Hello Isotopers,

We've been attempting to use our Gas Bench to do hydrogen analysis of 
waters using platinum catalysts and headspace equilibration at about 
32degreesC. When we started, we got acceptable numbers with good within 
analysis (10 peaks) standard deviations. Values plotted sensibly along (or 
very near) the meteoric water line. As time went on, our standard 
deviations have gotten much worse (as much as 7 permil) and our data very 
seldom plot reasonably close to the meteoric water line. Often, hydrogen 
data plot well above the meteoric water line. We have gotten great results 
from oxygen analyses of the same samples.

We've just spent a great deal of time and effort to ensure that our sample 
gas is totally dry before it enters the mass spectrometer. I also let the 
catalysts sit in our oven (60degreesC) for two days before placing them 
into the vials with our water samples. We got better standard deviations 
for our analyses this time (between 2 and 3), but still have unreasonable 
scatter around the meteoric water line.

Questions:

1) What is a generally acceptable standard deviation for d2H from water?

2) Could the problem be with our catalysts? I thought that, by definition, 
catalysts were reusable .. but maybe not. We've only done about 5 analyses 
with each catalyst.

3) Are we going to have this same problem when we start running waters 
using our TC/EA instead? or are we going to be happy that we made the change?

If anyone out there in cyberspace has dealt with and corrected this 
problem, we would love to hear from you!

Thanks,

~Penny

*******************************************************************
                       Dr. Pennilyn Higgins
                        Research Associate

                             "SIREAL"
   Stable Isotope Ratios in the Environment Analytical Laboratory

         Department of Earth and Environmental Sciences
         University of Rochester
         227 Hutchison Hall
         Rochester, NY 14627

                    [log in to unmask]

Office: 209b Hutchison Hall               Lab: 209 Hutchison Hall
Voice : (585) 275-0601              Outer lab: (585) 273-1405
FAX   : (585) 244-5689              Inner lab: (585) 273-1397

           http://www.earth.rochester.edu/SIREAL/index.html
*******************************************************************