AJP - Endocrinology and Metabolism, Vol 268, Issue 5 E1018-E1026, Copyright © 1995 by American Physiological Society
Direct determination of deuterium in untreated water and urine by NMR: application to DLW analysis
J. T. Brenna and K. E. Yeager
Division of Nutritional Sciences, Cornell University, Ithaca, New York 14853, USA.
Determination of deuterium (D) concentration in tap water and urine is
demonstrated to average error approximately 0.5% (coefficient of variation)
using a 400-MHz nuclear magnetic resonance (NMR) instrument. Time domain
data are obtained using 0.75-ml samples in a broadband probe. Peak areas
derived from absorption and magnitude mode Fourier transforms and
least-squares fitting of the time domain free induction decays (FIDs) are
all investigated as means to derive D concentrations from raw data.
Least-squares fits using a sum of exponentially damped sinusoids, which
yields estimates for the amplitude, damping constant (relaxation time),
wavelength (resulting from mixing of precession and reference frequencies),
and phase for each of the two components, are shown to provide the best
precision for unfiltered FID. Amplitudes are proportional to the number of
spins at each frequency, as analysis of untreated urine from doubly labeled
water experiments yield highly linear washout data (r2 > 0.99998) for
baseline-corrected log-transformed data. The procedure is general and
should extend to other body fluids with minimal modifications. These data
show that least-squares curve fitting is the most precise method of
quantitative NMR data reduction for a wide range of experimental
conditions.
