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6.
Why
are instrumental methods of detection so useful?
Typical
chemical tests are on a separate web page
and a page on Mass
spectrometry 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 are accurate, sensitive and
rapid and are particularly useful when the amount of a sample is very
small.
Mass spectroscopy
can
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).
-
Atomic
emission spectroscopy can be used to
identify elements and analyse element mixtures.
-
Basically atomic
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.
-
So
atomic spectroscopy is used to identify elements and analyse a
mixture of elements or detect traces of elements in a solid or
solution.
-
This
analytical method has many applications
e.g.
-
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
spectroscopy
-
Nuclear magnetic
resonance spectroscopy (nmr) is one of the most powerful analytical tools
for determining the molecular structure of
an organic compound.
-
Infra-red spectroscopy
can help to determine molecular structure
and identify an organic compound.
-
Ultra-violet
spectroscopy can be used to the determine purity or
concentration of solution of a substance that absorbs uv
light.
-
 Gas-liquid
chromatography (gc/glc) can be used to analyse liquid mixtures which can be
vapourised (e.g. petrol, blood for alcohol content). The instrument
is called a gas chromatograph.
-
-
A sample
of the substance under investigation is injected and vapourised
into a tube containing a carrier gas (called the mobile phase,
it moves).
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 absorbed by an absorbent
material held in the or column (called
the immobile phase or stationary phase, which doesn't move), but only temporarily. However different
substances 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
phases.
-
The column is
filled with a porous solid so gas can get through but passes over a
large surface area OR it is coated in a very high boiling organic
liquid which can also provide a large absorbing surface but still
allows gas flow.
-
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
column material,
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.
-
Therefore it
is possible to identify components in a mixture and calculate their
relative proportions in the mixture.
-
The chromatogram
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
> C5H12
-
The gas
chromatographic instrument can be calibrated with known
amounts of known substances.
-
Don't confuse
with 'non-instrumental'
paper/thin layer
chromatography.
-
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
and computing.
-
Various factors have influenced
the development of instrumental methods.
With modern methods
you get ...
-
greater sensitivity
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)
-
automation of
analysis,
multi-samples efficiently analysed
-
a greater range of
analytical techniques, today's laboratory is far more versatile
these days
-
greater
reliability and consistency once the instrument is set up and
procedures in place and checked.
 
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