5-8 Mass Spectrometry and Isotopic Abundance 189also have a similar carbon isotope ratio. Suppose now that an
animal, say a rabbit, has a diet comprising two plants, A and
B. Plant A has a ∂^13 C value of 240 / 00 , and plant B has a del
value of 100 / 00. If the rabbit eats equal amounts of the two
plants, the ∂^13 C value of the rabbit will be the average of the
two values, or 170 / 00. Values more positive than 170 / 00
would indicate a higher consumption of plant B than of plant
A, whereas more negative values would reflect a preference
for plant A.
Similar studies have been conducted with the stable iso-
topes of nitrogen. A major way in which nitrogen differs from
carbon in isotopic studies relates to how ∂^13 C and ∂^15 N val-
ues change as organic matter moves along the food chain—
from inorganic nutrient to plant, then to herbivore, to car-
nivore, and on to higher carnivores. It has been pointed out
that ∂^13 C remains nearly constant throughout successive lev-
els of the food chain. In contrast, on average there is a 3
to 50 / 00 shift in the value of ∂^15 N at each successive level
of the food chain. For instance, suppose a plant has a ∂^15 N
value of 1^0 / 00. If an herbivore, such as a rabbit, feeds exclu-
sively on that one type of plant, it will have a ∂^15 N value of
40 / 00. If another animal, such as a fox, feeds exclusively on
that particular type of rabbit, it in turn will have a ∂^15 N value
of 7^0 / 00. An important implication of this phenomenon is that
an organism’s nitrogen isotope ratio can be used as an indi-
cator of the level in the food chain at which that species of
animal feeds.
An interesting application of SIRA is the determination
of the adulteration of food. As already mentioned, the iso-
tope ratios of different plants and animals have been deter-
mined. For instance, corn has a ∂^13 C value of about 120 / 00
and most flowering plants have ∂^13 C values of about 260 / 00.
The difference in these ∂^13 C values arises because these plants
carry out photosynthesis by slightly different chemical reac-
tions. In the first reaction of photosynthesis, corn produces
a molecule that contains four carbons, whereas flowering
plants produce a molecule that has only three carbons. High-
fructose corn syrup (HFCS) is thus derived from a “C 4 ” plant,
whereas the nectar that bees gather comes from “C 3 ” plants.
The slight differences in the photosynthetic pathways of C 3
and C 4 plants create the major differences in their ∂^13 C val-
ues. Brokers who buy and sell huge quantities of “sweet”
products are able to monitor HFCS adulteration of honey,
maple syrup, apple juice, and so on by taking advantage of
the SIRA technique. If the ∂^13 C value of one of these prod-
ucts is not appropriate, then the product obviously has had
other substances added to it, i.e., has been adulterated. The
U.S. Department of Agriculture conducts routine isotope
analyses to ensure the purity of those products submitted for
subsidy programs. Similarly, the honey industry monitors
itself with the SIRA technique.
Another interesting use of SIRA is in the determination
of the diets of prehistoric human populations. It is known
that marine plants have higher ∂^15 N values than terrestrial
plants. This difference in ∂^15 N is carried up food chains, caus-
ing marine animals to have higher ∂^15 N values than terrestrial
animals. The ∂^15 N values of humans feeding on marine food
sources are therefore higher than those of people feeding on
terrestrial food. This phenomenon has been used to estimate
the marine and terrestrial components of the diets of historic
and prehistoric human groups through the simple determi-
nation of the ∂^15 N value of bone collagen collected from
excavated skeletons.
Stable isotope ratio analysis is a powerful tool; many of
its potential uses are only slowly being recognized by
researchers. In the meantime, the use of stable isotope meth-
ods in research is becoming increasingly common, and
through these methods scientists are attaining new levels
of understanding of chemical, biological, and geological
processes.Beth A. Trust