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the free encyclopedia.
An isotopic signature
(also isotopic fingerprint)
is a ratio of stable or unstable
isotopes of particular
elements found in an investigated
material. The
atomic mass of different
isotopes affect their
chemical kinetics behavior,
leading to natural
isotope separation processes.
For example, different sources
and sinks of
methane have different
affinity for the 12C
and 13C isotopes, which
allows distinguishing between
different sources by the 12C/13C
ratio in methane in the air.
Similarly, carbon in inorganic
carbonates shows little
isotopic fractionation, while
carbon in materials originated by
photosynthesis is depleted of
the heavier isotopes. In addition,
there are two types of plants with
different biochemical pathways;
the
C3 carbon fixation, where the
isotope separation effect is more
pronounced, and
C4 carbon fixation, where the
heavier 13C is less
depleted. The different isotope
ratios for the two kinds of plants
propagate through the
food chain, thus it is
possible to determine if the
principal diet of a human or an
animal consists primarily of C3
plants (rice,
wheat,
soybeans,
potatoes) or C4 plants (corn,
or
corn-fed
beef) by
isotope analysis of their
flesh and bones. Similarly, marine
fish contain more 13C
than freshwater fish, with values
approximating the C4 and C3 plants
respectively.
The
14C isotope is
important in distinguishing
biosynthetized materials from
man-made ones. Biogenic chemicals
are derived from biospheric
carbon, which contains 14C.
Carbon in artificially made
chemicals is usually derived from
fossil fuels like
coal or
petroleum, where the 14C
originally present has decayed
below detectable limits. The
amount of 14C currently
present in a sample therefore
indicates the proportion of carbon
of biogenic origin.
The ratio of 15N/14N
presents a characteristic
distinction between
herbivores and
carnivores, as the movement up
along the food chain tends to
concentrate the 15N
isotope, by 3-4‰ with each step of
the food chain (terrestrial
plants, with the exception of
legumes, has the isotopic
ratio 2-6‰ of N). The tissues and
hair of
vegans therefore contain
significantly lower percentage of
15N than the bodies of
people who eat mostly meat.
Isotopic analysis of hair is an
important source of information
for
archaeologists, providing
clues about the ancient diets; a
terrestrial diet produces a
different signature that a
marine-based diet and this
phenomenon has been used in
analysing differing cultural
attitudes to food sources.
Oxygen comes in two variants as
well. The ratio of 18O/16O
in water depends on the amount of
evaporation the water experienced
(as 18O is heavier and
therefore less likely to
vaporize). As the vapor tension
depends on the concentration of
dissolved salts, the 18O/16O
ratio shows correlation on the
salinity and temperature of water.
As oxygen gets built into the
shells of
calcium carbonate secreting
organisms, such sediments prove a
chronological record of
temperature and salinity of the
water in the area.
Hot particles, radioactive
particles of
nuclear fallout and
radioactive waste, also
exhibit distinct isotopic
signatures. Their radionuclide
composition (and thus their age
and origin) can be determined by
mass spectroscopy or by
gamma spectrometry. For
example, particles generated by a
nuclear blast contain detectable
amounts of 60Co
and 152Eu,
these isotopes were not present in
the particles released during the
Chernobyl accident contain
125Sb
and 144Ce,
absent in the particles from
nuclear tests. Particles from
underwater bursts will consist
mostly of irradiated sea salts.
Ratios of 152Eu/155Eu,
154Eu/155Eu,
and 238Pu/239Pu
are also different for fusion and
fission
nuclear weapons, which allows
identification of hot particles of
unknown origin.
