Penny,
      I think you have to determine the nature of the deviation.  If the
deviation of the standard value is a blank/contaminant issue then you
should be using the ratio of the measure to expected.  For example if your
blank/contaminant has a d13C of zero at .01 mole fraction (1%), by mass
balance a 100 permil d13C sample would measure 99, but a 10 permil would be
9.9, so the correction is not additive.  However if your deviation is a
fractionation associated with your method then  all samples run under the
same conditions would have a constant offset, and therefore to a good
approximation additive.  You should run three standards with different 13C
and 18O to see which case applies...

Cheers
Greg
  At 03:24 PM 7/1/2005 -0500, you wrote:
>The first method of simple addition and subtraction has no logical
>derivation.  If you have to do it, then the second method of
>normalization (multiplying by the ratio of true/measured) is
>appropriate.  Hope this helps.
>
>
>Paul
>Department of Earth and Environmental Science
>6900 N. Loop 1604 W.
>San Antonio, Texas 78249
>
>-----Original Message-----
>From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
>Behalf Of Penny Higgins
>Sent: Friday, July 01, 2005 2:57 PM
>To: [log in to unmask]
>Subject: [ISOGEOCHEM] Correcting to standards - procedure
>
>Greetings all,
>
>I hope this isn't a totally dumb question. What seemed pretty
>straight-forward to me has become a large discussion in our lab. Maybe
>I've
>been doing this wrong all along - I just want to be sure.
>
>We're running carbonate samples for d13C and d18O on a gasbench
>connected
>to a DeltaPlus XP. NBS-19 is the standard we're using.
>
>So, I run a bunch of samples with a few NBS-19s tossed in for good
>measure.
>The values for NBS-19 don't come out exactly as specified by Coplen 1994
>as
>d13C = 1.95 and d18O = -2.20. Instead, I get d13C = 1.98 and d18O =
>-2.22.
>
>Ordinarily, I'd just do simple adding or subtracting to all the samples
>in
>the run to make the NBS-19s "right." I subtract 0.03 from ALL the d13C
>values and subtract 0.02 from all the d18O values. My understanding is
>that
>then I can report my results as VPDB.
>
>I was just told that a more correct way to do it is to calculate the
>ratio
>between the real value and the measured value of NBS-19 and multiply
>that
>with all of my samples. That is, the ratio of "true" and measured d13C
>for
>NBS-19 is 0.986, which I multiply all my d13C values with. The ratio of
>"true" and measured d18O for NBS-19 is 0.990, and I multiply all my d18O
>values with that.
>
>This second method just seems wrong, because any measured sample near
>zero
>would not change. But it makes sense in the sense that everything we are
>measuring is done in terms of ratios.
>
>So, at the risk of wasting bandwidth, which is the correct method? Is
>there
>some instances where both methods are required? Am I delusional (it
>happens)?
>
>Stumped,
>
>~Penny
>
>*******************************************************************
>                        Dr. Pennilyn Higgins
>                         Research Associate
>
>                              "SIREAL"
>    Stable Isotope Ratios in the Environment Analytical Laboratory
>
>          Department of Earth and Environmental Sciences
>          University of Rochester
>          227 Hutchison Hall
>          Rochester, NY 14627
>
>                     [log in to unmask]
>
>Office: 209b Hutchison Hall               Lab: 209 Hutchison Hall
>Voice : (585) 275-0601               Outer lab: (585) 273-1405
>FAX   : (585) 244-5689              Inner lab: (585) 273-1397
>
>            http://www.earth.rochester.edu/SIREAL/index.html
>*******************************************************************

Greg Michalski
Stable Isotope Lab, 5108 Urey Hall
University of California, San Diego
9500 Gilman Dr.
La Jolla, Ca. 92093-0356
858-534-6053