Atomic Emission
Spectroscopy
Lecture 19
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Advantages of Plasma Sources
1. No oxide formation as a result of two
factors including
• Very high temperature
• Inert environment inside the plasma (no
oxygen)
2. Minimum chemical interferences
3. Minimum spectral interferences except for
higher possibility of spectral line
interference due to exceedingly large
number of emission lines (because of high
temperature)
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4. Uniform temperature which results in
precise determinations
5. No self-absorption is observed which
extends the linear dynamic range to
higher concentrations
6. No need for a separate lamp for each
element
7. Easily adaptable to multichannel
analysis
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Plasma Emission Instruments
Three classes of plasma emission instruments
can be presented including:
1. Sequential instruments
In this class of instruments a single channel
detector is used where the signal for each
element is read using the specific
wavelength for each element sequentially.
Two types of sequential instruments are
available:
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a. Linear sequential scan instruments where
the wavelength is linearly changed with
time. Therefore, the grating is driven by a
single speed during an analysis of interest
b. Slew scan instruments where the
monochromator is preset to provide
specific wavelengths; moving very fast in
between wavelengths while moving slowly
at the specific wavelengths. Therefore, a
two-speed motor driving the grating is thus
used.
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Radial vs. Axial Viewing
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Detector
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Slew scan spectrometer
• Two slewscan
gratings
• Two PMTs
for VIS and
UV
• Most use
holographic
grating
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2. Multichannel Instruments
This class of instruments is also referred
to as simultaneous instruments in
which all signals are reported at the
same time using two types of
configurations:
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a. Polychromators
Multiple detectors, usually
photomultiplier tubes are used. Beams
of radiation emerging from the grating
are guided to exit slits (each
representing the wavelength of a
specific element) are focused at several
PMTs for detection. Detection, thus,
takes place simultaneously
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Detectors
Grating
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b. Array-based systems
This multichannel type instrument uses a
multichannel detector like a charge injection
device or a charge-coupled device. Diffracted
beams from a grating pass through a prism
where further resolution of diffracted beams
takes place by a prism. The prism will
disperse the orders of each diffracted beam.
The multichannel detector can also be a
linear photodiode array as in the figure
below:
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Diode Array Detector
Grating
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CCD or CID Detector
Grating
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3. Fourier transform instruments
(FT)
Instruments in which the signal is coded will need a
decoding mechanism in order to see the signal. FT is
a very common technique for decoding time domain
spectra. In such instruments, the detector records
the change of signal with time, which is practically
not useful. However, Fourier transformation of the
time domain signal yield a frequency domain
spectrum, which is the usual signal, obtained by
conventional methods. Instruments that rely on
decoding a coded signal is also said to have a
multiplex design.
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Sample Introduction
There are several methods for sample
introduction; the most widely used is, of
course, the nebulization of an analyte
solution into the plasma. However, other
methods, as described earlier, are fine where
vapors of analyte molecules or atom from
electrothermal or ablation devices can be
driven into the torch for complete
atomization and excitation. For your
convenience, sample introduction methods
are summarized here again:
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Samples in Solution
Pneumatic Nebulizers
Samples in solution are usually easily
introduced into the atomizer by a simple
nebulization, aspiration, process.
Nebulization converts the solution into an
aerosol of very fine droplets using a jet of
compressed gas. The flow of gas carries the
aerosol droplets to the atomization chamber
or region.
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Ultrasonic Nebulizers
In this case samples are pumped onto the
surface of a piezoelectric crystal that
vibrates in the kHz to MHz range. Such
vibrations convert samples into
homogeneous aerosols that can be driven
into atomizers. Ultrasonic nebulization is
preferred over pneumatic nebulization since
finer droplets and more homogeneous
aerosols are usually achieved. However,
most instruments use pneumatic
nebulization for convenience.
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Electrothermal Vaporization
An accurately measured quantity of
sample (few mL) is introduced into an
electrically heated cylindrical chamber
through which an inert gas flows.
Usually, the cylinder is made of
pyrolytic carbon but tungsten cylinders
are now available. The vapors of
molecules and atoms are swept into the
plasma source for complete
atomization and excitation.
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Hydride Generation Techniques
Samples that contain arsenic, antimony,
tin, selenium, bismuth, and lead can be
vaporized by converting them to
volatile hydrides by addition of sodium
borohydride. Volatile hydrides are then
swept into the plasma by a stream of an
inert gas.
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