188 CHAPTER 5: The Structure of Atoms
CC The Development of Science
HEMISTRY IN USE
Stable Isotope Ratio Analysis
Many elements exist as two or more stable isotopes, although
one isotope is usually present in far greater abundance. For ex-
ample, there are two stable isotopes of carbon,^13 C and^12 C, of
which^12 C is the more abundant, constituting 98.89% of all
carbon. Similarly, there are two stable isotopes of nitrogen,(^14) N and (^15) N, of which (^14) N makes up 99.63% of all nitrogen.
Differences in chemical and physical properties that arise
from differences in atomic mass of an element are known as
isotope effects. We know that the extranuclear structure of
an element (the number of electrons and their arrangement)
essentially determines its chemical behavior, whereas the
nucleus has more influence on many of the physical proper-
ties of the element. Because all isotopes of a given element
contain the same number and arrangement of electrons, it
was assumed for a long time that isotopes would behave iden-
tically in chemical reactions. In reality, although isotopes
behave very similarly in chemical reactions, the correspon-
dence is not perfect. The mass differences between different
isotopes of the same element cause them to have slightly dif-
ferent physical and chemical properties. For example, the
presence of only one additional neutron in the nucleus of the
heavier isotope can cause it to react a little more slowly than
its lighter counterpart. Such an effect often results in a ratio
of heavy isotope to light isotope in the product of a reaction
that is different from the ratio found in the reactant.
Stable isotope ratio analysis (SIRA) is an analytical tech-
nique that takes advantage of the chemical and physical
properties of isotopes. In SIRA the isotopic composition of
a sample is measured using a mass spectrometer. This com-
position is then expressed as the relative ratios of two or more
of the stable isotopes of a specific element. For instance, the
ratio of^13 C to^12 C in a sample can be determined. This ratio
is then compared with the isotope ratio of a defined standard.
Because mass differences are most pronounced among the
lightest elements, those elements experience the greatest iso-
tope effects. Thus, the isotopes of the elements H, C, N, O,
and S are used most frequently for SIRA. These elements
have further significance because they are among the most
abundant elements in biological systems.
The isotopic composition of a sample is usually expressed
as a “del” value (∂), defined as
∂Xsample(^0 / 00 ) 1000
(RsampleRstandard)
Rstandard
where ∂Xsampleis the isotope ratio relative to a standard, and
Rsampleand Rstandardare the absolute isotope ratios of the sam-
ple and standard, respectively. Multiplying by 1000 allows the
values to be expressed in parts per thousand (^0 / 00 ). If the del
value is a positive number, the sample has a greater amount
of the heavier isotope than does the standard. In such cases
the sample is said to be “heavier” than the standard, or to
have been “enriched” in the heavy isotope. Similarly, if the
del value is negative, the sample has a higher proportion of
the lighter isotope and thus is described as “lighter” than the
standard.
The most frequently used element for SIRA is carbon.
The first limited data on^13 C/^12 C isotope ratios in natural
materials were published in 1939. At that time, it was estab-
lished that limestones, atmospheric CO 2 , marine plants, and
terrestrial plants each possessed characteristic carbon isotope
ratios. In the succeeding years,^13 C/^12 C ratios were deter-
mined for a wide variety of things, including petroleum, coal,
diamonds, marine organisms, and terrestrial organisms. Such
data led to the important conclusion that a biological organ-
ism has an isotope ratio that depends on the main source of
carbon to that organism—that is, its food source. For exam-
ple, if an herbivore (an animal that feeds on plants) feeds
exclusively on one type of plant, that animal’s carbon isotope
ratio will be almost identical to that of the plant. If another
animal were to feed exclusively on that herbivore, it would