David,

We do not have experience with carbonates but do routine size vs isotope 
ratio correction for CO in organic samples as generally IRMS systems do 
not appear to be linear for CO.  I will be teaching a 1.5 hour course on 
how to build a spreadsheet to do this at the 14th CF-IRMS workshop 
http://cfirms2008.webhop.org/ next week in Saskatoon Canada.  I and 
others have attempted to write up how this is done but each individuals 
post-analysis spreadsheet is so complex that they tend to be 
incomprehensible to anyone but the creator.  For an example you can 
download a rather complex example of our spreadsheet at  
http://ib.berkeley.edu/groups/biogeochemistry/downloads.php.  Download 
"Generic Template Berkeley 1.00", macros can be disabled.  An example 
with some description of the size correction is on page "smooth 1 std 
size adjust" (see the bottom of the spreadsheet window for the pages).

I and others have found these methods much easier to describe in person 
than to write down.  If you are planning on being in the Berkeley area 
we could meet to go over this method.

Sorry I cannot be of more help with a better written description!

Paul Brooks.



David Mrofka wrote:
> I, and maybe others, would greatly appreciate some feedback on correcting
> for linearity for Carbon and Oxygen in carbonates.
>
> We have been struggling for about a year now (since beginning collection of
> data on a Delta V using CF) on the best way to correct for linearity for C &
> O on carbonates…or even if we should.  Recently I was told by several other
> machine users that they made no correction for linearity.  Here are my
> concerns/questions below:
>
> Given the following: The “acceptable” linearity max limit for carbon, for
> instance, is 0.06 per mille/volt.  Say we are successful in keeping sample
> sizes in the range that produce peak amplitudes of 1-8 V.  After running 10
> internal stds we end up with an average amplitude for every group of sample
> gas peaks, or 10 averages ranging from 1V to 8V for our 10 internal stds. 
> Assume the cross-plot of average amplitude vs. delta 13-C gives a slope of
> 0.05 per mille/V.
>
> 1.	First Question: before getting into correcting actual samples for
> linearity, I have a question on correcting internal stds used in a run. 
> After using their amp vs delta cross plots to get a linearity slope, we
> typically use that linearity to correct them around a 5V “ideal” monitoring
> gas amplitude for mass=46.  In other words, if our linearity is +0.05 per
> mille/V, then the std at 8V is lessened by 0.15 per mille (3V x 0.05) and
> the std at 1V is increased in delta value by 0.2 per mille.  This results in
> a tighter grouping of stds when we plot the enriched and depleted stds vs
> their real values to make an offset correction.  Any criticisms of this
> procedure?
>
> 2.	Second Question: say we set 5V as that “ideal” monitoring gas pressure
> and we have 2 identical samples on the same run.  If one sample has peaks
> averaging 8V and a delta 13-C value of -2.0 and the second sample has peak
> amplitudes averaging 1V and the 0.05 per mille/V linearity holds true (I
> know this is much more ideal than what really happens) then currently we
> would use our linearity slope derived from stds to correct the 8V sample
> down from -2.0 to -1.85 per mille.  We correct our low amplitude 1V sample
> up from around -1.65 to -1.85 per mille.  Again, for the sake of argument
> just assuming this perfect linearity.  Surely to be able to report error
> less than ~0.1 per mille, we want to make this linearity correction?  
>
> 3.	Third and last question:  we are using the mass=46 amplitude of internal
> stds throughout for this correction.  Should we instead use mass=45
> amplitude for Carbon linearity and 46 for oxygen linearity?
>
> Thank you for any and all thoughts on this; perhaps their has been a short
> course on this issue of someone has something written up as a procedure
> they’d be willing to share?
>
> Thanks to all,
>
> Dave Mrofka
> UC Riverside
>