Sorry to add to the email burden, but I have to reply.
One thing that is especially troublesome in the original email is the
need to analyze 200 mg of soil in order to generate 75 micrograms of
N to get linearity. I own your standard Delta Plus XL, vintage 1999.
We routinely, and I stress routinely, analyze standards with 10
micrograms of N and get perfectly good isotope ratios. If there is a
need to push, then push we can, and often do. So, before trying to
cram, shove, and pummel 200 mg of soil into the EA throat, I would
have a look at why the mass spec is not linear unless it gets 75 mikes.
Second, Hillary's point about sample heterogeneity is very important.
What information one might derive from looking at a low organic/N
soil is a big question. There is no doubt that one can pick up
different bits of organic and inorganic reservoirs at very low sample
concentrations. What would the isotope ratios mean? and why are they
important for you to measure? If there were not some really good
serious questions, I would shrug and suggest that if N is that low in
a rock/soil sample, it is unclear about its origin and interpretation.
The other issue being discussed is how one would measure the effects
of a soil matrix on a small amount of organic matter. There is not an
easy answer to that. We've had in our lab a couple of projects that
demanded ultra low (non-ionprobe) measurements of carbon isotopes in
rocks. Blanks are important--especially the blanks on tin or silver
boats, the general level of carbon or nitrogen in lab air, etc. Tied
up in experiments to determine isotopes on soils/rocks is considering
what happens when a sample reacts in the combustion oven. I am no
Bruno Lavettre, but I do understand that the reaction is flash
combustion. My EA is not set up to flash combust 200 mg of anything,
as we have a small oxygen loop installed. The question of basalt vs.
quartz vs. other minerals is an important point as well. The matrix
for combustion is important.
Not withstanding all of these comments, I do commonly, but not
routinely, run rocky bits for N isotopes and concentrations. Old
Silverback John Hayes wrote a paper on signal to noise ratio
considerations for continuous flow isotope measurements. Maybe signal
to noise is what is causing non-linearity problems and the need for
75 micrograms. In my hands, it is signal to noise that typically
"gets" us on the nitrogen isotopes at low levels.
In summary, I'd go back to my original scientific question and see if
it could be answered by another method, or I would modify my question
if my technique did not satisfy it.
Marilyn
Dr. Marilyn L. Fogel
Geophysical Laboratory
Carnegie Institution of Washington
5251 Broad Branch Rd., NW
Washington, DC 20015
Phone (202) 478-8981
Main office (202) 478-8900
FAX (202) 478-8901
Website: http://fogel.gl.ciw.edu/
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