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Atomic absorption spectrometry
Atomic absorption spectrometry (AAS) is an
analytical technique that measures the
concentrations of elements. Atomic absorption is so
sensitive that it can measure down to parts per billion
of a gram ( μg dm–3) in a sample. The technique
makes use of the wavelengths of light specifically
absorbed by an element. They correspond to the
energies needed to promote electrons from one
energy level to another, higher, energy level.
Atomic absorption spectrometry has many uses in
different areas of chemistry.
Clinical analysis. Analysing metals in biological
fluids such as blood and urine.
Environmental analysis. Monitoring our
environment – eg finding out the levels of various
elements in rivers, seawater, drinking water, air,
petrol and drinks such as wine, beer and fruit drinks.
Pharmaceuticals. In some pharmaceutical
manufacturing processes, minute quantities of a
catalyst used in the process (usually a metal) are
sometimes present in the final product. By using
AAS the amount of catalyst present can be
determined.
Industry. Many raw materials are examined and
AAS is widely used to check that the major elements
are present and that toxic impurities are lower than
specified – eg in concrete, where calcium is a major
constituent, the lead level should be low because it is
toxic.
Mining. By using AAS the amount of metals such as
gold in rocks can be determined to see whether it is
worth mining the rocks to extract the gold.
Figure 2
atoms there is in the vapour, the more radiation is
absorbed. The amount of light absorbed is
proportional to the number of lead atoms. A
calibration curve is constructed by running several
samples of known lead concentration under the same
conditions as the unknown. The amount the
standard absorbs is compared with the calibration
curve and this enables the calculation of the lead
concentration in the unknown sample.
Consequently an atomic absorption spectrometer
needs the following three components: a light source;
a sample cell to produce gaseous atoms; and a means
of measuring the specific light absorbed.
The light source
The common source of light is a ‘hollow cathode
lamp’ (Fig. 1). This contains a tungsten anode and a
cylindrical hollow cathode made of the element to be
determined. These are sealed in a glass tube filled
with an inert gas – eg neon or argon – at a pressure of
How it works
Atoms of different elements absorb characteristic
wavelengths of light. Analysing a sample to see if it
contains a particular element means using light from
that element. For example with lead, a lamp
containing lead emits light from excited lead atoms
that produce the right mix of wavelengths to be
absorbed by any lead atoms from the sample. In
AAS, the sample is atomised – ie converted into
ground state free atoms in the vapour state – and a
beam of electromagnetic radiation emitted from
excited lead atoms is passed through the vaporised
sample. Some of the radiation is absorbed by the lead
atoms in the sample. The greater the number of
Figure 1
+-
1. Ionisation
Ne ・・ Ne+
+- 2. Sputtering
Ne +
+-
3. Excitation
M *
Ne +
+-
4. Emission
M ・・
M *
M ・・
M ・・ Light
between 1 Nm –2 and 5 Nm–2. The ionisation of some
gas atoms occurs by applying a potential difference of
about 300 –400 V between the anode and the
cathode. These gaseous ions bombard the cathode
and eject metal atoms from the cathode in a process
called sputtering. Some sputtered atoms are in
excited states and emit radiation characteristic of the
metal as they fall back to the ground state – eg
Pb* → Pb + h (Fig. 2). The shape of the cathode
concentrates the radiation into a beam which passes
through a quartz window, and the shape of the lamp
is such that most of the sputtered atoms are
redeposited on the cathode.
ν
2
A typical atomic absorption instrument holds
several lamps each for a different element. The lamps
are housed in a rotating turret so that the correct
lamp can be quickly selected.
The optical system and detector
A monochromator is used to select the specific
wavelength of light – ie spectral line – which is
absorbed by the sample, and to exclude other
wavelengths. The selection of the specific light allows
the determination of the selected element in the
presence of others. The light selected by the
monochromator is directed onto a detector that is
typically a photomultiplier tube. This produces an
electrical signal proportional to the light intensity
( Fig. 3).
Double beam spectrometers
Modern spectrometers incorporate a beam splitter so
that one part of the beam passes through the sample
cell and the other is the reference (Fig. 4). The
intensity of the light source may not stay constant
during an analysis. If only a single beam is used to pass
through the atom cell, a blank reading containing no
analyte (substance to be analysed) would have to be
taken first, setting the absorbance at zero. If the
intensity of the source changes by the time the
sample is put in place, the measurement will be
inaccurate. In the double beam instrument there is a
Figure 3
constant monitoring between the reference beam and
the light source. To ensure that the spectrum does not
suffer from loss of sensitivity, the beam splitter is
designed so that as high a proportion as possible of
the energy of the lamp beam passes through the
sample.
Atomisation of the sample
Two systems are commonly used to produce atoms
from the sample. Aspiration involves sucking a
solution of the sample into a flame; and
electrothermal atomisation is where a drop of sample
is placed into a graphite tube that is then heated
electrically.
Some instruments have both atomisation systems
but share one set of lamps. Once the appropriate lamp
has been selected, it is pointed towards one or other
atomisation system.
Reference beam
Sample beam
Sample cell
Beam recombiner
Monochromator Detector
Electronics
Source Beam splitter Readout
Sample Cell
Source Monochromator Detector Meter
Chopper
Flame
(or furnace)
Figure 4
Flame aspiration
Figure 5 shows a typical burner and spray chamber.
Ethyne/air (giving a flame with a temperature of
2200 –2400 ・・C) or ethyne/dinitrogen oxide (2600–
2800 ・・C) are often used. A flexible capillary tube
connects the solution to the nebuliser. At the tip of
the capillary, the solution is ‘nebulised’ – ie broken
into small drops. The larger drops fall out and drain
off while smaller ones vaporise in the flame. Only
ca 1% of the sample is nebulised.
3
Nebuliser
End cap
Impact bead
Mixing chamber
with burner head
Flow spoiler
Figure 7
Sample preparation
Sample preparation is often simple, and the chemical
form of the element is usually unimportant. This is
because atomisation converts the sample into free
atoms irrespective of its initial state. The sample is
weighed and made into a solution by suitable
dilution. Elements in biological fluids such as urine
and blood are often measured simply after a dilution
Electrothermal atomisation
Figure 6 shows a hollow graphite tube with a platform.
25 μl of sample (ca 1/100th of a raindrop) is placed
through the sample hole and onto the platform from
an automated micropipette and sample changer. The
tube is heated electrically by passing a current
through it in a pre -programmed series of steps. The
details will vary with the sample but typically they
might be 30 –40 seconds at 150 ・・C to evaporate the
solvent, 30 seconds at 600 ・・C to drive off any volatile
organic material and char the sample to ash, and with
a very fast heating rate ( ca 1500 ・・C s-1) to 2000–
2500 ・・C for 5–10 seconds to vaporise and atomise
elements (including the element being analysed).
Finally heating the tube to a still higher temperature
– ca 2700 ・・C – cleans it ready for the next sample.
During this heating cycle the graphite tube is flushed
with argon gas to prevent the tube burning away. In
electrothermal atomisation almost 100% of the
sample is atomised. This makes the technique much
more sensitive than flame AAS.
of the original sample. Figure 7 shows a flame atomic
absorption spectrometer with an autosampler and
flow injection accessory.
When making reference solutions of the element
under analysis, for calibration, the chemical
environment of the sample should be matched as
closely as possible – ie the analyte should be in the
same compound and the same solvent. Teflon
containers may be used when analysing very dilute
solutions because elements such as lead are sometimes
leached out of glass vessels and can affect the results.
Background absorption
It is possible that other atoms or molecules apart from
those of the element being determined will absorb or
scatter some radiation from the light source. These
species could include unvaporised solvent droplets, or
compounds of the matrix (chemical species, such as
anions, that tend to accompany the metals being
analysed) that are not removed completely. This
means that there is a background absorption as well as
that of the sample.
One way of measuring and correcting this
background absorption is to use two light sources, one
of which is the hollow cathode lamp appropriate to
the element being measured. The second light source
is a deuterium lamp.
The deuterium lamp produces broad band
radiation, not specific spectral lines as with a hollow
Light
Sample hole
Figure 5 Figure 6
4
Readers will find a more detailed explanation of atomic absorption spectrometry in the forthcoming R. Levinson, More
Modern Chemical Techniques , RSC. For further information contact The Education Department, The Royal
Society of Chemistry, Burlington House, Piccadilly, London W1J 0BA.
This leaflet is produced in association with The Royal Society of Chemistry Fine Chemicals and Medicinals Group.
cathode lamp. By alternating the measurements of the
two light sources – generally at 50 –100 Hz – the
total absorption (absorption due to analyte atoms plus
background) is measured with the specific light from
the hollow cathode lamp and the background
absorption is measured with the light from the
deuterium lamp. Subtracting the background from the
total absorption gives the absorption arising from only
analyte atoms.
Calibration
A calibration curve is used to determine the unknown
concentration of an element – eg lead – in a solution.
The instrument is calibrated using several
solutions of known concentrations. A calibration
curve is produced which is continually rescaled as
more concentrated solutions are used – the more
concentrated solutions absorb more radiation up to a
certain absorbance. The calibration curve shows the
concentration against the amount of radiation
absorbed (Fig. 8(a)).
The sample solution is fed into the instrument
and the unknown concentration of the element – eg
lead – is then displayed on the calibration curve
(Fig. 8(b)) .
Interferences and matrix modification
Other chemicals that are present in the sample may
affect the atomisation process. For example, in flame
atomic absorption, phosphate ions may react with
calcium ions to form calcium pyrophosphate. This
does not dissociate in the flame and therefore results
in a low reading for calcium. This problem is avoided
by adding different reagents to the sample that may
react with the phosphate to give a more volatile
compound that is dissociated easily. Lanthanum
nitrate solution is added to samples containing
calcium to tie up the phosphate and to allow the
calcium to be atomised, making the calcium
absorbance independent of the amount of phosphate.
With electrothermal atomisation, chemical modifiers
can be added which react with an interfering
substance in the sample to make it more volatile than
the analyte compound. This volatile component
vaporises at a relatively low temperature and is
removed during the low and medium temperature
stages of electrothermal atomisation.
Figure 8(b)
Concentration
Absorbance
Figure 8(a)
Concentration
Absorbance
A bad paint job
Atomic absorption spectrometry is sometimes used for
investigating unusual problems. One such case was
that of a seriously ill baby whose symptoms could not
be explained.
Lead is a toxic element that can cause poisoning
in children. A baby was brought to a hospital
suffering from vomiting and stomach pains, and was
very drowsy. There were no obvious reasons or signs
why the child should be ill.
As part of the routine tests performed, the lead
level in a blood sample from the child was measured
using electrothermal atomisation AAS. The lead
level was higher than normal and there was no
known source for the lead. However, the parents
explained that the child had been chewing the
painted wood on its cot. The paint was also examined
by dissolving it in nitric acid and then using flame
AAS to find out the lead content. A very high level
was found.
Other paints in the baby ’s bedroom were found to
have low lead levels. This identified the cot paint as
the source of lead in the baby. The baby ’s cot was old
and had been painted when leaded paint was very
common. This type of paint is now banned from
household use and by law all painted toys must be
examined for lead and other toxic metals to make
sure that they are safe for small children.
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