Dear Franco,

Determining the concentration of deuterium in hydrogen gas with
satisfactory accuracy is relatively easy.  One only needs to measure
the unknown hydrogen gas relative to hydrogen extracted quantitatively
from VSMOW and SLAP reference waters--uranium reduction has provided
excellent service during the last five decades for this quantitative
reduction.  The delta value of the unknown gas is calculated on the
VSMOW/SLAP normalized scale.

The absolute deuterium concentration can then be calculated because the
absolute isotopic composition of hydrogen is well calibrated to the
relative scale for hydrogen.  You may know that the value of -428 per
mill was assigned to SLAP reference water relative to VSMOW reference
water by considering the absolute isotope ratio measurements of both
reference waters performed by three groups:

  VSMOW              SLAP               Delta         References
  0.000 155 76(5)    0.000 089 02(5)    -428.5(4)   Hagemann et al, 1970
  0.000 155 75(8)    0.000 089 12(7)    -427.8(5)   De Wit et al, 1980
  0.000 155 60(12)   0.000 088 88(18)   -428.8(13)  Tse et al, 1980

Because many laboratories can measure hydrogen isotope ratios normalized
to the VSMOW/SLAP scale with an uncertainty of better than 1 per mill, an
uncertainty in the fraction of deuterium of the order of 0.000 000 1
can be obtained by careful stable isotope laboratories.

It should be noted that there is no evidence of low salinity water
changing its hydrogen isotopic composition over time when sealed in
glass ampules; thus, requesting a stable isotope ratio analysis of
hydrogen gas using uranium reduction of reference waters should provide
the needed accuracy in the absolute ratio determination.

Tyler Coplen
U.S. Geological Survey