The use of light for analysis is called spectroscopy.
Spectroscopy
Radiation from each portion of the electromagnetic spectrum has a specific
frequency, wavelength and energy associated with it. Ultraviolet light is very short
wave radiation with high energy, while radio waves have long wavelengths and low
energy.
We will be looking at the effects of the different levels of
electromagnetic radiation on atoms and molecules. We can use
this information to provide us with details about the composition of
compounds and the structure. The energy of the radiation
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determines what part of an atom or molecule is affected.
See table 7.1 page 78. Discuss.
Flame tests and spectra
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

When certain metal salts are placed in a flame, distinctive colours are often obtained.
Sodium, for example, gives a yellow colour.
These colours are caused by the electrons returning to lower energy levels after being
excited by the flame. When such light is viewed through a spectroscope, a characteristic
pattern of lines, called an emission spectrum, is observed. As each element has its own
spectrum, such patterns are like fingerprints.
Note: It is because each element has a different number of protons in the nucleus that we
see different spectra, No two elements have the same number of protons and therefore the
same level of attraction to the nucleus.
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Atomic emission spectroscopy
Flame tests can give us only limited qualitative information about the likely
elements present in a sample. Only a few elements give a coloured flame.
Spectroscopy
Two modifications greatly improve the usefulness of the technique:

using a hotter flame, so that sufficient energy is available to excite
electrons in a wider range of elements

passing the light through a prism, as shown in the Figure, The
different energies in the light emitted by a heated sample are separated into a
series of coloured lines, called an emission spectrum.
No two elements will therefore have energy levels of exactly the same energy, so a
spectrum is characteristic of a particular element.
Very few elements are excited by even the hottest laboratory flame, and flame
atomic emission spectroscopy is therefore useful for identifying only a limited
number of metals, particularly the group 1 and 2 elements.
These discrete lines appear as each line corresponds to radiation of a specific
wavelength and frequency and energy exactly equal to the difference in energy of
the electron energy levels. Qualitative analysis.
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Atomic absorption spectroscopy (AAS)
The instrument used is called an atomic absorption spectrometer. It uses the
absorption of light to measure concentrations of metal ions.
Spectroscopy
It works on the principle that atoms will absorb light if the frequency (and therefore
the energy) of this light is of the correct value to promote an electron from its
ground state energy level to a higher energy level.
How atomic absorption spectroscopy works.
 Each element to be analysed requires its own light source that will emit light
of the correct wavelength. The light is provided by a special lamp called a hollow
source cathode lamp.
 Solutions of known concentration are analysed first, followed by the solution
being tested. These solutions are sprayed into the flame. Where it is converted into
an atomic vapour.
 Atoms of the element being analysed absorb some of the radiation.
 The amount of light absorbed by the flame is measured and can be used to
determine the concentration of the substance being analysed.
Applications of AAS
 urine and blood analysis to detect an excess or deficiency of metals
 detection of toxic metals such as lead or copper in blood
 analysis of toxic metals in food and drink
 environmental sampling—testing for metal ion pollution of air, soil and water
 assays of mineral and soil samples to determine the type and amount of
metals present
 analysis of metals present in engine oil to predict the possibility of engine
failure.
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Example
A brand of fruit salad makes the claim that a 100 g serving contains 6 mg of
sodium. AAS was used to check this claim as follows. A 50 g sample of the fruit
salad was treated in such a way as to extract all the sodium. The volume of solution
remaining after this procedure was 22.3 mL.
A 10 mL sample of this solution was then accurately diluted to 100 mL for analysis.
A set of sodium standards were then analysed, followed by the diluted sample. The
results are shown in the table below.
Absorbance reading
0.367
0.712
1.110
1.470
0.989
Spectroscopy
Solution concentration (ppm)
5
10
15
20
Test sample
Value off this graph revealed that the test sample had a concentration of 13.5 ppm
Calculations
Concentration (diluted extract) = 13.5 ppm = 13.5 mg/ L (as diluted)
100
Concentration (undiluted extract) = 13.5 X 10 = 135 mg /L(in the undiluted extract)
22.3
Mass of sodium in extract = 135 X 1000 = 3.01 mg (in the 22.3 mL of extract)
mass of sodium in 50 g sample = 3.01 mg (which came from the 50 g sample)
mass of sodium in 100 g sample = 6 mg (as claimed on the label)
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UV–visible spectroscopy [colorimetry]
Colorimetry is the process of comparing the intensity of a coloured solution with a
set of standards of known concentration.
Spectroscopy
When a substance absorbs visible light, it appears coloured. The colour observed is
the complement of the absorbed colour because this is what remains to reach our
eyes
UV–visible spectroscopy is a more sophisticated development of instrumental
colorimetry. The basic idea is the same — the amount of absorption is related to
the concentration of the substance being tested. However, this technique is far
more selective and therefore less likely to suffer interference from similarly coloured
compounds.
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
For qualitative analysis, the sample to be analysed is dissolved
using a suitable solvent. The subsequent solution is then placed in the test cell,
and a blank (a pure solvent) is placed in the reference cell.
.

The blank in the reference cell usually contains just pure solvent. It
is therefore the same as the sample in the test cell, except for the presence of the
substance being analysed. This substance is therefore the only variable which
can affect the absorbance.
Spectroscopy

The cells themselves cannot be made from normal glass as this is
opaque to UV light. Instead they are made from a special type of glass which is
able to transmit UV radiation.
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