Environmental Engineering Reference
In-Depth Information
reducing agent and purged onto the amalgamation media
and then eluted into the detector. It is important to note
that reactive mercury is operationally defi ned as readily
reducible mercury, and in a nonoxidized sample, the Hg
that is reducible is greatly impacted by the method and
duration of reduction. There is some evidence to suggest
that a neutral pH reduction (or certainly something less
acidic than standard SnCl2) provides a better measure of
“methylatable” mercury (Bloom, 1994).
to use because some preservatives contain an Hg compound
or may be contaminated with Hg impurities. If organic
species are also to be determined in a blood sample it is
important to note that preservatives and/or anticoagulants
are likely to break up the original Hg species. Most antico-
agulants are either polyanions (e.g., heparin) or metal che-
lators (e.g., EDTA, citrate) and therefore have a high affi nity
for metal species. After blood has been sampled without an
anticoagulant, it will clot spontaneously and separate into
serum and packed cells. For determination of total Hg, it is
therefore advisable to collect only about 200 µL of blood
and to digest the entire sample, thus avoiding any chance
of bias if the sample coagulates or separates.
Solid-phase samples must be digested to liberate Hg into
aqueous solution for analysis for all analytical methods,
except those based on direct thermal decomposition, A
common digestion media for sediments is strong mixed
acids such as aqua regia. A review of the older literature
shows that the strong oxidant potassium permanga-
nate was often used in the digestion process to promote
complete digestion. It is essential to carefully assess the
reagents for Hg content prior to use. Potassium perman-
ganate in particular is notorious for Hg contamination,
which can be inconsistent even within the same container
of dry reagent. When methods using digestion reagents
like permanganate were fi rst described, the instrument
detection limits for Hg were such that this contamina-
tion was not always evident. In the past 20 years, instru-
ment detection limits have improved to the point at which
reagent or method blanks are now usually the limiting
factor in setting the sensitivity of the analytical method
for Hg. This improvement in instrumental sensitivity has
resulted in high-biased data for low concentration samples.
Note that other digestion techniques, such as those writ-
ten for the digestion of tissue and sediment samples for
the National Oceanic and Atmospheric Administration
(NOAA) Status and Trends Program (NOAA, 1997) work
well with these analytical techniques and do not use potas-
sium permanganate.
Total Mercury in Solid Matrices
In sediments and soils, Hg is frequently associated with
humic matter. In contaminated sites (particularly mining
areas), it is mostly present as cinnabar (HgS). Because the
Hg can be adsorbed to carbon or strongly bonded within
sediment grains, it is essential that the samples be properly
digested to release all of the Hg present in the samples. In
biologic samples, Hg concentrations can range widely, from
less than 0.5 ng/g (0.5 ppb) to over 100 µg/g (100 ppm) and
be present in both inorganic and organo-Hg forms. At low
trophic levels in a food chain, the percentage of mercury
as inorganic mercury is much higher than at high trophic
levels, where the dominant form of mercury is usually
MMHg (Bloom 1992; Boudou and Ribeyre, 1997; Morel
et al., 1998; Wiener et al., 2003). The wide variety of bio-
logic samples, from invertebrates and feathers to blood and
muscle tissue, requires that the sample-preparation tech-
niques be tailored to the sample type. Frequently, tissue
samples such as algae or single invertebrates have very small
masses combined with low Hg concentrations, in which
case it is essential to use analytical methods that are sensi-
tive enough to detect the Hg.
SAMPLING AND STORAGE
Sediment samples should be collected in acid-cleaned glass,
plastic, or Tefl on containers. Samples should be stored fro-
zen until analysis or freeze-dried to increase sample hold-
ing time. For biologic samples, the sample-collection and
storage methods must be modifi ed to suit the tissue type.
Dry samples such as feathers and hair (or fur) can be sealed
in plastic bags and stored at room temperature in a clean
laboratory environment. It may be necessary to gently
wash hair or feather samples with high-purity deionized
water and/or acetone to remove any dirt on the surface of
the samples. Muscle or organ tissue should be collected in
precleaned glass, polypropylene, or polyethylene contain-
ers and frozen or freeze-dried until digestion. Total Hg in
freeze-dried biologic samples, such as biologic certifi ed ref-
erence materials (CRMs), are stable for years. Any required
dissection or homogenization should be done with Tefl on,
titanium, or Tefl on-coated stainless steel utensils.
Blood samples require special care. Ideally, they should be
collected without the addition of any preservative or coagu-
lant. If preservatives or coagulants must be used, it is impor-
tant to verif y low Hg concentrations in the preservative prior
Determination of Total Mercury in Solid-Phase
Materials
COLD-VAPOR ATOMIC ABSORPTION SPECTROMETRY
METHODS
The most commonly used standard methods for total Hg
in tissue and sediment (and high concentration wastewa-
ters) uses a CVAAS technique similar to those described
above for water samples. An aliquot of the digested sample
is diluted in water, reduced with stannous chloride or other
reducing agent, and the evolving elemental Hg is passed
through an adsorption cell and quantifi ed against stan-
dards of known concentration. The typical detection limit
for these methods is ~10 ng/g (ppb) as Hg for both tissues
and sediments. When fi rst described, CVAAS systems for
