Analytical Methods for Measuring Mercury in Water, Sediment, and Biota - Mercury in the Environment - page 37

table 3.4
Recommended Containers and Preservation Methods for Organo-Mercury

Analyte Matrix Preservative Container Hold time

MMHg Freshwater 0.5% HCl/dark Tefl on or glass 180 days a

MMHg Seawater 0.5% H 2 SO 4 /dark Tefl on or glass 180 days a

MMHg Sediment Frozen Glass or plastic Undetermined

MMHg Tissue 4 2°C b , frozen c /freeze-dried Glass or plastic Undetermined

MMHg Blood 4 2°C b /frozen c Glass or plastic Undetermined

a. EPA Method 1630.
b. Not recommended for long-term storage.
c. Samples should never be thawed and refrozen.

1990; Parker and Bloom, 2005). Some authors claim that
for MMHg determinations, storage of unpreserved sam-
ples at low temperatures (or even deep-frozen) is better
than adding acid (Bloom, 1989; Horvat et al., 2003). Horvat
and Byrne (1992) noted that long periods of deep-freeze stor-
age do not affect the methylmercury concentration in fi sh
muscle, but losses of up to 30% were observed in shellfi sh.
Freeze-drying is therefore recommended as the best method
for long-term preservation of tissue samples. Table 3.4 lists
recommended sample-collection containers, hold times,
and preservation methods for the most common environ-
mental samples collected for organo-Hg analysis.

reagent in which the organo-Hg species present are con-
verted to volatile analogs for isolation from solution by
gas-phase stripping. A critical part of this procedure is the
preparation of samples prior to derivatization. Often, a pre-
separation and preconcentration step is necessary for this
isolation method to be effective, regardless of which detec-
tion system is used to quantify the organo-Hg compounds.
Monomethylmercury compounds must be removed from
bound sites to facilitate the ethylation reaction. Interfering
compounds (such as sulfi des) must also be removed. Two
basic approaches have been described to prepare the sample
prior to the derivatization step. The fi rst method is based
on extraction of MMHg compounds into methylene chlo-
ride and then back-extraction into water by solvent evapo-
ration (Bloom, 1989). The second is based on water vapor
distillation (see, e.g., De Wild et al., 2002). Distillation has
advantages, since it quantitatively releases MMHg from
sulfur and organic-rich water samples (Horvat et al., 1993).

Determination of Organo-Mercury Compounds in
Aqueous Media

A large number of articles describing methods for the deter-
mination of MMHg compounds in biologic and sediment
samples have been published. Far fewer analytical meth-
ods have been described for the reliable determination of
organo-Hg species in water samples at ambient concen-
tration levels. In many studies, Hg compounds in aquatic
environments are “speciated” according to their ability to
reduce the various forms of Hg present to the elemental
state. Because dimethylmercury and Hg 0 are highly vola-
tile, they must be isolated immediately after sampling to
avoid gas-phase exchanges. The most common isolation
approach used with volatile species is direct aeration and
adsorption on a suitable adsorbent (e.g., Carbotrap or
Tenax), coupled with noble metal amalgamation for Hg 0
(Horvat et al., 2003). Alternatively, volatile species can be
directly isolated by cryogenic trapping, separation on a gas
chromatography (GC) column and detected by one or more
suitable Hg detectors, most recently by ICP-MS (Hintelmann
and Ogrinc, 2003; Monperrus et al., 2004; Stoichev et al.,
2004). It is important that samples not be acidifi ed prior to
such separations to avoid conversion of dimethylmercury
and Hg 0 into MMHg and Hg(II), respectively.
A common method used for the determination of
organo-Hg species (usually focusing on MMHg) involves
a derivatization step using an ethylating or other similar

ORGANO-MERCURY DETERMINATIONS USING
A DISTILLATION STEP

Analytical methods for the determination of organo-Hg
species based on a distillation step are perhaps the most
widely used approaches for the determination of MMHg
(Bloom, 1989; Horvat et al., 1993; Bloom and von der
Geest, 1995; Olsen et al., 1997; De Wild et al., 2002). The
method is based on the distillation of a water sample to
isolate MMHg (as a chloride formed by acidifi cation with
hydrochloric acid) (Figure 3.3).
Following isolation by distillation, the sample is ethyl-
ated with sodium tetraethylborate, 3NaB(C 2 H 5 ) 4 . During
the derivatization step using 3NaB(C 2 H 5 ) 4 , both inorganic
Hg and organo-Hg species in the sample become ethylated:

[CH 3 Hg ] [Hg 2 ] 3NaB(C 2 H 5 ) 4 → [CH 3 HgC 2 H 5 ]

[Hg(C 2 H 5 ) 2 ] 3Na 3B(C 2 H 5 ) 3

Ethylated Hg species are volatile and therefore can be
purged from solution at room temperature using a sparg-
ing vessel; the volatile species are collected on adsorbent