I had excellent results for 0.1 to 0.2 ml samples with the modified VG
equilibrator Tyler Coplen and I used in his USGS lab in Reston.  We used 15
cc flat-bottomed vessels, weighed in the exact amount of water wanted (or
make corrections for the differences in water volume), added 500 umol CO2,
and equilibrated the samples for 10 h instead of the usual 5 h.  As I
remember, these samples needed a slightly larger "fudge" factor for
fractionation of the water during pumping than for larger samples, and the
analytical precision was worse than usual (0.1 permil instead of 0.05).  

We have also analyzed 0.2 ml samples with the modified Finnigan equilibrator
here at the USGS lab in Menlo Park.  Here we carefully pipet in the water,
use 200 umoles of CO2 in 15 cc vessels, equilibrate for some 30 hours (the
shaker is not very efficient), and also get a 1 sigma precision of 0.1
permil for sets of replicates, instead of the normal 0.05 permil. Again,
there is a larger evaporative fractionation than with larger samples.

I have not tried smaller samples with the automated equilibrators (I used
Sakai's method instead), but I suspect that you could successfully analyze
even smaller samples if you didn't mind longer equilibration times and
impaired precision.  Good luck.

--Carol



>Dear Subscribers,
>
>I am conducting some tracer experiments using 18O labeled water, which
involve a large number of water samples. Due to the nature of the experiment
I would really like to minimize the water sample size into the 10's-100's of
microliter water range, if at all possible. With our commercial water
CO2-H20 equilibrator (VG-Isoprep18 - Optima) the typical sample size is 2-3
ml.  
>
>I have heard rumors that some people have successfully modified their
commercial equilibration systems to run microliter sized samples in the
range I'm interested in. If so, I would be interested in how this was done,
what modifications were required to vessels, instrument parameters, etc. The
worst precision I can live with is  +/- 1 permil.
>
>Any advice, suggestions and experience greatly appreciated.
>
>Len Wassenaar
>National Hydrology Research Institute
>Environment Canada
>11 Innovation Blvd.
>Saskatoon, SK
>Canada  S7N 3H5
>tel: (306) 975-5747
>
>
Carol Kendall                                           Phone:
+1-415-329-4576
U.S. Geological Survey                                Fax:       +1-415-329-4538
345 Middlefield Road, MS 434                      
Menlo Park, CA, 94025 USA                         email:   [log in to unmask]