Isoprime 100 - Isotope Corrections for CO2 and Nitrogen Analysis

IsoPrime User’s Guide
Version 1.01 Background Information Page 147 of 219
90
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2



mean
mean
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
CARBON DIOXIDE CORRECTIONS (CRAIG FORMULAE)
Craig corrections are widely used to correct measured delta values of CO2 into
13
C and
18
O
values.
When analysing CO2, different isotopic species of the same element can produce contributions
at certain masses, an interference correction must be made. For example, the international CO2
standard PDB has a 6% contribution at mass 45 from
12
C
17
O
16
O. Similarly, 0.2% of mass 46 is
derived from the species
13
C
17
O
16
O. Species with masses higher than 46 have negligible
abundances.
For a triple collector instrument measuring a CO2 sample similar to PDB, the correction formulae
are:
OC
1813
0338.0)44/45(067.1  
and:
CO
1318
0021.0)44/46(0010.1  
NITROGEN CORRECTIONS
When analysing nitrogen at low enrichments (less than 5%), trace nitrous oxide (NO) may cause
an interfering peak at mass 30. This interference can adversely affect the calculation of
15
N
Atom%.
Two formulae are typically used to overcome this problem: