are instrumental methods of detection so useful?
chemical tests are on a separate web page
and a page on Mass
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.
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
resonance spectroscopy (nmr) is one of the most powerful analytical tools
for determining the molecular structure of
an organic compound.
can help to determine molecular structure
and identify an organic compound.
spectroscopy can be used to the determine 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).
The instrument for doing
gas chromatography is called a gas chromatograph.
of the substance under investigation is injected and vapourised
into a tube containing a carrier gas (called the mobile phase,
The gas carries the vaporised substance through a long 'separating'
tube or column wound around inside a thermostated oven.
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. A
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
temperature etc.). 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.
is possible to identify components in a mixture and calculate their
relative proportions 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.
with 'non-instrumental' paper/thin layer
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. You can get the molecular mass of each
component from the molecular ion peak (see
mass spectroscopy further up
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.
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