Environmental Engineering Reference
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it does not need to be heated in a flame or furnace during spectroscopic analysis. It
can be analyzed by a ''cold vapor'' atomic absorption (CVAA) technique. Arsenic
(As) and selenium (Se) are not enough volatile at room temperature to be analyzed
by the ''cold vapor'' techniques, but they are too volatile to be analyzed by FAA or
GFAA. The hydride generation atomic absorption (HGAA) technique, introduced
in Chapter 7 as a sample preparation method (Section 7.2.3), is applicable in atomic
absorption spectroscopy for the analysis of As, Se, and several other elements (Bi,
Ge, Pb, Sb, Sn, Te).
Cold Vapor Atomic Absorption (CVAA) Spectroscopy for Hg
Free mercury (Hg) atoms can exist at room temperature and, therefore, Hg can be
measured by atomic absorption without a heated sample cell. In the CVAA technique,
Hg is chemically reduced to the free atomic state by reacting the sample with a strong
reducing agent like stannous chloride (SnCl
2
) or sodium borohydride (NaBH
4
)ina
closed reaction system (Eq. 7.7). The volatile-free mercury is then driven from the
reaction flask by bubbling air or argon through the solution. Mercury atoms are
carried in the gas stream through tubing connected to an absorption cell, which is
placed in the light path of the AA spectrometer (Fig. 9.5). Sometimes the cell is
heated slightly to avoid water condensation but otherwise the cell is completely
unheated. As the mercury atoms pass into the sampling cell, measured absorbance
rises indicating the increasing concentration of mercury atoms in the light path.
The sensitivity of the cold vapor technique is far greater than can be achieved by
conventional flame AA. This improved sensitivity is achieved, first of all, through a
100% sampling efficiency. All of the mercury in the sample solution placed in the
Figure 9.5
Schematic diagram of cold vapor mercury analyzer (Girard, JE, Principles of
Environmental chemistry, 2005, Jones and Bartlett publishers, Sudbury, MA. WWW.jbpub.com.
Reprinted with permission.)
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