16
A. Theoretical and Instrumental Background
where R represents the isotope ratio. If lJ A> lJ B , we speak of A being
"heavier" than B.
Unfortunately, not all of the lJ-values cited in the literature, are given
relative to one of the world-wide standards, so that often several standards of one element are in use. To convert lJ-values from one standard to
another, CRAIG (1957) has given the following equation
lJ x - A and lJ x - B are lJ-values of sample x with relation to standards A
and B; lJ B - A is the lJ-value of B with relation to A.
For each element each laboratory uses a convenient "working
standard". However, all values measured relative to the "working
standard" should be calculated and given in the literature as relative to a
universal standard. Unfortunately, there often has been a general lack
of agreement among stable isotope researchers as to what standard
should be assigned as the universal standard.
The question of which standard is the most suitable for stable isotope
determinations is very complicated. The National Bureau of Standards
(NBS) has undertaken a program for the preparation and distribution of
isotopic reference samples. The NBS serves as a clearinghouse for data,
and distributes the accumulated data with each sample requested.
A standard should fulfil the following requirements: It should be
1) used world-wide as the zero point,
2) homogeneous in composition,
3) available in relatively large amounts,
Table 3. World-wide standards in use for the isotopic composition of hydrogen,
carbon, oxygen and sulfur
Element
Standard
Standard
abbreviated
H
Standard Mean Ocean Water
SMOW·
C
Be1emnitella americana from the Cretaceous
PDB
Pee Dee formation, South Carolina
0
Standard Mean Ocean Water
SMOW·
S
Troilite (FeS) from the Canyon Diablo iron
CD
meteorite
• SMOW is defined in terms of NBS-1, a distilled water sample, such that
D/H (SMOW) = 1.050 D/H (NBS-1). ISOrO (SMOW)= 1.008 ISOrO (NBS-1)
(CRAIG, 1961b).
A. Theoretical and Instrumental Background
where R represents the isotope ratio. If lJ A> lJ B , we speak of A being
"heavier" than B.
Unfortunately, not all of the lJ-values cited in the literature, are given
relative to one of the world-wide standards, so that often several standards of one element are in use. To convert lJ-values from one standard to
another, CRAIG (1957) has given the following equation
lJ x - A and lJ x - B are lJ-values of sample x with relation to standards A
and B; lJ B - A is the lJ-value of B with relation to A.
For each element each laboratory uses a convenient "working
standard". However, all values measured relative to the "working
standard" should be calculated and given in the literature as relative to a
universal standard. Unfortunately, there often has been a general lack
of agreement among stable isotope researchers as to what standard
should be assigned as the universal standard.
The question of which standard is the most suitable for stable isotope
determinations is very complicated. The National Bureau of Standards
(NBS) has undertaken a program for the preparation and distribution of
isotopic reference samples. The NBS serves as a clearinghouse for data,
and distributes the accumulated data with each sample requested.
A standard should fulfil the following requirements: It should be
1) used world-wide as the zero point,
2) homogeneous in composition,
3) available in relatively large amounts,
Table 3. World-wide standards in use for the isotopic composition of hydrogen,
carbon, oxygen and sulfur
Element
Standard
Standard
abbreviated
H
Standard Mean Ocean Water
SMOW·
C
Be1emnitella americana from the Cretaceous
PDB
Pee Dee formation, South Carolina
0
Standard Mean Ocean Water
SMOW·
S
Troilite (FeS) from the Canyon Diablo iron
CD
meteorite
• SMOW is defined in terms of NBS-1, a distilled water sample, such that
D/H (SMOW) = 1.050 D/H (NBS-1). ISOrO (SMOW)= 1.008 ISOrO (NBS-1)
(CRAIG, 1961b).