Atomic Spectroscopy for Metal Analysis - Fundamentals of Environmental Sampling and Analysis - page 229

the aerosol with fuel oxidant thoroughly for introduction into the flame. The
flame then destroys any analyte ions and breakdown complexes, and creates
atoms (the elemental form) of the element of interest into vapor form such as
Fe 0 ,Cu 0 ,Zn 0 (Fig. 9.1). As shown in Figure 9.4a, the rapid flow of the
reactant gases, along with the spinning baffles in the chamber, acts to reduce
the droplet size of the sample to a fine aerosol. Excess liquid that is no
longer in the gas stream collects on the bottom of the premix chamber and
flows out as waste. In a flameless graphite furnace system (Fig. 9.4b),
instead of using an aspiration device, both liquid and solid samples (a few
microgram or microliter sample) can be deposited directly to a graphite boat
using a syringe inserted through a cavity. The graphite furnace can hold an
atomized sample in the optical path for several seconds, compared with only
a fraction of a second in the flame system. This results in a significantly
higher sensitivity of the GFAA as compared with FAA.
3. Monochromator: The role of a monochromator is to isolate photons of
various wavelengths that pass through the flame or furnace and remove
scattered light of other wavelengths from the light source. In doing this, only
a narrow spectral line impinges on the photomultiplier tubes (PMT)
detector. The monochromator of atomic absorption uses UV-VIS radiation,
which is governed by the same principles as those described in Section 8.2.1
for a UV-Visible spectrometer. These two devices are similar in mechanical
construction in the sense that they employ slits, lenses, mirrors, windows,
and either gratings or prisms.
4. Detector: The PMT detector determines the intensity of photons in the
analytical line exiting the monochromator. Since the basis for both the
flame and flameless system is atomic absorption, the monochromator seeks
to allow only the light not absorbed by the analyte atoms in the flame or
furnace to reach the PMT. That is, before an analyte is atomized, a measured
signal is generated by the PMT as light from the HCL passes through
the flame or furnace. When analyte atoms are present in the flame or
furnace—while the sample is atomized—some part of that light is absorbed
by those atoms. This causes a decrease in PMT signal that is proportional to
the amount of analyte. In a double-beam system, the readout represents
the ratio of the sample to reference beams. Therefore, fluctuations in source
intensity do not become fluctuations in instrument readout, and signal
stability is enhanced.
9.2.2 Cold Vapor and Hydride Generation Atomic
Absorption
The two atomic absorption spectrometric methods using flame and graphite furnace
can measure most elements with satisfactory detection limits. There are notable
exceptions to these techniques, including several elements (mercury, selenium,
and arsenic) that are of environmental importance. Mercury (Hg) is a volatile metal;