Geoscience Reference
In-Depth Information
whereas the methane molecule can be reproduced identically to itself by rotating the CH
4
tetrahedron of 2
12 indistinguishable configurations are
therefore accessible to the same molecule. Methane thus has access to 12 times as many
rotational states with respect to CO and, everything else being equal, its relative stability
will be greatly enhanced with respect to that of carbon monoxide.
π
/3 around any C-H bond: 4
×
3
=
3.2 Delta notation and stuff
Using raw isotopic ratios to describe natural variability is very inconvenient because of
their small amplitude. Also there is a common difficulty with interlaboratory biases. The
same sample processed and analyzed by different groups on different instruments will
come out differently and even sometime more differently than the actual natural variabil-
ity. This is not a real issue because the knowledge of “true” isotopic ratios is practically
pointless. Reproducibility is the magic word, not accuracy. The practice for all laboratories
is to measure the same reference material so that each and every analyst can report his or
her own data on unknown samples with respect to the same standard. Reference samples
must be multiple, broadly available, homogenous, and they should be analyzed using the
same analytical procedure as the unknown samples. Under these conditions, we expect that
deviations of the isotopic composition from that of the reference, usually small numbers,
should be known with great precision, typically 0.02 to 1
depending on the element and
sample size.
The delta notation with respect to a particular reference material is generally used for
isotopic ratios of stable nuclides;
17
O represent the deviations of the isotopic
18
O/
16
O and
17
O/
16
O ratios in the sample in parts per thousand relative to the same ratio
in the reference material.
18
O and
δ
δ
18
O is defined as:
δ
18
O
16
O
sample
18
O
1
/
18
O
16
O
ref
δ
=
−
×
1000
(3.21)
/
18
O
with the reciprocal expression of the isotopic ratio of a sample as a function of its
δ
value:
18
O
16
O
sample
=
18
O
16
O
1
18
O
1000
+
δ
(3.22)
ref
It is common practice to use a light but abundant isotope in the denominator. Jargon refers
to heavy oxygen for high
18
O/
16
O ratios and to light oxygen otherwise.
A useful approximation of the fractionation properties is to compare the difference of
18
O values between two co-existing phases 1 and 2 with the fractionation coefficients
δ
α
of the same phases. The following notation is used:
1000
18
O
16
O
2
−
18
O
16
O
1
/
/
18
O
2
−
δ
18
O
1
=
18
O
16
O
ref
δ
(3.23)
/
