Environmental Engineering Reference
In-Depth Information
conditions and matrices. Analytical protocols using
enriched stable and radioactive isotopes of Hg in combi-
nation with the ICP-MS are increasingly used (Monperrus
et al., 2004). Multiple stable tracer experiments allow stud-
ies of the fate of Hg species in the environment and in bio-
logic systems. This concept allows the investigation of mul-
tiple transformation processes simultaneously (Demuth and
Heumann, 2001; Hintelmann and Ogrinc, 2003; Monperrus
et al., 2004). The use of radioisotopes to trace different trans-
port and transformation processes is also widespread; in the
case of Hg, the most frequently used radiotracer is
203
Hg
(t
1/2
than 10 ng/mL should be prepared in Tefl on and moni-
tored frequently for titer.
Methods using dual amalgamation can also be calibrated
using the Hg saturated air calibration method (Fitzgerald
and Gill, 1979; Dumarey et al., 1985). Elemental Hg is
placed in a closed glass container, and the temperature is
kept at a constant value. The temperature dependence of
the saturated Hg vapor concentration is calculated from the
ideal gas law. An appropriate volume of Hg vapor is taken
by gas-tight syringe and injected into the gas train through
the chromatographic septum on the sampling gold trap.
This approach has the advantage of preventing problems
associated with the stability of aqueous Hg solutions and is
ideal for calibration of methods for determining total Hg in
air. However, when total Hg is measured by the reduction-
aeration method, calibration of the analytical instrument
with aqueous standards is preferable because it acts as a
control for the reduction amalgamation conditions and
represents less danger for possible contamination in the
case of damage of the calibration vessel (Horvat, 2005).
There are very few commercially available standards for
organo-Hg compounds. Most must be made in the labora-
tory from pure liquid or powder compounds. Care must be
taken to ensure that the compounds are prepared in sol-
vents and containers that will keep them stable as long as
possible. There have been quite a few studies performed
concerning the stability of organo-mercurials in standard
solutions (Lansens et al., 1990; Meuleman et al., 1993). A
decrease of MMHg in aqueous solutions can be caused by
adsorption onto the container walls. Losses of MMHg chlo-
ride due to volatilization is unlikely to occur [K
d
(gas-liquid
distribution coeffi cient: C
gas
/C
H2O
) is 1.07
46 days) (Guimaraes et al., 1995; Stordal et al., 1996;
Gilmour et al., 1998; Mauro et al., 2002). However, when
adequate facilities are available
197
Hg (t
1/2
64.14 hours)
can also be used successfully, as it was demonstrated in Hg
methylation-demethylation studies in soils and sediments
(Guevara et al., 2004). Demethylation has been studied by
the use of
14
CH
3
Hg
, where the
14
CH
4
produced indicates
reductive demethylation, and
14
CO
2
oxidative pathways of
detoxifi cation mechanisms (Hines et al., 2000; Oremland
et al., 1991). A review of improvements on this topic is pre-
sented by Bjorn et al. (2007).
Calibration and Quality Control
It is essential that any analytical method used be carefully
calibrated using standards available from various certify-
ing agencies such as the National Institute of Standards and
Technology (NIST) or manufacturers such as High Purity
Standards. Because most commercially available standards
must be diluted to be in the working range for most envi-
ronmental Hg samples, it is recommended that two or
more dilutions be used for calibration, as described in EPA
Method 1631 (EPA, 2002), so that any loss of titer in one of
the standards will be obvious when the two standards are
plotted together. In addition, the calibration should be ver-
ifi ed using a CRM from NIST, the National Research Coun-
cil of Canada (NRCC), the IAEA, or another trusted certify-
ing agency. CRMs are available for total Hg and MMHg for
many sample types. It is best to match the CRM matrix as
closely as possible to the samples being analyzed.
The diffi culty of preventing losses of low-level Hg from
aqueous solutions is well known, so care must be taken that
standards prepared for instrument calibration are stable.
Losses during storage are due to adsorption on container
walls and volatilization losses due to reduction of inorganic
Hg to elemental Hg. Numerous papers have been written
describing various treatments of samples to prevent losses
of inorganic Hg during storage (Coyn and Callins, 1972;
Carr and Wilkniss, 1973; Lo and Wai, 1975). Strong acids
and oxidants (such as HNO
3
, HCl, or BrCl) can be added
as preservatives. Losses are very much dependent on the
container materials used; the best materials are quartz,
Pyrex glass, and Tefl on. Polyethylene or polypropylene
containers are not recommended for storage of standards.
Any working standard solution with a concentration less
10
-5
at 20°C].
The stability is strongly dependent on the concentration,
the container materials, and the storage temperature. Stock
MMHg standard solutions made up in organic solvents and
stored in Pyrex glass bottles are very stable and can last
for years. Working dilutions of the stock standards can be
made in a solution of 0.2% hydrochloric acid and 0.5% ace-
tic acid that, if stored in Tefl on at 4
2°C, will last at least
a year. Aqueous MMHg solutions are somewhat less stable.
An aqueous solution with a concentration of 10 µg/L,
stored in Pyrex at low temperature (e.g., in a refrigerator)
is stable for approximately one month. If Tefl on containers
are used, the solution is stable for several months if stored
in the dark at room temperature. A new CRM for calibra-
tion of Me
202
Hg was produced by the European Institute for
Reference Materials and Measurements (Snell et al., 2004)
for calibration and quality control using ICP-MS detection
and isotope-dilution analysis.
Volatile organo-Hg compounds (in particular
dimethlymercury) can also be prepared in the gas phase.
This is important for the optimization of methods for
organo-Hg speciation in air and dimethylmercury in
water. An aliquot of vapor is removed from a temperature-
stabilized vessel using a gas-tight syringe. The concentra-
tion can be calculated from data on the partial pressure
