Marilyn and Bill,

In my experience, the C-18 cartridges work well if you have 
particularly dirty samples.  But our TOC analysis shows that they do 
not remove 100% of organics, so you will likely have some small 
amount of organic contamination left even after filtering through 
C-18 cartridges.  It is meant to remove non-polar organics, so I 
expect it is the polar compounds are not retained in the 
cartridges.  It should certainly help to lengthen the lifetime of 
your mol. sieve column though.  We recently tried another type of 
cartridge from Dionex (IC-RP).  They claimed that it would remove the 
polar organics in addition to non-polar, but our TOC analysis showed 
that it did not.  We were experimenting with relatively clean samples 
(Greenland snow).

As for what it is that is gumming up your column...
At one point I tried to measure O-18 in organics with a high C/O 
ratio using a TCEA reduction furnace.  I had the same experience as 
you did, and at the time I guessed that I was making bucky balls in 
the reduction furnace and that was the culprit.  Adding a liquid 
nitrogen trap before the mol. sieve column didn't help.

Regards,
Becky

At 07:05 AM 11/17/2006, you wrote:
>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
>
>For express mail: 1125 Faucette Dr.
>
>(919) 515 - 7143 Office
>(919) 515 - 7802 Fax
>(919) 515 - 3689 Lab
>(919) 515 - 7911 Field Lab
>[log in to unmask]

Becky Alexander
Assistant Professor
Department of Atmospheric Sciences
University of Washington
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http://www.atmos.washington.edu/~beckya/

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