Hi, David:

The inhomogeneity  of one of our working standards, a mixture of D2O and tap
water, initiated the discussion last year. The isotope value was
fluctuating by +-3 per mil (the expected value is +34 per mil) although that
water had stayed in a refrigerator at 4 degree C  for about five years.

I hope this information is helpful.

Dachun Zhang
Stable Isotope Lab
Global Geochemistry Corp.