Nitrogen
41
1956, 1957), MAYNE (1957), PARWEL et ai. (1957), HOERING and MOORE
(1958).
Nitrogen isotope ratios are measured in the form ofN2. The standard
gas used is atmospheric nitrogen.
Different preparation techniques are used to convert organic nitrogen, inorganic nitrates, and nitrogen from rocks and minerals. It is imperative that N2 be free of carbon monoxide, which gives an interfering
mass-spectrum.
Very little can be said about the fractionation mechanisms that might
be responsible for the variations in the 15Nj14N ratios. Only a few equilibrium processes appear to fractionate the nitrogen isotopes. It is wellknown that biological processes play the key role in the nitrogen cycle,
but the extent to which the abundance of 15N relative to 14N may be
altered is not known. WELLMANN et ai. (1968) have studied the behavior
of several microorganisms participating in the nitrogen cycle. In all experiments, 14NOi and 14NOi have been preferentially reduced.
MAYNE (1957) and SCALAN (1957) have analyzed the isotopic composition of magmatic nitrogen, present in trace amounts in all igneous
rocks (WLOTZKA, 1961). MAYNE (1957) found large variations in igneous
rocks from -15.8 to 30.9%0. He assumed he had found a correlation
between the nitrogen content and its isotopic composition in which the
more depleted the rock is in nitroge.n, the more enriched it is in 15N.
SCALAN (1957), however, reports a very narrow range in isotopic
composition from + 3.0 to + 5.0%0 in igneous rocks.
Small variations exist in biogenic matter, from a given locality and
even among the various parts of given species. CHENG et al. (1964)
studied the isotope abundance in different soils and in various forms of
soil nitrogen. The 15Nj14N isotope variation appears to be small.
HOERING and MOORE (1958) and EICHMANN (1969) have measured a
number of N2 samples from natural gas and crude oil and found wide
variations. The origin of the N2 is not known, but several sources are
possible. It may have originated from the decomposition of the nitrogenous compounds in crude oils or sedimentary material. This is suggested
by the fact that the nitrogen in natural gas is lighter than the nitrogen in
the accompanying crude oil. (The molecules with the lighter isotope
would decompose more readily, enriching the gas phase in 14N relative to
the remaining oil.) Entrapped air may also be a source in some cases
(EICHMANN,1969).
In summary, nitrogen isotope determinations seem to be a fruitful
field, but much more data are needed to obtain a better understanding of
the naturally occurring variations.
41
1956, 1957), MAYNE (1957), PARWEL et ai. (1957), HOERING and MOORE
(1958).
Nitrogen isotope ratios are measured in the form ofN2. The standard
gas used is atmospheric nitrogen.
Different preparation techniques are used to convert organic nitrogen, inorganic nitrates, and nitrogen from rocks and minerals. It is imperative that N2 be free of carbon monoxide, which gives an interfering
mass-spectrum.
Very little can be said about the fractionation mechanisms that might
be responsible for the variations in the 15Nj14N ratios. Only a few equilibrium processes appear to fractionate the nitrogen isotopes. It is wellknown that biological processes play the key role in the nitrogen cycle,
but the extent to which the abundance of 15N relative to 14N may be
altered is not known. WELLMANN et ai. (1968) have studied the behavior
of several microorganisms participating in the nitrogen cycle. In all experiments, 14NOi and 14NOi have been preferentially reduced.
MAYNE (1957) and SCALAN (1957) have analyzed the isotopic composition of magmatic nitrogen, present in trace amounts in all igneous
rocks (WLOTZKA, 1961). MAYNE (1957) found large variations in igneous
rocks from -15.8 to 30.9%0. He assumed he had found a correlation
between the nitrogen content and its isotopic composition in which the
more depleted the rock is in nitroge.n, the more enriched it is in 15N.
SCALAN (1957), however, reports a very narrow range in isotopic
composition from + 3.0 to + 5.0%0 in igneous rocks.
Small variations exist in biogenic matter, from a given locality and
even among the various parts of given species. CHENG et al. (1964)
studied the isotope abundance in different soils and in various forms of
soil nitrogen. The 15Nj14N isotope variation appears to be small.
HOERING and MOORE (1958) and EICHMANN (1969) have measured a
number of N2 samples from natural gas and crude oil and found wide
variations. The origin of the N2 is not known, but several sources are
possible. It may have originated from the decomposition of the nitrogenous compounds in crude oils or sedimentary material. This is suggested
by the fact that the nitrogen in natural gas is lighter than the nitrogen in
the accompanying crude oil. (The molecules with the lighter isotope
would decompose more readily, enriching the gas phase in 14N relative to
the remaining oil.) Entrapped air may also be a source in some cases
(EICHMANN,1969).
In summary, nitrogen isotope determinations seem to be a fruitful
field, but much more data are needed to obtain a better understanding of
the naturally occurring variations.