carbon and nitrogen stable isotopes
(^) Measurements of (^15) N/ (^14) N in tissue are made by drying and grinding it; burning the powder with
pure oxygen at 1000°C in a carbon–nitrogen analyzer; drying water from the gases produced; passing
the carbon and nitrogen oxides through a reduction column (shredded copper) to obtain N 2 ;
separating N 2 from CO 2 in a gas-chromatographic column; and finally determining relative isotope
abundances (^14 N, ^15 N, ^16 N) in a mass spectrometer. Measurements of ^13 C/^12 C are made in the
same general way: the CO 2 from the burn is directly analyzed by mass spectrometry, with C^18 O^16 O
separated from ^13 C^16 O 2 by mass difference. Multiple heavy-isotope combinations are rare, but also
have distinctive masses.
(^) Both (^15) N/ (^14) N and (^13) C/ (^12) C ratios are conventionally characterized by comparison to standards. For
nitrogen, it is atmospheric nitrogen, for which ^15 N/^14 N = 0.003660 (3.66‰). Since these ratios
amount to parts per thousand, ‰, the comparison of a sample to the standard is made as a ratio of
ratios in that unit and symbolized as δ^15 N:
(^) This ratio of ratios equation is usually used to present δat.wtX values. However, if you multiply the
quantity in brackets by (^15 N/^14 Nstandard)/(^15 N/^14 Nstandard) = 1, you will see that it is really a
difference of the sample and standard ratios.
(^) One standard for carbon in early work was (^13) C/ (^12) C from fossil cephalopod shells, Belemnitella
americana, found in the Pee Dee Cretaceous limestone formation in South Carolina, USA, and thus
called Pee Dee Belemnite (PDB). The ratio is very high for a natural substance, 0.01111 (11.1‰),
such that the δ^13 C values for natural organic matter are mostly negative:
(^) The advantage of limestone is obvious: add a little acid and you get CO 2 instantly. It is too late to
consider whether it is advantageous for almost all relative ^13 C contents to be reported as negative
numbers. The standards must be repeatedly analyzed to check the mass spectrometry so, when stable
isotope studies became very popular, the original PDB standard ran out. A laboratory in Vienna
generated a cross-calibrated substitute limestone standard now commonly used, termed Vienna-Pee
Dee Belemnite (V-PDB). There are several other standards that are precisely calibrated to PDB.
Unless a report specifies otherwise, δ^13 C values are made equivalent to PDB as standard.
Changes in both δ^13 C and δ^15 N occurring at each trophic-level step are quite
strongly variable (Post 2002), which is most often handled by using overall averages
from a mixture of laboratory studies and food-web studies for which trophic position
is relatively obvious. Changes in δ^13 C are small from diet to consumer, ∼+0.39‰, but
δ^13 C varies for other reasons, and we will return to it. According to Vanderklift and
Ponsard (2003) and Caut et al. (2009), δ^15 N changes by ∼+2.4‰ at each trophic
transfer, termed the trophic enrichment factor (TEF). Because this is much larger than