Ruben-
This may be one of those, is it worth it analysis. I will leave this up
to the individual because I'm sure this comment alone will get folks
going which is not my point. Consider this more a cautionary note than a
statement.
You can combust this much material, but you will not be able to do many
of them before having to ash the top of the oxidation column (inserts
will help). You will need to use perhaps 9x10mm capsules (double wrap
them in 5x9mm will work) to get enough tin to help the combustion. I
would do some tests by mixing up some sample in the lab of similar C:N
ratio e.g., some organic matter with low N illite or similar clay to
check reproducibility at this sample size. Do your best to homogenize
your sample. You will also need a carousel that has a zero N blank
as well as your capsules or this N will/can be a significant contribution
too.
That said, if you have a lot of carbon associated with your sample (i.e.,
high double digit or higher C:N ratios) this could cause problems with
the combustions efficiency.
Now the cautionary note: Muller, 1977 on Geochim Cosmochim Acta v.41 p.
765-776 (there are other papers too), pointed out the issue of
interpreting C:N data with low N in marine sediments. The point carries
over to soils as well especially when clays are present. The high CEC of
clays will absorb inorganic N (e.g., such as NH4) and other things that
will be released along with the organic N when you combust the sample. If
you are after an predominate organic N signal (obvious both inorganic and
organic N are released during the the bulk combustion), results in the
less than 0.0?% range will be more influenced by the presence of
inorganic N skewing the isotopic and concentration result.
We do numerous C/N analyses on sediments and soils (among other things)
and samples with low N can be difficult to interpret. This of course is
data set dependent. In some cases the N present is all from organic
matter.
So the answer to your question is, yes, you can combust your 200mg sample
with some playing around. However, the interpretation of a bulk isotope
values at this low an N may be another matter. I leave the matter of
convincing reviewers up to you.
Good luck,
Dave
At 08:20 PM 2/20/2007, you wrote:
Dear all,
I am analyzing SOIL NC in EA and I need at least 75ugN in the sample to
get a
good linearity. With this amount of N I need, I need to weigh up to 200
mg of
finely ground sample due to low %N in the sample. Please help me on how
to
know if I have a good combustion of the sample. Is there a specific
combustion catalyst for plant or soil combustion?
Many thanks for your help.
Ruben G. Chavez
Officer
Analytical Service Laboratories
Mail: DAPO Box 7777, Metro Manila, Philippines
Tel: +63(2) 580-5600 ext. 2419
Fax: +63(2) 580-5699
Web:
www.irri.org
***********************************************************
David A. Mucciarone
Stanford University
Dept. of Geological & Environmental Sciences
450 Serra Mall
Building 320, Room 118
Stanford, CA 94305-2115
650-723-0817 (O) 650-725-2199 (F)
http://pangea.stanford.edu/isotope/dam/sil_home.html
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