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Worksheet 12.2
Chapter 12: Modern analytical chemistry – fast facts
A.1 Analytical techniques
 Chemical analysts identify and characterize unknown substances, determine the composition of
a mixture and identify impurities.
Their work can be divided into:

Qualitative analysis: the detection of the presence, but not the quantity, of a substance in
a mixture.

Quantitative analysis: the measurement of the quantity of a particular substance in a
mixture.

Structural analysis: a description of how the atoms are arranged in a molecule.
Modern analytical methods generally work by analyzing the effect of different forms of energy
on the substance analyzed. They are generally faster and more precise than traditional methods,
such as those based, for example, on volumetric analysis.

Infrared spectroscopy is used to identify the bonds in a molecule.

Mass spectrometry is used to determine relative atomic and molecular masses. The
fragmentation pattern can be used as a fingerprint technique to identify unknown substances or
for evidence of the arrangements of atoms in a molecule.

Nuclear magnetic resonance spectroscopy is used to show the chemical environment of
certain isotopes (usually 11 H ) in a molecule.
Information from only one technique is usually insufficient to determine or confirm a structure,
so a combination of methods is needed.

A.2 Principles of spectroscopy
 The electromagnetic spectrum includes, in order of decreasing frequency/energy:, γ rays,
X-rays, ultraviolet radiation, visible light, IR radiation, microwaves and radio waves (see Table
3 of the IB Data booklet).

Frequency (f) and wavelength (λ) are related by: c (speed of light) = fλ.

The energy of a photon (Ephoton) is related to the frequency (f) of the radiation by Planck’s
equation:
Ephoton = hf (The equation is given in Table 1of the IB Data booklet). h is Planck’s constant
(Table 2 of the IB Data booklet).
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
An emission spectrum is produced
when an atom/molecule moves from a
higher to a lower level with the
emission of a photon of energy:
∆E = Ephoton = hf.

An absorption spectrum shows the
radiation absorbed as atoms/molecules move from a lower to a higher energy level.
Different regions give different types of information, as they interact with substances in
different ways.

Radio waves can be absorbed by certain nuclei (usually 11 H ) causing them to reverse their
spin. They are used in NMR and can give information about the chemical environment of H
atoms.

Microwaves cause molecules to increase their rotational energy. This can give information
about bond lengths. You are required to know the details at this level.

Infrared radiation is absorbed by certain bonds causing them to stretch or bend. This gives
information about the bonds in a molecule.

Visible and ultraviolet light can produce electronic transitions and give information about the
electronic energy levels within the atom or molecule. In organic chemistry it gives information
about the π bonds in a molecule. In inorganic chemistry it gives information about the
concentration of metal ions.
A.3 Infrared (IR) spectroscopy
 Many spectroscopic methods use a
double-beam method in which one
beam is passed through the sample
under investigation and the other
through a reference sample.

In the double-beam IR spectrometer, IR
radiation from a heated filament is split
into two parallel beams.

Radiation is absorbed by the sample
when it has the same frequency as the
natural bond frequencies in the sample
molecule.

When the radiation is not absorbed by the sample, the transmittance is 100% but when
radiation is absorbed the transmittance falls to lower values.
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
Each bond vibrates and bends at a natural frequency which depends on the bond strength and
the masses of the atoms. Light atoms vibrate at higher frequencies than heavier atoms and
multiple bonds vibrate at higher frequencies than single bonds.

The IR spectrum can be used to identify bonds present, but not always the functional groups
present.

A molecule can only be excited by infrared light if a change in vibrational motion produces a
change in the dipole moment of the molecule.

The three vibrational modes of
H2O are IR active as they
produce a change in dipole
moment.

Only three of the vibrational modes of CO2 are IR active.

The symmetric stretch produces no change dipole and so is IR inactive.
The IB Data booklet contains a table of IR absorptions for some bonds in organic molecules.
A.4 Mass spectrometry
 The stages of operation:

vaporization of sample

ionization to produce M + ions by electron bombardment

acceleration of positive ions by electric field

deflection of ions by magnetic field perpendicular to their path

detection of ions.

The degree of deflection depends on the charge/mass ratio: the smaller the mass or the greater
the charge, the greater the deflection.

The peak with largest mass represents the molecular (parent) ion and its mass gives the relative
molecular mass.
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
The fragmentation
pattern can help
determine molecular
structure.
(Mr – 15)
(Mr – 17)
(Mr – 29)
(Mr – 31)
loss of CH3
loss of OH
loss of C2H5 loss of
or CHO
CH3O
(Mr – 45)
loss of
COOH
Examples of fragments
A.5 Nuclear magnetic resonance (NMR) spectroscopy
 A spinning hydrogen nucleus can be thought of as a small bar magnet. The energy between its
two spin states depends on the strength of the external magnetic field applied by an
electromagnet and its chemical environment.

Electrons in an external magnetic field orbit in such a way as to set up a magnetic field that
opposes the applied field and so the frequency at which the hydrogen nuclei absorb radiation
depends on the electron density near to the nucleus and therefore on the chemical environment.

The frequency of the NMR signal relative to this standard is called the chemical shift (δ) of
the proton.
δ = ( (f – f0)/f0)  106 ppm
Hydrogen nuclei in particular environments have characteristic chemical shifts. A list is given
in Table 18 of the IB Data booklet.

The number of peaks in an NMR spectrum equals the number of different environments of
hydrogen atoms.

The integrated trace indicates the relative number of hydrogen atoms in the different
environments.

NMR spectroscopy is non-invasive as the sample is recovered unchanged.

In magnetic resonance imaging (MRI) the patient is placed in a strong magnetic field
chamber and bombarded with pulses of radio waves. Different parts of the body have different
water-lipid ratios in the tissue and therefore absorb radio waves at different frequencies. The
signals produced are decoded by a computer to produce a two- or three-dimensional image of
organs in the human body.

MRI is ideal for detecting brain tumours, infections in the brain, spine and joints, and in
diagnosing strokes and multiple sclerosis.
A.6 Atomic absorption (AA) spectroscopy
 Atomic absorption spectroscopy determines the concentration of atoms, irrespective of how
they are combined together.
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
It is an extremely sensitive method, allowing concentrations as low as one part per billion
(ppb) to be determined.

It is used to measure the concentration of metals in water, soils, food and blood.

The non-excited vaporized metal atoms absorb radiation at a characteristic wavelength from an
external source and are excited to a higher energy level. The log of ratio of the intensity of the
transmitted light (I) to that of the incident light energy (Io) is proportional to the concentration
of the metal atoms present.

A calibration curve is obtained using standard
solutions of known concentrations, from which
unknown concentrations can be determined.

The graph is linear for low concentrations in
agreement with the Beer–Lambert Law: The
absorbance of a given wavelength of light by a
dilute solution is proportional to the concentration.
A = log10 (Io/I) = εcl

ε is molar absorptivity, c is concentration and l is
the length of the radiation through the substance.

The atomic absorption spectrometer:

The monochromatic source has a
hollow cathode with the vapour of
the element under investigation.
Radiation with wavelength of the
maximum absorbance (λmax) for the
element under investigation is
generally used.

In the atomizer a flame is used to
separate the sample into gaseous
metal atoms and the solvent is
evaporated. Ethyne (C2H2),
hydrogen (H2) and propane (C3H8)
are used as fuels.

Modern machines use the double-beam detection principle to find the amount of light
absorbed by atoms in the flame.
A.7 Chromatography
 Chromatography separates and identifies the components of a mixture.

It is used in qualitative analysis: the identification of components present; and quantitative
analysis: the determination of the amount a component is present.
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
The components have different affinities for the stationary and mobile phases and so are
separated as the mobile phase moves through the stationary phase.

A component which has a strong attraction for the mobile phase will move quickly, whereas a
component with a strong attraction for the stationary phase will be held back.

Partition chromatography uses a non-volatile liquid stationary phase held on an inert solid
surface. The components distribute themselves between the two phases according to their
relative solubility. Examples of partition chromatography include paper and gas–liquid
chromatography.

Adsorption chromatography uses a solid stationary phase and a mobile liquid or gas phase.
Some components of the mixture are attracted to the solid surface and other components,
which are less strongly bonded, travel faster with the mobile phase. As the stationary phase is
generally a polar solid, the more polar solutes are more readily adsorbed than the less polar
solutes. Thin-layer chromatography is an example.

In paper chromatography, the stationary phase is water bonded to the cellulose in paper and
the liquid moving up the paper is the mobile phase. A spot of the mixture is placed on
absorbent paper, which is then dipped in the liquid, which acts as the mobile phase. As the
solvent soaks through the paper by capillary action, it moves past the spot. The components
that are more soluble in the liquid are carried along with it leaving components that are more
soluble in the water stationary phase. When some of the solvent has almost reached the top of
the paper, the level is marked as the solvent front. The paper is then removed from the solvent
and dried. The resulting chromatogram is treated with a locating agent or ultraviolet light if
the different components are not visible. The organic dye ninhydrin is used to identify the
different amino acids.

The different components are identified by their Rf value (retention factor).
Rf = distance moved by component above the base line/distance moved by solvent front.

In thin-layer chromatography (TLC) the stationary phase is a thin layer of silica (silicon
dioxide, SiO2) or alumina (aluminium oxide, Al2O3) on a glass or plastic support. The
separation of the components is based on the differences in how the components are adsorbed
by the stationary phase. The separation depends on the relative polarity of the components.

TLC has four advantages over paper chromatography:


It is quicker than paper chromatography as the silica or alumina particles are finer than
the pores in paper.

It is more efficient, works on very small samples and the separated components can be
easily recovered in a pure form.

The results are more easily reproduced.

A range of mixtures can be separated by changing the mobile and stationary phases.
TLC gives better and faster separation than paper chromatography and works with fairly small
samples. The silica or alumina particles are finer than the pores in paper.
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
In column chromatography the stationary phase is silica or alumina and the separation
depends on whether a component is strongly adsorbed onto the surface or dissolves in the
mobile solvent phase. The oxide powder is packed into a column with the solvent and the
mixture applied at the top. The solvent slowly drips out of the bottom of the column, controlled
by a tap, and fresh solvent added at the top so that the packing never becomes dry. As the
mixture moves down it separates out into its components.
A.8 Ultraviolet and visible (UV–vis) spectroscopy
 The d orbitals in a transition metal
complex are split into two levels by
the electric field created by the lone
pair of electrons of the surrounding
ligands.

Electrons in d orbitals localized
along the axes have higher energy
than electrons occupying d orbitals
between the axes.

Ligands have a lone pair of
electrons and include NH3, H2O and
Cl–.

The energy separation, ∆E, between the d orbitals in the sub-levels and hence the colour of the
complex depends on:

the nuclear charge and hence the identity of the central metal ion;

the charge density of the ligand;

the geometry of the complex ion (the electric field created by the ligand’s lone pair of
electrons depends on the geometry of the complex ion);

the number of d electrons present and hence the oxidation number of the central metal ion.

The part of an organic molecule responsible for absorbing UV and visible radiation is called a
chromophore, and generally includes unsaturated groups such as C = C, C = O, N = N, NO2
and the benzene ring.

A compound is more likely to absorb visible light and so appear coloured when it contains a
conjugated system of alternate C – C and C = C bonds, with the π electrons delocalized over
a larger area. Benzene rings and other double bonds can also form part of a conjugated system.

Chlorophyll has extensive conjugation and has a green colour as it absorbs blue and red light.
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
Carotene has a conjugated system of π electrons and is an orange colour because it absorbs
blue and green light.

The conjugated π system of retinol (right)
absorbs visible light. The energy gained
allows retinol to change its geometry,
which allows an electrical signal to be
sent along the optic nerve to the brain.

In acidic solution, the benzene
rings in phenolphthalein are
isolated and the compound is
colourless.

In alkaline solution, the π
system is extended throughout
the molecule and it appears
purple.

The concentration of metal ions in solution can be determined by measuring how much light, at
the characteristic wavelength of the metal, is absorbed by the sample. A = log10 (Io/I) = εcl
Io

I
This is known as the Beer–Lambert Law, ε is molar absorptivity and c is concentration.
A.9 Further NMR spectroscopy
 NMR frequencies are measured against the standard signal produced by the twelve hydrogen
nuclei in tetramethylsilane (TMS).

TMS produces a strong signal which, as silicon has lower electronegativity than carbon, does
not overlap with any signals from the sample under investigation.

TMS is chemically inert and soluble in most organic solvents. It can be easily removed from
the sample as it has a low boiling point.

In high resolution 1H NMR spectroscopy the peaks split as the effective magnetic field,
experienced by particular nuclei, is modified by the magnetic field produced by neighbouring
protons. This effect is known as spin–spin coupling.
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
The splitting patterns produced
from different numbers of
neighbouring protons can be
deduced from Pascal’s triangle.

If a proton has N protons as nearest
neighbours, its NMR peak is split
into a group of N + 1 peaks.

When analyzing high-resolution
NMR spectra, the following additional points should be noted:

Protons bonded to the same atom do not interact with one another as they are equivalent
and behave as a group.

Protons on non-adjacent carbon atoms do not generally interact with one another.

The O–H single peak in ethanol does not split unless the sample is pure, as rapid exchange
of the protons between ethanol molecules averages out the different possible spins.
A.10 Further chromatography

GLC is used to separate and identify small samples of gases and volatile liquids.

The sample is injected through a self-sealing cap into an oven where it is vaporized. The
vapour is carried by an unreactive gas, the mobile phase, over a non-volatile liquid stationary
phase.

Nitrogen and helium are typical carrier gases; long chain alkanes of high boiling point
supported on a silicon dioxide surface act as the stationary phase.

The components of the sample are partitioned between the stationary and mobile phases,
depending on their relative boiling points and relative solubilities in the two phases. They pass
through the column at different rates, and so leave after different retention times.

Volatile components with low solubility in the liquid stationary phase have shorter retention
times.
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
Unknown compounds can be identified by comparing the retention time with that of known
compounds measured under the same conditions.

The area under each peak is a measure of the amount of each substance present.

The temperature of the oven is carefully controlled as it affects the rate at which molecules
move through the apparatus.

GLC cannot be used for non-volatile substances or for substances which decompose at
temperatures near their boiling points.

High-performance liquid chromatography (HPLC) is used when the sample is non-volatile
or decomposes near its boiling point. High pressure is used to force the mixture through a
column tightly packed with very fine solid particles.

The method can be used to separate components which are very similar to each other.

The choice of chromatographic technique depends on whether a quantitative or qualitative
method is needed and on the nature of the sample.

Column chromatography can be used with large amounts, but GLC is very sensitive and so it
can be used with smaller samples.

HPLC is the preferred method when the sample is non-volatile or decomposes near its boiling
point.

GLC is used in the analysis of urine samples from athletes for illegal drugs; blood samples for
ethanol levels and underground mine gases.

HPLC is used in the analysis of oil pollutants; alcoholic beverages; and antioxidants, sugars
and vitamins in foods. It is used in the pharmaceutical and polymer industries, for quality
control of insecticides and herbicides, and in biochemical and biotechnology research.
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