Environmental Engineering Reference
In-Depth Information
FIGURE 3.5. Schematic diagram of the CVAFS detector interfaced with the isothermal GC and pyrolytic decomposition column. ( Source : EPA.
1998b.)
the chromatographic separation techniques use such small
sample volumes, a preconcentration step is required to
bring the analytes to a detection range suitable for natu-
ral waters. Cairns et al. (2008) describe a high-performance
liquid chromatography (HPLC)-ICP-MS technique using
online microcolumn preconcentration for speciation of
mercury in seawater. The method achieved a detection
limit of 0.07 ng/L for inorganic mercury and 0.02 ng/L for
MMHg. Other examples of hyphenated methods include
ion chromatography (IC)-ICP-MS as described by Chen, K.-J.
et al. (2009) and a HPLC-ICP-MS technique described by
dos Santos et al. (2009). However, these methods had detec-
tion limits of 30 ng/L (MMHg) and 100 ng/L (inorganic
Hg) and 5.2 ng/L (MMHg) and 4.6 ng/L (inorganic Hg),
respectively. These latter methods do not have suffi cient
sensitivity to detect natural levels of mercury in water and
illustrate the necessity to use an appropriate preconcentra-
tion technique in order to detect natural levels of inorganic
mercury and MMHg in water samples.
There are also methods for determination of “total”
organo-Hg compounds. Inorganic and organic Hg are
preconcentrated on a dithiocarbamate resin and are sub-
sequently eluted with thiourea. Separation of organic and
inorganic Hg is achieved by differential reduction and
detection by CVAAS (Minagawa et al., 1979). Inorganic
and organic Hg can also be separated using anion exchange
resins. Organic Hg is then decomposed (by UV irradiation)
and measured by CVAAS. However, it has been shown that
the concentration levels obtained by this method do not
necessarily correspond to MMHg (owing to the lack of
specifi city of the protocol). The method has been improved
by the introduction of more specifi c separations of organic
and inorganic Hg species by water vapor distillation
(Padberg and Stoeppler, 1991).
In recent years, ICP-MS is being used more and more fre-
quently as a detector for inorganic and organo-Hg determi-
nations. Propylation has been shown to be an even more
suitable derivatization procedure, being free from interfer-
ences caused by halide ions (Demuth and Heumann, 2001).
Hydration was also proven to be a useful derivatization
method, in particular when coupled with preconcentration
by cryotrapping (Tseng et al., 1998; Monperrus et al., 2004;
Stoichev et al., 2004).
Determination of the Chemical and Phase
Speciation of Mercury in Natural Waters
The interaction of Hg with organic matter in natural waters
is important in controlling the solubility, mobility, and
bioavailability of Hg (Ravichandran et al., 1998, 1999). The
recognition that Hg can strongly interact with dissolved
organic matter and aquatic colloids to form both solution
and colloidal phases has spawned an interest in developing
methods to determine the chemical and phase speciation
of Hg in natural waters and in assessing the signifi cance of
Hg-DOM interactions. Many of the methods that have been
described in recent years are based on the pioneering work
HYPHENATED TECHNIQUES
Within the past decade, a number of methods have been
described that interface a gas or liquid chromatographic
separation technique to a sensitive detector such as CVAFS
or ICP-MS. These methods have the potential advantage
of allowing for the simultaneous determination of both
inorganic Hg and a variety of organo-Hg species. Because
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