2. Spectroscopy
• Spectrometric methods are a large group of analytical
methods
• Spectroscopy is the science that deals with the
interactions of radiation with matter (atomic and
molecular).
• The most widely used spectrometric methods are based
on electromagnetic radiation (light, gamma rays, X-rays,
UV, microwave, and radio-frequency).
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3. The electromagnetic nature of all photons is the same,
but photons can have different frequencies
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4. The relationship between the light velocity, wavelength, and
frequency is :
The energy, E, of one photon depends on its frequency of
oscillation :
where h is Planck's constant (6.62618x10-34 J·s)
v=
E = h = hv /
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6. Absorption of Radiation
• When radiation passes through a layer of solid, liquid, or
gas, certain frequencies may be absorbed, a process in
which electromagnetic energy is transferred to the
sample.
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7. • Absorption promotes these particles from their ground
state to more higher-energy excited state.
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8. Beer’s Law
• Many compounds absorb radiation. The diagram
below shows a beam of monochromatic radiation of
radiant power P0 directed at a sample solution.
• Absorption takes place and the beam of radiation
leaving the sample has radiant power P.
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10. • The amount of radiation absorbed may be measured
in a number of ways:
• Transmittance, T = P / P0
% Transmittance, %T = 100 T
• Absorbance,
• A = log10 P0 / P
A = log10 1 / T
A = log10 100 / %T
A = 2 - log10 %T
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11. • The last equation, A = 2 - log10 %T , is worth
remembering because it allows you to easily
calculate absorbance from percentage
transmittance data.
• The relationship between absorbance and
transmittance is illustrated in the following
diagram:
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12. • The equation representing the Beer’s law:
A = ε b c
• Where
• A is absorbance (no units, A = log10 P0 / P ).
• ε is the molar absorptivity (is a measure of the
amount of light absorbed per unit concentration)
with units of L mol-1
cm-1
.
• b is the path length of the sample that is, the path length of the cuvette
in which the sample is contained. We will express this measurement in
centimeters.
• c is the concentration of the compound in solution, expressed in mol L-1.
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13. • Beer’s law tells us that absorbance depends on the total
quantity of the absorbing compound in the light path through
the cuvette. If we plot absorbance against concentration, we
get a straight line passing through the origin (0,0).
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14. The working curves are used to
* Determine the concentration of an
unknown sample.
* To calibrate the linearity of an analytical
instrument.
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15. •Atomic absorption spectroscopy is a
quantitative method of analysis that is applicable
to many metals and a few nonmetals.
•A few examples include:
Al in blood serum
Ca in blood serum, plants, soil, water
Cu in alloys
Cr in sea water
Fe in plants
•Only a drop of sample needed
•The metals need not be removed from other
components (AA is a highly selective technique)
Atomic Absorption Spectroscopy
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16. •When metals are exposed to heat, they
absorb light.
•Each metal absorbs light at a characteristic
frequency. For example:
Describe the principles of AA.
Metal Zn Fe Cu Ca Na
λ (nm) 214 248 325 423 589
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17. •The metal vapor absorbs energy from an external
light source, and electrons jump from the ground to
the excited states
•The ratio of the transmitted to incident light energy
is directly proportional to the concentration of metal
atoms present
•A calibration curve can thus be constructed
[Concentration (ppm) vs. Absorbance]
Describe the principles of AA.
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