method described here makes use of a multinuclear Fourier transform nuclear magnetic resonance spectrometer. T h u s both ratios can be obtained in standard commercial instruments. No treatment of the sample is necessary, other than filtering if t h e sample contains particulates. T h e deuterium/hydrogen ratio is commonly determined by first reducing the water to hydrogen and then measuring the HDf/H2+ ratio in a mass spectrometer. An H3+signal dependent on pressure or residence time in the ion source is always present. For the separation of HD+ from H3+,a resolution ( M I A M where M is the mass-to-charge ratio m / z ) of at least 2000 is required; however, this can be compromised with suitable calibration in specially constructed instruments. If the water sample is not first reduced and the HDO+/H20+ ratio measured, the resolution requirement would be even higher. HDO+, lsOH+, and H30+all have m / z values close t o 19. T o separate HDO+ from H30+requires a resolution of over 12000. I n the present method, the deuterium/hydrogen ratio is determined by deuteron magnetic resonance. The authors are not aware of any previously reported attempt at using an FT-NMR spectrometer for such determination.
magnetic resonance signals are measured, using an internal working standard and calibration based on standard additions to the sample. A 2-mL water sample is needed. An internal working standard in the form of a sealed 1.7 mm diameter capillary containing a saturated lithium iodide solution in approximately 2 % deuterium-enriched acetone is placed coaxially in the 10 mm diameter NMR sample tube. The lithium solution provides a signal for the internal locking of the instrument (JEOL Model FX-9OQ). The value of 2% deuterium enrichment in acetone has been selected to match the size of deuteron resonance signals from natural waters. The ratio of integrated deuteron magnetic resonance signals from the internal working standard and the water sample is obtained. Calibration of the internal working standard is achieved by adding known volumes of deuterium oxide to a sample. The accuracy depends, of course, on the knowledge of the purity of the standard deuterium oxide that has previously been determined by proton NMR. We normally obtain our data after accumulating 400 transients, which require about 24 min. Under these conditions, the width at half maximum of the water signal is about 1.3 Hz, the digital resolution is about 0.12 Hz, the signal-to-noise ratio is about 118, and the reproducibility is good to about 1.3%. The last figure can be reduced to
