are instrumental methods of detection so useful?
chemical tests are on a separate web page
and a page on
Instead of testing for
chemicals using standard laboratory equipment such as test tubes etc.
Special instruments have been developed to carry out such testing. These are
quick, accurate and can be used on very small samples.
Many instrumental methods of
analysis are available and that these can improve sensitivity, accuracy
and speed of tests.
Elements and compounds can
also be detected and identified using a variety of instrumental methods.
Some instrumental methods are suited to identify elements while other
instrumental methods are suited to the identification of compounds.
Instrumental methods have
several advantages over traditional analytical methods e.g.
(i) very accurate -
as analysis technology has improved
sensitive - only a small amount of a sample is
rapid - again. through improved
(iv) methods can be fully
automated - so large numbers of samples can be dealt with
be used to identify elements and their
relative ratio of isotopes and
for molecules it can help to
determine a molecular structure (its expensive, and NMR is much
better for molecular structure analysis - especially organic molecules, see below).
The atoms or molecules
are vapourised and converted to positive ions (based on a single
atom or molecular fragment) by bombardment with high energy
electrons an instrument called a mass spectrometer.
The gaseous ions (e.g. Na+ or CH3+
etc.) are analysed according to their mass in a powerful magnetic
The highest mass ion,
known as the molecular ion peak, corresponds to the molecular
mass of the molecule.
It can be measured to
four decimal places and can even distinguish between molecules with
a similar molecular mass e.g. nitrogen N2 and
carbon monoxide CO, both 28, but not the same to four
emission spectroscopy can be used to
identify elements and analyse element mixtures.
spectroscopy is about 'exciting atoms' with heat or electrical
energy until they emit the
absorbed energy as visible light. You see this effect
when fireworks go off, most of the colour comes from the 'excited'
metal atoms in the salts added to the explosive powder mixture.
In a simple way flame
colour tests in the school laboratory are used to identify elements e.g.
sodium is yellow, barium green etc. BUT these colours are formed
from many specific frequencies of visible light added together, so
how do you sort out e.g. two shades of greens from copper or barium?
The answer is that
detailed analysis of the different emitted
frequencies of visible light (e.g. using a
prism) gives a 'finger print pattern'
by which to identify elements.
AND the greater
the relative intensity of light frequency the
more there is of that element.
atomic spectroscopy is used to identify elements and analyse a
mixture of elements or detect traces of elements in a solid or
analytical method has many applications
Its used in
the steel industry to monitor the composition of steel as the molten
mixtures are being made
Astrophysicists can identify elements in
distant stars from the light emitted.
Tiny traces of
metal ions can be detected in water e.g. for pollution monitoring.
Advanced level notes on the
A non–chemical test method for
identifying elements – atomic emission line spectroscopy
FLAME EMISSION SPECTROSCOPY - an instrumental method for METALS from LINE SPECTRA
the atoms of an element are heated to a very high temperature in a flame they emit
light of a specific set of frequencies (or wavelengths) called the
line spectrum. These are all
due to electronic changes in the atoms, the electrons are excited and
then lose energy by emitting energy as photons of light. These emitted
frequencies can be recorded on a photographic plate, or these days a
Every element atom/ion has its own unique and particular set of electron
energies so each emission line spectra is unique for each element
(atom/ion) because of a unique set of electron level changes. This
different pattern of lines i.e. a 'spectral fingerprint' by which to
identify any element in the periodic table .
e.g. the diagram above on the left
shows some of the visible emission line spectra for the elements
hydrogen, helium, neon, sodium and mercury - all the wavelengths become
reference data, either in a book or computer. A modern spectrometer will
be linked to a computer system of spectral analysis and database for
immediate element identification.
Each line results from a particular electronic energy level change - so
each line depends on the electron arrangement of the excited particle, which
may be an atom, or an ion of specific charge - the mechanism is
illustrated below for the formation of the yellow lines of sodium's line
spectra - the excitation can be caused by a very high temperature e.g.
in a bunsen flame of the Sun!
For more on
theory of light emission from atoms see
- including excitation of atoms gcse physics
the double yellow line for sodium, hence the dominance of yellow in its
flame test colour. In fact the simple flame test colour observations for
certain metal ions relies entirely on the observed amalgamation of these
yellow spectral lines.
The intensity of the line is a measure of the
atom/ion's concentration (see 2nd section on emission spectroscopy
This is an example of an
instrumental chemical analysis called spectroscopy and is performed using an instrument
called an optical spectrometer (simple ones are called
spectroscopes). This method, called
spectroscopy, is a fast, reliable, accurate and sensitive (can detect
minute traces of elements) method of chemical analysis.
This type of optical spectroscopy has enabled scientists to discover new
elements in the past and today identify elements in distant stars and
The alkali metals caesium (cesium) and rubidium were discovered by
observation of their line spectrum and helium identified from spectral
observation of our Sun.
technique has another important advantage.
the lines can be accurately measured and each element has characteristic
spectral lines, you can analyse mixtures - which I've tried to
illustrate with the diagram on the left.
I've superimposed the
spectra of hydrogen, helium and neon. Although some lines may overlap,
you can easily pick out lines that match one element, but no other
From the individual intensities you can analyse a mixture of
You can use the flame emission effect to measure the concentration of
metal ions in solution.
photometer instrument you can do quantitative analysis based on the
light emitted from a solution of a metal ion. The intensity of light
emission is proportional to the amount of element in the sample and
therefore you can measure concentration using flame emission
The sample is evaporated at high temperature in a flame and the
light emitted is measured with a special detector.
determine the precise concentration of a metal ion in dilute
solution by using a calibration curve (right).
known concentration are tested and a measure of the emitted
light (flame photometer signal intensity) can be plotted against the concentration to produce a linear
with an x,y origin of 0,0
can be tested with the
same set-up, and from
the emitted light value you can obtain the unknown concentration from the
You can use special light filters to exclude
the colour produced by other ions that may be present so improving the accuracy of a
specific metal ion measurement.
Many instrumental methods of analysis are available and
that these can improve sensitivity, accuracy and speed of tests.
Index of advanced
A level chemistry pages on SPECTROSCOPY
Other methods of instrumental analysis
spectroscopy can be used to the determine the purity or
concentration of solution of a substance that absorbs uv
chromatography (gc/glc) can be used to analyse liquid mixtures
which can be vapourised (e.g. petrol, blood for alcohol content).
of the substance under investigation is injected and vapourised
into a tube containing a carrier gas (called the mobile phase,
The substances in the mixture are partially
and temporarily absorbed by an absorbent
material held in the column.
The material in the
column consists of fine particles of solid or a layer of very
high boiling liquid, and is called
the immobile phase or stationary phase - which doesn't move.
Depending on the
strength of interaction between the different
substances in the mixture and the column material, they are held back, or 'retained', for different times so that
the mixture separates out in the carrier gas stream.
There is a dynamic equilibrium
between the stationary and mobile phases and the separation of the
components of a mixture by chromatography depends on the
distribution of the components in the sample between the mobile and stationary
The gases emerge
from the oven into a detector system which electronically
records the different signal as each substance comes through.
printout or computer display of the results from the gas chromatograph, called the
gas chromatogram, shows a
series of peaks in the graph
line imposed on a steady baseline when only the carrier gas is
passing through the detector.
The time it
takes for a substance to come through is called the retention
time and is unique for each substance for a particular
set of conditions (flow rate, length of separating column, nature of separating
Generally speaking, the greater the molecular mass of
the mixture molecule, the longer the retention time.
This is because
the component molecule - immobile phase intermolecular force of
attraction increases with the size of the component molecule, so it
is absorbed/retained temporarily a bit more strongly (see right of diagram).
The height of
the peak, or more strictly speaking, the area under
the peak, is proportional to the amount of that particular
substance in the mixture.
shown above (right of diagram) illustrates the separation of some alkane hydrocarbons in
petrol (in reality it is far more complicated with dozens of
hydrocarbon molecule peaks on the chromatogram). The different peak heights give the relative proportions
i.e. hexane >pentane > heptane.
The retention time order follows the
trend of increasing molecular mass gives increasing retention time
i.e. in time heptane C7H16 > C6H14
chromatographic instrument can be calibrated with known
amounts of known substances.
You can also have more
sophisticated analysis by attaching a mass spectrometer to the
gas chromatograph and analyse each separated molecule as they
exit the separating column.
spectrometry you can get the molecular mass of each
component molecule from the molecular ion peak (see
mass spectroscopy further up
Mass spectrometry can be used to determine the
relative atomic mass of a new element.
Industry requires rapid and
accurate methods for the analysis of its products. There have also been
increasing demands from society for safe and reliable monitoring of our
health and environment. The development of modem instrumental methods
has been aided by the rapid progress in technologies such as electronics
Various factors have influenced
the development of instrumental methods.
With modern methods
you get ...
i.e. smaller amounts of material can be used OR much smaller amounts of a
trace element or compound can be detected in a bulk mixture (drug
testing of athletes)
more accurate data
(perhaps analysed by computer)
multi-samples efficiently analysed
a greater range of
analytical techniques, today's laboratory is far more versatile
reliability and consistency once the instrument is set up and
procedures in place and checked.
selected pages describing industrial processes:
and Life Cycle Assessment
- uses, thermal decomposition of carbonates, hydroxides and nitrates
Contact Process, the importance of sulfuric acid
metals be made more useful? (alloys of Al, Fe, steel etc.)
Instrumental Methods of Chemical Analysis
Chemical & Pharmaceutical Industry Economics & Sustainability
Products of the
Chemical & Pharmaceutical Industries & impact on us
The Principles & Practice of Chemical
Production - Synthesising Molecules
Extraction of Metals
- electrolysis and cells
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