Environmental
Mercury Speciation Analysis by Flow-Hyphenation Techniques
C. M. Tseng1,2*, D. Amouroux2,
O.F.X. Donard2, W.F. Fitzgerald1
1Department
of Marine Sciences, University of Connecticut, Avery Point, Groton, CT 06340.
(Email: ctseng@uconnvm.uconn.edu)
2Laboratoire de Chimie Bio-Inorganique et
Environnement, CNRS EP 132, Centre Hélioparc, Université de Pau et de l’Adour,
Pau, 64000, France.
Abstract
The behaviour and fate of mercury in the
environment are closely related to its ambient speciation. In order to follow
the pathways of mercury in the environment, several analytical approaches were
developed to determine mercury species in different ecosystem compartments. For
gas-phase samples, mercury species are concentrated on chromatographic phases
and desorbed in a cryotrapping/separating (CT-GC) unit coupled to a detector.
For water samples, the mercury species are determined with a field cryofocusing
device using flow injection and hydride generation. For environmental solid
samples, mercury species are detected by an on-line D-CT-GC-QFAAS system after
sample preparation by a microwave-assisted technique. The proposed approaches
based on the hyphenation technique by means of flow analysis meet the need of
environmental investigation in terms of rapid and accurate analysis and field
operation. They were successfully applied to the estuarine environmental
matrices for investigating the occurrences and fates of Hg.
1.
Introduction
The
need for mercury speciation analysis instead of total concentration analysis
results from the viewpoints of bioavailability, toxicity, physico-chemical
behavior and understanding of environmental Hg cycling. Additionally, the
concentration of Hg species (Hgo, Hg(II), MMHg and DMHg) has been
measured at subpicomolar levels in air, water and precipitation of marine,
terrestrial and atmospheric ecosystems. How to obtain reliable results in the
determination of trace amounts of Hg species is, therefore, a challenge in
environmental studies.
The current methodologies used for Hg speciation
analysis, including sample preparation and speciation technique, in
environmental samples are available to identify individual mercury species in
natural ecosystems. However, major drawbacks related to the quality control of
the analytical results still remain in aspects of duration, complexity and
efficiency. For example, the techniques for Hg species determination in
environmental samples, either off- or on-line hyphenation of different
analytical steps, usually take 40-90 mins for each analysis (Bloom, 1989;
Horvat et al., 1993; Liang et al., 1994). The more analysis time and manual
handling involved, the more likely it is that more analytical error and poor
reproducibility will be obtained. To simplify complex analytical procedures, to
provide in-situ field operation and to improve accurate speciation analysis,
the new approaches based on the principle of a flow-hyphenation technique are
presented.
2. Concept of flow-hyphenation technique
The principle of the analytical approaches presented here is an on-line
coupling of the techniques of hyphenation and flow analysis. The technique of
flow analysis is applied to all analytical steps of metal speciation analysis
(e.g., sampling, sample preparation, preconcentration, derivatization, separation
and detection), which are combined (hyphenated) together. The sample transfer (e.g., sampling) and chemical
reaction (e.g., derivatization (D)) take place in a flow manifold made by
Teflon tubing. The final detection is achieved either by quartz furnace atomic
absorption spectroscopy (QFAAS), atomic fluorescence spectroscopy (AFS) or
inductively coupled plasma-mass spectrometry, or atomic emission spectrometry
(ICP-MS or -AES) after on-line cryofocussing/separation. This concept of the
flow analysis - hyphenation technique applied to metal speciation analysis is
simply seen in Fig. 1. As a whole, this technique can provide a reliable and
accurate analysis of metal speciation in regard to sampling, sample
pretreatment and determination. This process also provides advantages such as
automation, high simple throughput and low analytical costs.
3. New
approaches for environmental matrices
3.1. Solid
samples (sediments and biotissues)
3.1.1. Total
analysis
Fig. 2a shows a simple manifold used for total Hg
analysis of environmental solid samples by FI-ICP/MS after sample preparation
by a microwave-assisted technique. Both methods of Triton X-100 and Au
amalgamation are adapted here for direct analysis of total Hg using an ICP/MS
as a detector to improve transfer efficiency and to avoid memory effect. They
can be easily combined with the flow injection technique. The elution peaks of
Hg obtained by FI-ICP/MS are, as an example, seen in Fig. 2b.
In addition, an on-line flow microwave digestion
set-up (CF-MD-D-CVAAS) for total mercury analysis in biological fluid (e.g.,
urine, blood etc.) or solid sample extract (e.g., sediment and biotissue) is
shown in Fig. 3. A microwave system can
be combined with other apparatus to flow continuously. Organomercury compounds
in the sample can be decomposed to inorganic Hg by microwave. The total Hg,
including the originally inorganic Hg and the decomposed Hg, was subsequently
measured by an atomic detector after the derivatization through hydride
generation or stannous reduction.
3.1.2.
Speciation analysis
An automated on-line speciation
analyser (D-CT-GC-QFAAS) hyphenated several analytical steps for Hg speciation
analysis was well described in the literature (Tseng et al., 1998). All
analytical steps, including sample delivery, derivatization, preconcentration,
separation, detection and data acquisition are hyphenated together and then
programmed by a Browin computer software. Due to this automatic operation, a
high reproducibility of Hg speciation analysis can be simply achieved. A
complete sample run starting from sample delivery to data acquisition only
takes 10 to 20 min, depending on the type of derivatization and sample volume.
3.2. Water
samples
3.2.1.
Dissolved Hg speciation
Fig.
4a shows a schematic of an in-situ field hydride generator (LVHG-CT-GC) for Hg
speciation analysis (Tseng et al. 2000). The technique of flow injection
analysis was applied to the setup of purge and trap. The setup permits hydride
generation to perform with a large volume sample (0.5~1 l). It is used for
samples like estuarine and sea waters containing low levels of Hg species. In
addition, this device can be easily operated on board with a simple AFS
detector or in a laboratory with a multi-element detector like the ICP/MS. A typical
chromatogram of Hg species obtained by ICP-MS detection was presented in Fig
4b.
That device (CF-HG-CT-GC-D) shown in Fig. 5 is
modified from the setup of Fig. 4 (Tseng et al. 2000). It is a setup in the
mode of continuous analysis in order to increase the analytical performance in
terms of precision, accuracy and sample throughput. In addition, it becomes
more compact and more easily to assemble for in-situ shipboard operations. The
derivative reaction takes place during the flow. Volatilisation of elemental Hg
and Hg hydrides through the purge is contained in a small gas-liquid separator.
Volatile species are trapped in a U-shaped glass column and separated by gas
chromatography and detected by an atomic spectrometer.
3.3 Gas-phase
samples
3.3.1. Air
and dissolved volatile Hg species
Fig. 6. shows an in-situ volatile mercury speciation
analyzer (VMSA) which can be used to determine volatile Hg species in both air
and water samples. The upper part is for air Hg species (Fig. 6a); the lower
one is for dissolved volatile Hg species (Fig. 6b) (Amouroux et al. 1998). For
the analysis of dissolved volatile Hg species, the sample can be transferred
into the bubbler by He pressure in the closed environment. The injection valve
is used to control the He flow. Gas Hg is then purged by He and trapped in a
cryogenic trap and finally detected by a detector through thermal desorption.
In the analysis of gas Hg species, the diagram shows that preconcentration,
separation and detection of Hg species are easily handled by an injection
valve. Consequently, we can separately measure dissolved gas Hg and air Hg
species at the same time during a sample run. One analysis run for both will
take about 30 min for all Hg species determination. In-situ determination may
also allow us to calculate the Hg exchange flux between the interface of water
and air.
3.4 Figure of
merits
The
analytical performance of these approaches with optimum working conditions for
Hg species determination in environmental matrices is summarized in Table 1.
Other details of the experimental work have been described elsewhere (Tseng et
al. 1998, 2000; Amouroux et al., 1998) in terms of reproducibility and
validation.
3.5.
Estuarine environmental analysis
Samples
of air, water and sediments collected in the European macrotidal estuaries
(e.g. Gironde, Scheldt, Rhine) during 1996 and 1998 campaigns within the frame
of the BIOGEST project were analysed for mercury speciation following the
techniques described here. The results obtained were submitted to environmental
journals. All in all, the approaches developed appear adequate to quantify
processes in the cycle of Hg in estuarine environments. In addition, the
methods based on the technique of flow-hyphenation shall be the future for
in-situ field investigation of Hg speciation.
4. References
Amouroux,
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Horvat, M., Liang,
L., Bloom, N. (1993), Anal. Chim. Acta, 282, 153-168.
Liang, L., Horvat,
M., Bloom, N. S. (1994), Talanta, 41,
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Tseng,
C.M., de Diego, A., Pinaly, H., Amouroux, D. and Donard, O.F.X. (1998), J.
Anal. Atom. Spectro., 13, 755-764.
Tseng,
C. M., Amouroux, D., Brindle, I.D. and Donard, O.F.X. (2000), J. Environ. Mon.,
In press.
