Environmental Engineering Reference
In-Depth Information
option for many modern FAA instruments. The HGAA system shown in Figure 9.6,
for illustration, is a batch flow system, but continuous flow systems are more
commonly used. Less-expensive systems use manual operation and a flame-heated
cell. The most advanced systems combine automation of the sample chemistries
and hydride separation using flow injection techniques with decomposition of the
hydride in an electrically heated, temperature-controlled quartz cell.
Similar to the cold vapor mercury systems, samples in a hydride generation
module are reacted in an external system with a reducing agent (usually NaBH
4
).
Gaseous reaction products are then carried to a sampling cell in the light path of
the AA spectrometer. Unlike the mercury technique, the gaseous reaction products
are not free analyte atoms but the volatile hydrides (SeH
3
and SeH
2
). These
molecular species are not capable of causing atomic absorption. To dissociate the
hydride gas into free atoms, the sample cell must be heated. In some hydride sys-
tems, the absorption cell is mounted over the burner head of the AA spectrometer,
and the cell is heated by an air-acetylene flame. In other systems, the cell is heated
electrically. In either case, the hydride gas is dissociated in the heated cell into free
atoms (Se
0
and Se
0
), and the atomic absorption rises and falls as the atoms are
created and then escaped from the absorption cell. The maximum absorption
reading, or peak height, or the integrated peak area is taken as the analytical signal.
The elements determinable by hydride generation techniques can not only
measure As and Se, but also measure several other metals and metalloids such as Bi,
Ge, Pb, Sb, Sn, and Te. For these elements, detection limits well below the mg/L
range are achievable. Like cold vapor mercury, the extremely low detection limits
result from a much higher sampling efficiency. In addition, separation of the analyte
element from the sample matrix by hydride generation is commonly used to
eliminate matrix-related interferences.
The major limitation to the hydride generation technique is that it is restricted
primarily to several elements listed above. Results depend heavily on a variety of
parameters, including the valence state of the analyte, reaction time, gas pressures,
acid concentration, and cell temperature. Therefore, the success of the hydride
generation technique will vary with the care taken by the operator in attending to the
required detail. The formation of the analyte hydrides is also suppressed by a
number of common matrix components, leaving the technique subject to certain
types of chemical interference.
9.2.3 Inductively Coupled Plasma Atomic Emission
Figure 9.7 is a representation of the layout of a typical ICP-OES. The sample is
nebulized and entrained in the flow of plasma support gas, which is typically argon
(Ar). The plasma torch consists of concentric quartz tubes. The inner tube contains
the sample aerosol and Ar support gas and the outer tube contains flowing gas to
keep the tubes cool. A radio frequency (RF) generator (typically 1-5 kWat 27 MHz)
produces an oscillating current in an induction coil that wraps around the tubes.
The induction coil creates an oscillating magnetic field, which in turn sets up an
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