>Hi,
>I am a Master's student at Colorado State University, and I am getting 
>ready to do some sulfur isotope work in a subalpine wetland complex.  We 
>also have some nitrogen isotope work in forest soil-water starting up.  
>We are interested in the isotopes of sulfur on sulfate and nitrogen on 
>nitrate, and are currently running our water samples through anion 
>exchange resin to adsorb the sulfate or nitrate.  The waters we are about 
>to begin sampling have high DOC (about 10mg/L) and this will interfer 
>with the anion adsorption.  A cation column preceeding the anion column 
>cannot remove enough DOC, and we have thought about using organic 
>specific resin (XAD8) or activated carbon.  But, we have also thought 
>about some sort of membrane which would selectively remove the DOC and 
>allow the water and our anions to move through and adsorb onto the anion 
>exchange resin.  I am interested if anyone might know of something like 
>this that work.  It seems as if most reverse osmosis-type membranes 
>remove everything, and that would not be good...
>
>any suggestions?
>Cyndi Brock
>

Dear Cynthia:

I am not sure if this helps you very much but I thought I simply let you
know that we are planning to do similar things as discribed above.

So far we achieved very good results by using Bio-Rad AG 1-X8 resin (also
available in prefilled Poly-Prep colums) for removing sulfate and nitrate
simultaneously from test solutions with various concentrations. By eluation
with 3M HCL we got close to 100% recovery. However our test solutions were
free of DOC. So far, I was under the impression that as long as you keep the
resin capacity large enough to accommodate sulfate, nitrate, and organics
there should be no interference with the sulfate and nitrate adsorption even
at high DOC contents.

However, we are also interested in a quantitative removal of DOC for another
reason. We want to determine not only the isotope ratios of sulfate-S and
nitrate-N from our samples but also the oxygen isotope compositions of both
sulfate and nitrate. For this purpose you need to remove as much DOC as
possible since it constitutes an additional oxygen source (DOC contains as
much as 20% oxygen) which can interfer with the oxygen isotope analysis. Our
plans are to use consecutive colums filled with XAD-4 and XAD-8 resins but
we know that they will not remove DOC completely. Therefore, I am very
interseted to hear if you find any better solutions for this problem (such
as membranes etc.).

For further information you might want to look at

Wassenaar L. Appl. Geochem. Vol. 10, 391-405 (1995) 

or contact Steven Silva ([log in to unmask]) or Cecily Chang
([log in to unmask]) at the USGS, 345 Middlefield Rd, MS 434, Menlo Park, CA
94025. Both of them are much more experienced in running "real" samples
(such as soil seepage water or throughfall samples) than I am.

Regards,

Bernhard
>
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Bernhard Mayer
Lehrstuhl f. Sediment u. Isotopengeologie
Institut fuer Geologie
Ruhr-Universitaet Bochum
Universitaetsstr. 150
44780 Bochum

Germany

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