Experimental - Spectroscopy

ADVERTISEMENT
Atomic Spectroscopy
Determination of Hg in Environmental Samples Using a Direct
Mercury Analyzer
Johan Nortje, Milestone Inc.
As emphasis is continually being placed on the monitoring
of mercury emissions, both private and public
institutions are looking to characterize the soil, and
in some cases water, on their facilities. Direct mercury
analysis, an alternative to these methods, has been
used successfully to determine total mercury in soil
and other environmental matrices. This technique requires
no sample preparation and delivers results in
as little as 6 min per sample.
Experimental
Determine mercury content at various concentrations in environmental
samples. In addition to the environmental samples, NIST
2709 San Joaquin Soil, a matrix-matched standard reference material
(SRM) is periodically analyzed to ensure accuracy.
Principle of Operation
A direct mercury analyzer, as referenced in EPA Method 7473, incorporates
the following techniques in sequence to perform a direct
measurement in a solid or liquid sample: thermal decomposition,
catalytic conversion, amalgamation, and atomic absorption
spectrophotometry. Controlled heating stages are implemented
to first dry and then thermally decompose a sample introduced
into a quartz tube. A continuous flow of oxygen carries the decomposition
products through a hot catalyst bed where halogens,
nitrogen, and sulfur oxides are trapped. All mercury species are
reduced to Hg(0) and are then carried along with reaction gases
to a gold amalgamator where the mercury is selectively trapped.
All nonmercury vapors and decomposition products are flushed
from the system (by the continuous flow of gas) the amalgamator
is then subsequently heated which releases all mercury vapors to
the single beam, fixed wavelength atomic absorption spectrophotometer.
Absorbance is measured at 253.7 nm as a function of
mercury content.
Figure 1: Principles of operation of direct mercury analyzer
(DMA-80).
with the certified value. Discrepancy on the soil samples can be
attributed to sample homogeneity, and possibly analytical errors,
during the sample preparation step for CVAA analysis.
Table I: Unknown Environmental Samples on DMA-80 vs.
Contract Laboratory (Using CVAA)*
Sample DMA-80 Conc. (µg/kg) CVAA Conc. (µg.kg)
200 ppt standard 0.21
1 ppb standard 0.91
5 ppb standard 4.72
Soil 5.09 5
Soil 0.54 0.63
LCS (5 ppb) 5.54 n/a
Soil 398.01 520
Soil 603.39 1000
Soil 3282.22 2000
Soil 7684.28 6590
*Samples provided by an independent contract laboratory.
Results
See Table I.
Discussion and Conclusion
The DMA-80 successfully processed the environmental and powdered
standard reference material and is an acceptable alternative
to traditional techniques like CVAA and ICP-MS. The SRM concentrations
obtained on the DMA-80 were in excellent agreement
Milestone Inc.
25 Controls Drive, Shelton, CT 06484
tel. (203) 925-4240, toll free (866) 995-5100, fax (203) 925-4241
Website: www.milestonesci.com, Email: mwave@milestonesci.com