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The noble gases are any of the chemically inert
elements found on the far right of the periodic
table in the helium group. The noble gas family
consists of six different elements, five of which
have stable isotopes (helium [He], neon [Ne],
argon [Ar], krypton [Kr], and xenon [Xe]), and
one that only has radioactive isotopes (radon
[Ra]). Table 1 lists the stable isotopes in the
noble gas family and their abundances per element.
The radioactive isotopes of argon
and krypton and the
stable isotopes of helium
are described separately on this website.
Source: WebElements.com
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Gas Chromatography/Mass
Spectrometry:
Price ranges for noble gas analysis are not currently
posted. Many research labs perform this analysis
but on an in-house basis only. For more information,
visit web sites for individual labs such as:
 Noble
Gas Lab of Columbia University
Rosenstiel
School of Marine and Atmospheric Science Noble
Gas Isotope Lab
UA
Center for Thermochronology and Noble Gas Studies
MIT
Noble Gas Laboratories
Reston
Chlorofluorocarbon Lab
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Atmospheric Composition
Noble gases are naturally occurring and present
in the earth's atmosphere.
Table 2 displays the abundances for each of these
gases within the atmosphere:
Source: http://www.c-f-c.com/charts/atmosph.htm
Most of the noble gases have been in the earth's
atmosphere since the earth formed. Two exceptions
are radon and helium. Radon is continuously produced
as a daughter product during uranium decay. It
is radioactive and decays quickly to other elements.
Due to its light weight, most helium originally
present in the atmosphere has escaped to space.
The helium currently present in the earth's atmosphere
is largely the product of alpha decay of heavy
isotopes (to 4He)
and the beta decay of cosmogenic tritium (to 3He).
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Sampling
Samples are typically taken from wells through
a plastic hose into a copper tube (~1cm in diameter,
<1m long). Water is run through the tube for
several minutes before it is sealed using stainless
steel clamps or special pliers. A regulator valve
at one end of the copper tube is typically used
to increase the pressure in order to minimize
bubble formation. Detailed instructions for sampling
are outlined at the USGS
Reston Chlorofluorocarbon Laboratory FAQ page.
Measurement
Noble gas concentrations in groundwater samples
can currently be measured in several ways: gas
chromatography, mass spectrometry, or a combination
of both.
The precision of mass spectrometry leads to a
reproducibility (including all steps in the sampling
and measurement procedures) of ± 1-2% for
the abundance of each gas and ±0.1-1% for
ratios of more abundant isotopes. Gas chromatography
is slightly less precise.
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The behavior of noble gases is governed by three
factors: diffusion, partition between phases,
and nuclear transformation.
Diffusion is the process by which the gases move
from areas of higher concentration to areas of
lower concentration until equilibrium is reached.
Lighter isotopes are more mobile and thus more
affected by diffusion than heavier isotopes. In
gases, the ratio of diffusion coefficients is
equal to the square root of their masses.
Partitioning between phases is an important process
in bodies of water in contact with the atmosphere.
Lakes, rivers, oceans, and groundwater at the
water table are all generally assumed to be in
equilibrium with the atmosphere with respect to
their concentrations of noble gases. The atmosphere
is the greatest source of noble gases for these
bodies of water. The equilibrium relationship
between the atmosphere and the body of water is
described in Henry's Law:
pi
= ki(T,S)xi
where pi
is the partial pressure of the noble gas in the
gas phase,
ki
is a proportionality coefficient (Henry's coefficient,
dependent on temperature [T] and salinity [S]),
and xi
is the equilibrium concentration of the dissolved
gas in water (or other liquid) as a mole fraction.
In other words, the equilibrium concentration
of the noble gases depends on three parameters:
temperature, salinity, and atmospheric pressure.
Nuclear transformation through radioactive decay
may be important for some isosopes. For example,
4He
[link] is a stable product of many U-series decay
paths. In contrast, 222Rn
gas is a radioactive intermediate.
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Assuming that salinity and atmospheric pressure
are known, water that is at equilibrium with the
atmosphere at the time of recharge and is subsequently
sealed off from the atmosphere may be used to
reconstruct the temperature at the time of the
recharge. Since noble gases tend to not react
with other elements, the concentrations of noble
gases in groundwater remain fairly constant once
sealed off from the atmosphere.
Two additional parameters need to be considered
in the reconstruction of these past or paleo-temperatures:
amount of excess air (A) and degree of re-equilibration
(R). Excess air is dissolved gas in the groundwater
that does not originate with the initial equilibration
with the atmosphere. One possible scenario for
the formation of this excess air is as follows:
the water table rises, quickly trapping air bubbles
in the groundwater; the air bubbles partially
dissolve in the water (with the heavier noble
gases partially dissolving first, leading to fractionation);
the gas slowly exchanges with the atmosphere (re-equilibration)
until new recharge seals off the exchange (Stute
and Schlosser 2000). The resulting gas component
in the water is considered excess air. It can
be recognized when the ratio of N2
to Ar in the sample is different from the equilibrium
solubility ratio of N2
to Ar in water. High concentrations of excess
air are caused by a rapid rise in the water table
and are indicative of fractured rock aquifers
and aquifers in semi-arid lands.
Because the solubility of gases varies as a function
of temperature, noble gas concentrations can be
used to determine recharge temperature of a groundwater
sample. (Studies generally use stable neon, argon,
krypton, and xenon gases in reconstructing the
temperature of recharge from years to millions
of years ago; due to high radiogenic production,
helium is not often considered). This may provide
a clue as to where the recharge occurred. Likewise,
if combined with an age-dating isotope such as
3H,
36Cl,
or 14C,
a record of temperature can be reconstructed for
the aquifer system. Such information is useful
for determining change in groundwater recharge
and the relationship to climate change. This technique
deals with total gas concentration, not one particular
isotope. Several noble gases must be measured
for the best temperature determination. The solubilities
of noble gases are highly temperature dependent
but each isotope has a unique relationship to
temperature. Therefore, by determining the ratios
of several noble gas concentrations in groundwater
samples, recharge temperature can be determined:
 |
Variation in noble gas solubilities with temperature
relative to their solubility at 0ºC (modified
from Benson and Krause 1976 and Andrews 1992).
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- Aeschbach-Hertig, W., et al, Interpretation
of dissolved atmospheric noble gases in natural
waters, Water Resour. Res. 35(9), 2779-2792,
1999.
- Andrews, J.N., Mechanisms for noble gas dissolution
by groundwaters, in Isotopes of Noble Gases
as Tracers in Environmental Studies, pp.
87-110, International Atomic Energy Agency,
Vienna, Austria, 1992.
- Attendorn, H.G., and R.N.C. Bowen, Radioactive
and Stable Isotope Geology, Chapman and
Hall, New York, 1997.
- Benson, B.B., and D. Krause, Empirical laws
for dilute aqueous solutions of nonpolar gases,
J. Chem. Phys. 64, 639-709, 1976.
- Petrucci, R.H., Nuclear chemistry, in General
Chemistry, 5th ed., pp. 936-941, Macmillan
Publishing, New York, 1985.
- Stute, M., and P. Schlosser, Atmospheric noble
gases, in Environmental Tracers in Subsurface
Hydrology, edited by P. Cook and A.L. Herczeg,
pp. 298-348, Kluwer Academic Publishers, Norwell,
MA, 2000.
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Chemical
Elements.com
EAWAG
(Swiss Federal Institute for Environmental Science
and Technology, Environmental Isotopes Group,
Interpretation
of dissolved noble gases in groundwater.
Lawrence
Berkeley National Laboratory, Chemistry
and Material Science Division
Winter,
Mark, WebElements
Periodic table
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