At 02:15 PM 3/24/2000 +0000, you wrote:
>Fellow Isogeochemists,
>
>Can anybody tell me what 99.9 atm% carbon 13 would convert to if
>expressed as the delta notation?
>Regards
>Andy Stott
    Deadly to your 45 cup amplifier.

    The point of the delta scale is to express small fractional differences
away from a standard value.  This scale works well for calculation of
isotopic fractionations when the reference value is the unfractionated or
starting value, which is often not far from the original standard value.
    However, the delta nomenclature is somewhere between unusable and absurd
when applied to any tracer situation.  Why? because the denominator is a
ratio value specific to the element & isotope you are measuring.  Thus a 5
ppm increase in 2H produces a much greater change in delta 2H than it does a
5 ppm increase in 13C upon delta 13C.
    In tracer applications (I doubt that anyone is finding 99.9% 13C in the
field in either geochemistry or environmental sciences), you are interested
in the amount of tracer present against existing (natural) material.  Thus,
tracer enrichments of 5 mole % excess of [2H3]acetate and [1-13C]acetate or
even [1,2-13C2]acetate should be equivalent if they are each 5 moles of the
labeled tracer per 100 moles of acetate -- as defined by 5 mpe.  BUT, if you
do the delta calculations for these species as H2 or CO2 you get remarkably
different values.
    BOTTOM LINE: If you are doing tracer applications (where you might have
99% 13C material around), convert your delta measurements to mole fractions
or mole ratios of tracer immediately after the numbers come out of your
instrument.  The deltas mean almost nothing.

---------------------------------------
Dwight E. Matthews, Ph.D.
Professor of Medicine and Chemistry
The University of Vermont
Department of Medicine
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