Update on the silver nitrate contaminant story:

We have not nailed this absolutely conclusively yet, but are close. The
C-18 may help remove organic matter, especially if it is of the type that
is found in soils and streams. I have measured %C via EA for a number of
pre- and post- C-18 prepped samples. As Bao pointed out to me, sometimes
the column does not decrease the amount of C, and sometimes it does. I
find that the %N from EA was higher than via TC/EA, for contaminated
samples.

In our samples (and maybe yours as well) there are often some black
crystals before the C-18 step. Also, if your silver nitrate is not white,
look under the microscope, there may be another mineral phase in there.
Our samples were often pink, resulting from white and black dried down
together.

The black crystals are a calcium, titanium silicate. At the temperatures
of the TC/EA furnace, one can form TiN (stable at high temperaturea) and
CaO (also stable). Based on the appearance of the main TC/EA furnace,
these reactions were occuring in the lower regions of the furnace assembly
and also at the head of the GC column, probably because the molecular
sieve was the first point of seriously slow flow rate.

The presence of contaminants like this (common in soils and streams around
here) will result in lower N and O yields, because of cross reactions
forming more stable compounds.

To prep samples for C-18 or any other columns, the "silver nitrate" is
redissolved, and often the CaTi contaminant is not. By virtue of its
solubility it is "removed" from the purified silver nitrate.

Marilyn Fogel

> Bill
>
> I'm interested in your cleanup of silver nitrate using C-18 (is this the
> 'Discovery DSC-18'?). I tried this myself some time ago: 'dirty' silver
> nitrate (as evidenced by low N2/CO ratios from the TC/EA) re-dissolved in
> about 10 mL water and put through 100 mg DSC-18 at about 5 mL per minute
> (too fast?). The DSC-18 had been pre-conditioned with methanol and water.
> A single pass cleanup like this appeared to make no difference to the
> purity of the nitrate (by comparing N2/CO ratios on the TC/EA in before
> and after samples)
>
> Any ideas on this would be most welcome. What volumes of silver nitrate
> solution, volume of resin, and flow rates are you using? (and how many
> passes are necessary?). Do you pre-condition with methanol?
>
> Thanks for any tips
>
> Best wishes
>
> Tim H.E. Heaton
>
> NERC Isotope Geosciences Laboratory
> British Geological Survey
> Keyworth, Nottingham NG12 5GG, England
> (www.bgs.ac.uk/nigl/index.htm)
>
> Tel. +44(0)115 936 3401
> Email: [log in to unmask]
>
>
>
> -----Original Message-----
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]]On
> Behalf Of Bill Showers
> Sent: 17 November 2006 15:06
> To: [log in to unmask]
> Subject: Re: [ISOGEOCHEM] Contaminant to Molecular Sieve 5A column
>
>
> 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]
>
>
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