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Spectroscopy
Chapter 7
The electromagnetic spectrum
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All forms of spectroscopy use a part of the electromagnetic
spectrum to give us information about the materials
around us.
Electromagnetic radiation interacts with atoms and
molecules.
The nature of this interaction depends upon the energy of
the electromagnetic radiation.
Spectroscopy
► Various
spectroscopic techniques provide us
with information about:
 The type of atom of molecule that is present
 How much of a particular atom or molecule is
present
 The structure and bonding of the molecule.
Spectroscopy
► Spectroscopy
that:
techniques utilise the fact
 Atoms or molecules absorb and emit
electromagnetic radiation of specific energies
 Atoms and molecules undergo a change when
they absorb electromagnetic radiation
 Different parts of the electromagnetic spectrum
affect different parts of the atom or molecule
Energy
► In
each of the spectroscopic techniques we
will look at this chapter, the atom or
molecule absorbs a specific quantum of
energy which causes the atom or molecule
to move to a higher energy level.
► With atoms we look at the movement of
electrons to higher energy levels.
Energy
► With
molecules, as well as the movement of
electrons to high energy levels
► We observe the movement of molecules to
higher
 Vibrational
 Rotational
 Nuclear spin
Energy levels
Analysis of Atoms
► The
following 3 techniques that use
radiation from the visible region of the
electromagnetic spectrum to give us
information about the elements present in a
sample.
 Flame tests
 Emission Spectroscopy
 Absorption absorption spectroscopy
Flame Tests
►A
simple form of qualitative analysis
► Identifies certain atoms in a compound.
► Atoms of different elements have different
electron arrangements and hence different
capacities to absorb and emit
electromagnetic radiation
How Flame Tests Work
► When
atoms become excited, by heating, they
absorb energy as electrons move to higher energy
levels, further from the nucleus.
► Excited atoms emit energy as electrons return to
lower energy levels, closer to the nucleus. This
energy is emitted in the form of photons of light.
► These photons correspond to a particular
wavelength.
► The unique electron arrangement of each metal
means it produces a unique flame colour.
Atomic Emission Spectroscopy
► Turn
to page 80
► As a class we are going to read.
► Also don’t forget ROYGBIV
► Then it is your turn
► Page 81
► Question 3
► Page 106
► Question 20
Atomic Absorption Spectroscopy
► Atomic
Spectroscopy is:
 One of the most widely used modern
instrumental techniques
 An Australian invention
 Very versatile, being capable of detecting over
70 elements
 Extremely sensitive, detecting concentrations of
elements as part per million (ppm) levels or, in
some cases, part per billion (ppb)
AAS
► AAS
can be used to:
 Measure the amount of mercury in fish
 Detect toxic metals such as copper in blood
 Test urine and blood to detect an excess or
deficiency of metals
 Environmental sampling
 Soil sampling
How does AAS work
► Atoms
will absorb light if the energy of the
light is exactly that required to promote an
electron from its normal energy level to a
higher energy level
► Each element has its own absorption
spectrum
► Each element to be analysed requires its
own light source that will emit light of the
correct wavelength
AAS
How does AAS work?
A solution of the sample to be analysed is sprayed into a
flame
► It is converted into atomic vapour
► Light containing the chosen wavelength is passed through
the flame
► Atoms of the element being analysed that are present
absorb some of the radiation
► The light beam is then passed through a filter
(monochromator) to select for the light of the chosen
wavelength
► Its intensity is measured by an elecrontronic detector
►
Interpreting atomic absorption
spectra
► Worked
Example 7.3 on page 83
Your Turn
► Page
84
► Question 5
► Page 106
► Question 23 and 24
UV-Visible Spectroscopy
► Makes
use of the fact that many substances
absorb light of characteristic wavelengths
► The wavelengths of the light absorbed by
compounds can be useful for their
identification
UV-Visible Spectroscopy
► Whereas
Flame Emission and AAS require that the
sample being analysed is sprayed into a flame,
UV-visible spectroscopy involves the sample, in
aqueous solution, being placed in a glass holder.
► Ultraviolet or visible light at a wavelength
strongly absorbed by the species being
analysed for, is passed through the solution and
the amount of light absorbed is directly
related to the amount of that species
present in the sample
UV-Visible Spectroscopy
► When
a substance absorbs visible light, it appears
coloured.
► The colour observed is the compliment of the
absorbed colour because this is what remains to
reach our eyes.
Visible spectrum
Of chlorophyll
UV-Visible Spectroscopy Applications
► Although
it can be useful for qualitative analysis.
UV-visible spectroscopy is usually used for
determining concentration of a substance in a
sample
► The procedure involves recording the spectrum of
the pure substance and selecting a wavelength at
which the substance absorbs strongly but other
components of the sample do not.
► The absorbance of the sample is then measured at
this wavelength and compared to the absorbance
of a series of standard solutions.
Uses
► Clinical
analysis, such as the haemoglobin
content and sugar levels in blood
► Determining the amount of coloured dyes in
plastics
► In qualitative analysis of DNA and proteins
in the field of molecular biology
► Determining the levels of nutrients,
additives and contaminants in water and
foods.
UV-Visible Spectroscopy
► The
substance to be analysed is place in a
special vial.
► A reference cell must be used which
contains pure solvent. This is used to
compensate from any reflection, scattering
or absorbance of the light by the solvent.
Interpreting ultraviolet-visible spectra
► To
interpret spectra’s from UV-visible
spectrometry the absorbance by a series of
standard solutions is measured and a
calibration graph is drawn.
► The graph shows absorbance on the y-axis
and concentration on the x-axis
► If the absorbance of an unknown solution is
known we can use this to find the
concentration.
Worked Example 7.4
► Page
87
Your Turn
► Page
88
► Questions 6-8 and 12
► Pg 106-107
► Question 23, 24 and 26