Environmental Engineering Reference
In-Depth Information
of the individual compound and the gas-law equation
(Dumarey et al., 1985).
Because there are very few certifi ed calibration standards
for organo-Hg compounds, it is essential to test the titer of
the standards frequently by cross-calibrating with multiple
standards, exchanging standards with other laboratories,
and analyzing frequent standard reference materials
(there are several tissue and sediment Standard Reference
Materials from IAEA and National Research Council
Canada certifi ed for MMHg).
It is recommended that the quality-control samples be
analyzed with each batch of 20 or fewer samples to verify
the validity of the data. Field blanks, method blanks and
bottle blanks, blank spikes, matrix spikes/spike duplicates,
analytical duplicates, and CRM (when available) are all
recommended. All of the standard EPA methods stipulate
recovery and precision data quality objectives.
reports of even lower concentrations, in the 0.02-0.04 ng/L
(0.1-0.2 pM) range (Laurier et al., 2004; Kotnik et al., 2007;
Sunderland et al, 2009). More typically, aqueous total Hg
analytical sensitivity will be limited by control of the
method blank, which mandates that extremely strict con-
tamination control measures be implemented in order to
achieve the sensitivity necessary to study total Hg at ambi-
ent levels, and especially so in open ocean seawater.
Aqueous MMHg measurements in seawater are even
more challenging given current sensitivity levels. Typical
sample volumes used for a MMHg analyses of an aqueous
sample is 100-200 mL, which result in detection limits
between 0.01 and 0.03 ng/L (50 and 150 fM). This level of
sensitivity is suffi cient for most freshwater (e.g. Hurley et al.,
1995; Dennis et al., 2005; Brigham et al., 2009; Scudder et al.,
2009) and estuarine and coastal marine systems (Choe and
Gill, 2003; Hammerschmidt and Fitzgerald, 2006; Han et al.,
2007), but appear to be insuffi cient for some oceanic areas
where concentrations have been reported to fall below
0.010 ng/L (50 fM) (Mason and Fitzgerald, 1990, 1993; Mason
et al., 1998; Cossa et al., 2009; Kotnik et al; 2007).
A common approach to improve analytical sensitivity with
mercury determinations is simply to preconcentrate a larger
volume of sample prior to analysis. However, this approach
will likely also result in sacrifi ce of sample throughput and
use of currently available instrumentation and automa-
tion approaches. Without continued improvements in new
methodology and enhanced analytical sensitivity for Hg and
especially MMHg measurements of aqueous samples, our
ability to study biogeochemical processes in some natural
waters and particularly open ocean areas will be limited.
Summary and Conclusions
A comparison of typical analytical sensitivities given in
Table 3.2, with typical ranges of concentrations of Hg and
MMHg in biological samples and sediments given in Table 3.1,
indicate that current analytical methods for the determina-
tion of Hg and MMHg in most cases are sensitive enough
to achieve a reliable quantifi cation of most solid matrices as
long as suffi cient mass is available for analysis.
Determination of aqueous Hg and Hg species is a different
story. Manufacturers who make CVAFS analytical instru-
ments for aqueous Hg determinations report that instru-
mental sensitivities of 0.02 to 0.03 ng/L (0.1 to 0.15 pM)
can be achieved using small sample volumes (
50 mL).
Assuming that instrument sensitivity is the limiting fac-
tor controlling Hg detection, then this level of sensitivity
should be suffi cient for most freshwater systems, but it is
just barely suffi cient for open ocean seawater determina-
tions. Total Hg concentrations in open ocean seawater
around 0.2 ng/L (1 pM) are often observed, with a few
Acknowledgments
We would like to acknowledge Dr. Eric Crecelius and Mr.
Nicolas Bloom for their pioneering research and analytical
methods development that opened the door to understanding
speciation, transport, and fate of mercury in the environment.
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