Environmental Engineering Reference
In-Depth Information
reaction flask is chemically atomized and transported to the sample cell for
measurement. The sensitivity can be further increased by using very large sample
volumes. Since all of the mercury contained in the sample is released for
measurement, increasing the sample volume means that more mercury atoms are
available to be transported to the sample cell and measured. The detection limit for
mercury by this cold vapor technique is approximately 0.02 mg/L. Where the need
exists to measure even lower mercury concentrations, some systems offer an
amalgamation option. Mercury vapor liberated from one or more sample aliquots
in the reduction step is trapped on a gold or gold alloy gauze. The gauze is then
heated to drive off the trapped mercury, and the vapor is directed into the sample cell.
The only theoretical limit to this technique would be that imposed by background
or contamination levels of mercury in the reagents or system hardware.
Although the cold vapor system is the most sensitive and reliable technique for
determining very low concentrations of mercury by atomic absorption, this tech-
nique requires a specialized instrument that can be used to only analyze mercury
since no other element offers the possibility of chemical reduction to a volatile-free
atomic state at room temperature.
Hydride Generation Atomic Absorption (HGAA) Spectroscopy
for As and Se
In Chapter 7 (Section 7.2.3), we discussed the chemical principle of how to generate
volatile hydride (AsH
3
and SeH
2
) from samples containing various species of As
or Se. This hydride generation module can be incorporated into an atomic absorp-
tion instrument, namely the HGAA spectrometer. As depicted in Figure 9.6, many
of the main parts of the HGAA system are identical to that of FAA including a
hollow cathode lamp, air-acetylene flame, and optical system. The nebulizer
required in FAA is not used in HGAA, and the only required addition is the hydride
generation module. Consequently, HGAA spectrometer is available through an
Sample
0.35% NaBH
4
50% HCl
Hollow
cathode
lamp
As
0
or Se
0
AsH
3
or SeH
2
(hydride)
Detector
Quartz absorption tube
Inert gas
Hydride
Hydride
Monochromator
To fume
hood
To fume
hood
Air-acetylene
burner
Magnetic stirrer
Figure 9.6
A hydride generation and atomization system in an atomic absorption spectrometer
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