Use of Stable Isotopes in Mercury Research - Mercury in the Environment - page 59

sample in known proportions prior to the measurement
(Albarède and Beard, 2003). While mercury has a total of
seven isotopes, it appears that at least two ( 199 Hg and 201 Hg)
are potentially subject to anomalous fractionation effects
(see below), leaving fi ve isotopes for the double-spike pro-
cedure. The performance of a double-spike method using
196 Hg and 204 Hg has been demonstrated for Hg isotope ratio
measurements, achieving very similar precision as com-
pared with the more widely used standard-sample brack-
eting technique with internal mass-bias correction (Mead
and Johnson, 2010).

infl uences reactions, where s-electron occupations change
between reactants and products. Smaller (lighter) isotopes
are concentrated in reduced species. Hence, redox reactions
involving changes in oxidation states of Hg(II) to Hg(0)
([Xe]4f 14 5d 10 6s 0 ˠ [Xe]4f 14 5d 10 6s 2 ) or vice versa show a
strong NVE effect. Such redox conversions are extremely
important reactions in the environmental Hg cycle, mobi-
lizing Hg as Hg(0) and transporting it globally through the
atmosphere. However, nuclear volume, and thus NVE frac-
tionation, does not scale linearly with mass. Instead, odd
isotopes ( 199 Hg and 201 Hg) are slightly smaller (appear to
be lighter) than their mass suggests, leading to even-odd
staggered isotope effects and creating mass independent
fractionation (MIF). It is expressed in both kinetic and
equilibrium fractionation. While Schauble was basing his
calculations in part on a compilation of published nuclear
radii (Angeli, 2004), another group of authors (Ghosh
et al., 2008) used a different set of experimental data
(Hahn et al., 1979) for their recalculation of nuclear radii
and arrived at values that are close to those obtained by
Schauble, with the notable exception of 204 Hg. According to
Schauble, the nuclear radius of 204 Hg is slightly larger than
its mass would predict, while the calculations by Ghosh et
al., put 204 Hg in a linear relationship with the other even-
numbered Hg isotopes. As a consequence, the latter data
would predict no MIF of 204 Hg, which is consistent with the
currently available experimental results for this isotope.
Another mechanism by which isotopes are fractionated
independently of mass is the magnetic isotope effect (MIE)
(Buchachenko, 2001; Buchachenko et al., 2004, 2006). MIE
is usually induced after hemolytic cleavage of bonds, lead-
ing to pairs of radical triplets, which are spin-forbidden
to recombine. However, fragments with magnetic nuclei
( 199 Hg, I

STANDARD-SAMPLE BRACKETING

This proven strategy was originally developed for the fi eld
of light isotope ratio measurements. Before and after mea-
surement of the unknown sample, a standard of known
isotopic composition is measured and the ratios are com-
pared to compute a relative difference between the two.
This comparison of isotope ratios generates a scale of devia-
tions relative to the standard material, which is set to zero.
In fact, exact knowledge of the standard's isotope ratio is
not even required, as long as its isotope composition is
identical in every measurement. The procedure implicitly
requires the instrumental mass bias not to change during
the measurement of the sample sequence, a requirement
that is not necessarily fulfi lled. In addition, for the proce-
dure to work, exact matrix and concentration matching is
mandatory, because the instrumental mass bias can be very
sensitive to matrix and concentration changes.
For these reasons, it has been shown that a combina-
tion of external mass bias correction and standard bracket-
ing delivers the most accurate and precise data. External
correction compensates for any subtle drifts in mass bias
during the measurement, while sample bracketing and
comparison to a common standard avoids the challenges
associated with calculating correct absolute isotope ratios.

3/2) undergo very fast triplet-
singlet conversion. Radical pairs can now recombine in the
singlet state to the starting reagents. This spin conversion
is much slower in diamagnetic nuclei, typically leading to
an enrichment of odd (paramagnetic) isotopes in starting
reagents and of even (diamagnetic) nuclei in the products.
MIE is a purely kinetic effect, not observed for equilib-
rium fractionation processes, and should be predominant
in reactions with radical intermediates. Indeed, photode-
methylation and photoreduction experiments have already
shown to create large MIFs (Bergquist and Blum, 2007).

1/2; 201 Hg, I

Theory of Mercury Isotope Fractionation

Isotope fractionation is a proven tool for examining
biogeochemical pathways of light element isotope systems
(H, C, N, O, and S), in which fractionation is caused by
classical isotope effects (CIEs). CIEs are based on differ-
ences in zero-point vibrational energies of isotopes (i.e.,
differences in bond strengths), which is a function of
their mass differences (Bigeleisen and Mayer 1947; Urey
1947). Because of the small relative difference in mass of
heavy isotope systems, scientists were initially skeptical
that Hg isotope fractionation would be detectable at all.
However, the dominant source of fractionation in heavy
isotopes (mass

Delta Notation

To minimize the effects of instrumental fractionation, iso-
tope ratios are commonly determined by standard-sample-
standard bracketing (and internal mass bias correction for the
most accurate data) and reported using the δ (‰) notation:

40) was suggested to be the nuclear vol-
ume effect (NVE, or nuclear fi eld shift effect) (Bigeleisen,
1996; Schauble 2007). Schauble modeled the expected frac-
tionation for Hg species in equilibrium with Hg(0) vapor
and found the NVE to be much larger than CIE. The NVE

δ

xxx Hg/ yyy Hg (‰)

[( xxx Hg/ yyy Hg) sample /
( xxx Hg/ yyy Hg) standard 1] 1000

While looking deceptively simple, this equation has the
potential of generating great confusion, particularly for