Hi Marilyn,

We have been running a lot of silver nitrate samples in our TCEA and 
have noticed that some samples come out of the freeze drier "fluffy" 
or dingy brown in color.  We redissolve these samples and put them 
through a SPE C-18 resin filter, lyophilize them again, and repeat 
these steps until they come out a pure white granular powder - just 
like the reagent grade stuff.  It appears that the NO3/DOM ratio in 
the sample determines how many C-18 cleanings are required, we notice 
problems in forest stream samples or groundwater with small amounts 
of nitrate.  The agriculture samples that are contaminated with 
nitrate seem to process very well?

We generally run 70 - 140 samples through the TCEA and then bake out 
the GC column.  We originally had to replace the GC column a few 
years back, before we started the C-18 resin cleanings. But since we 
have used the C-18 cleaning step we have had no problems, and we have 
run many 1000's of samples of silver phosphate and silver nitrate.

The SPE C-18 cleaning technique was first mentioned for silver 
nitrates in Michalski et al., 2004, Environ. Sci. Technol. 38, 
2175-2181. Greg is quite clever and worked this out while he was at 
UCSD, so he get's the credit for this important step in AgNO3 prep 
practices (but I think he found this referenced in some other  obscure paper).

You can get the SPE C18 resin filters from Aldrich (Cat # 
205250).  We have bought the resin in bulk and made our own columns, 
but it makes a mess. It is a more efficient use of the tech's time to 
process samples - so now we just use the pre-packaged columns fro 
Aldrich.  The guys in the lab have rigged up a vacuum setup so the 
samples can be process in a matter of minutes before freeze drying.

So don't give up on those ag nitrate samples!  Any other specific 
things we can help you with just call or contact me directly.

All the best,
Bill


At 05:45 PM 11/16/2006, you wrote:
>(An Aside: When last I wrote to you concerning the trials of Isodat,
>one young, astute reader of Isogeochem asked me if I maintained an
>isotope blog, as he would like to read it. Naw, we older folk know
>little of blogs, as that sounds too much like the shape of ourselves
>as we get older.)
>
>This message concerns a contaminated molecular sieve column that is
>on-line in a TC/EA instrument. I have used the TC/EA for analyzing
>all sorts of things including rocks, plants, carbonates, etc. With
>silver phosphates, we learned to install a trap prior to the Conflo
>to keep elemental P and Si from entering the capillaries and clogging
>them.
>
>For several years, I have been analyzing silver nitrates. Always a
>"joy" to prep and run, nonetheless silver nitrates produce nice N2
>and CO peaks that are typically straightforward to analyze, if they
>are pure. My latest challenge has been a series of silver nitrates
>prepared from nitrates leached from agricultural soils and nitrates
>found in well waters adjacent to agricultural fields.
>
>Standards run fine, nice peaks, ample separation. Silver nitrates of
>stream waters are no problem. But after 2 or 3 soil-derived or farm
>field-derived AgNO3s are reacted, the molecular sieve GC column
>clogs. Baking at 300 C overnight, 24 hrs, 36, or 48 hours does not
>help the problem. In fact, whatever is on the column to start with
>manages to move along a bit and stop flow completely.
>
>I have become adept at repacking molecular sieve columns, baking them
>out, and starting afresh, each time with eager anticipation of data
>to come, big ideas on revamping agricultural practices dancing in my
>head.
>
>I am stumped. We know that there is some organic contamination on the
>silver nitrates: maybe 1% C or so. This means of course that there is
>a bit of Organic N and O as well, which we are well aware of and are
>doing tests to see what the effects are on isotopic composition or
>yield.
>
>I don't know about S, or Fe, or K, or P. X-ray crystallography
>examinations of the offending AgNO3 crystals show only AgNO3.
>
>If you have any information on 1) what can contaminate a Molecular
>Sieve 5A that can not be baked off at 300C? or 2) what might
>contaminate an agricultural sample other than dissolved organic
>matter? I would be happy to know this.
>
>The postdocs in my lab have been making silent and secret bets on
>when I will give up this Fight with the Nitrates. Not yet.
>
>Regards, Marilyn Fogel

William J. Showers
Dept of Marine, Earth & Atm Sciences
Box 8208
North Carolina State University
Raleigh NC 27695

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