Atomic Emission Spectroscopy
Molecular Absorption Spectroscopy
Lecture 21
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Spectrograph
Beginning 1930s
• photographic film detector
–Cheap
–Long integration times
–Difficult to develop/analyze
–Non-linearity of line "darkness“
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Potential
Source
Graphite Electrodes
Photographic Film
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4
The blackness of the lines on the photographic
film is an indication of the intensity of the
atomic line and thus the concentration of the
analyte. The location of emission lines as
compared to standard lines on a film serves
to identify the wavelengths of emission lines
of analyte and thus its identity. The use of
spectrographs is not very convenient since a
lot of time and precautions must be spent on
processing and calibrating the photographic
film.
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Qualitative analysis is accomplished by
comparison of the wavelengths of
some emission lines to standards while
the line blackness serves as the tool for
semiquantitative analysis.
Polychromators are also available as
multichannel arc and spark
instruments. However, these have fixed
slits at certain wavelengths in order to
do certain elements and thus they are
not versatile.
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Potential Source
Detectors
Grating
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Recently, arc and spark instruments
based on charge injection and charge
coupled devices became available.
These have extraordinarily high
efficiency and performance in terms of
easier calibration, short analysis time,
as well as superior quantitative results.
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CCD or CID Detector
Potential Source
Grating
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Characteristics of Arc Sources
1. Typical temperatures between 4000-5000 oC are high
enough to cause atomization and excitation of
sample and electrode materials.
2. Usually, cyanogens compounds are formed due to
reaction of graphite electrodes with atmospheric
nitrogen. Emission bands from cyanogens
compounds occur in the region from 350-420 nm.
Unfortunately, several elements have their most
sensitive lines in this same region which limits the
technique. However, use of controlled atmosphere
around the arc (using CO2, Helium, or argon) very
much decreases the effect of cyanogens emission.
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3. The emission signal should be integrated over a
minute or so since volatilization and excitation of
atoms of different species differ widely. While
some species give maximum signal, others may
still be in the molecular state.
4. Arc sources are very good for qualitative analysis of
elements while only semiquantitative analysis is
possible. It is mandatory to compare the emission
spectrum of a sample with the emission spectrum
of a standard. In some cases, a few milligrams of a
standard is added to the sample in order to locate
the emission lines of the standard and thus
identify the emission wavelengths of the different
elements in the sample. A comparator
densitometer can be used to exactly locate the
wavelengths of the standard and the sample
components.
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Standard
Sample
The lines from the standard are projected on the lines
of the combined sample/standard emission spectra in
order to identify sample components. Only few lines
are shown in the figure.
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Why use Carbon in Atomic
Spectroscopy?
We have previously seen the use of graphite in
electrothermal AAS as well as arc and spark
AES, even though molecular spectra are real
problems in both techniques due to
cyanogens compounds absorption and
emission. The reasons after graphite
common use in atomic spectroscopy can be
summarized below:
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1.
2.
3.
4.
5.
6.
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It is conductive.
It can be obtained in a very pure state.
Easily available and cheap.
Thermally stable and inert.
Carbon has few emission lines.
Easily shaped.
Spark Sources
Most of the instruments in this category are arc
based instruments. Spark based instruments
are of the same idea except for a spark
source substituting an arc source. The spark
source is constructed as in the figure below
where an AC potential in the order of 10-50
KV is discharged through a capacitor which
is charged and discharged through the
graphite electrodes about 120 times/s;
resulting in a discharge current of about
1000 A.
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This very high current will suffer a great deal of
resistance, which increase the temperature to
an estimated 40000 oC. Therefore, ionic spectra
are more pronounced.
Potential Source
Transformer
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An introduction to Ultraviolet/Visible
Absorption Spectroscopy
Chapter 13
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In this chapter, absorption by molecules,
rather than atoms, is considered.
Absorption in the ultraviolet and visible
regions occurs due to electronic
transitions from the ground state to
excited state. Broad band spectra are
obtained since molecules have
vibrational and rotational energy levels
associated with electronic energy
levels. The signal is either absorbance
or percent transmittance of the analyte
solution where:
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Absorption measurements based upon ultraviolet
and visible radiation find widespread application
for the quantitative determination of a large
variety species.
Beer’s Law:
A = -logT = logP0/P = bc
A = absorbance
 = molar absorptivity [M-1 cm-1]
c = concentration [M]
P0 = incident power
P = transmitted power (after passing through
sample)
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Measurement of Transmittance and
Absorbance:
The power of the beam transmitted by the analyte
solution is usually compared with the power of the
beam transmitted by an identical cell containing
only solvent. An experimental transmittance and
absorbance are then obtained with the equations.
Psolution P
T

Psolvent P 0
Psolvent
P0
A  log
 log
Psolution
P
P0 and P refers to the power of radiation after it has
passed through the solvent and the analyte.
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