Introduction
Atomic
Absorption Spectroscopy (AAS)
Atomic absorption spectroscopy is
commonly used in calculating the concentration of a
particular element present in a sample. It is
based on the principle which requires standards with
the element concentration
is already known to establish the relationship between the element concentration and the
absorbance of the light and therefore relies on the Beer-Lambert Law which states that absorbance is directly proportional to the concentration of the element in the sample.
Beer-Lambert Law
Different atoms of elements absorb different wavelengths of light. The radiation may either be absorbed or transmitted depending on the
wavelength of the radiation when a beam of electromagnetic radiation is passed through
a sample. The
absorption of radiation of specific wavelength would increase the energy of the
molecule. These electrons can be excited to higher energy
level by absorbing a specific quantity of energy. The energy gained by the molecule
is directly proportional to the wavelength of radiation. This amount of energy is
specific for a particular element. In general, each wavelength
corresponds to only one element.
Electrons are excited to higher state
The unknown concentration
is usually determined by comparing the amount of light absorbed by the
sample to the amount of light absorbed using a series of standards of
known concentration. This involves the use of
a calibration graph.
Example of obtaining an unknown concentration from the calibration graph
For example with calcium, a light source containing calcium emits light with the wavelengths to be absorbed by any calcium atoms from the sample and is passed through a
vaporized sample. In AAS, the
sample which contains calcium ions is atomized (converted
into atoms in ground state
vapour form) by heating using a flame. Some of the radiation is absorbed by the calcium atoms, while others are transmitted. The greater the number of atoms there is in the form of vapour, the more light will be absorbed. The the number of calcium atoms in vapour form is proportional to amount of light absorbed. A calibration curve is
constructed by using some samples of known calcium concentration under the same conditions as the
unknown. The absorbance of the unknown sample is determined by comparing with the standard by using a calibration curve. This
enables the concentration of the calcium in the unknown sample to be calculated.
Calibration graph for Calcium
Light source: Hollow cathode
lamp are the most common radiation source in AAS. It
contains a tungsten
anode and a hollow cylindrical cathode made of the
element to be
determined. These are sealed in a glass tube filled with an
inert gas (neon or
argon) at a pressure of between 1Nm-2 and 5Nm-2. The
ionisation of some
gas atoms occurs by applying a potential difference of about
300-400V between
the anode and cathode and eject metal atoms from the
cathode in a
process called sputtering. Some sputtered atoms are in excited
states and emit
radiation characteristics of the metal as they fall back to the
ground state. 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. 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.
Nebulizer: Suck up liquid
samples at controlled rate. Create a fine aerosol spray for
introduction into
flame. Mix the aerosol, fuel and oxidant thoroughly for
introduction into
flame.
Atomizer: Elements to be
analysed needs to be in atomic state. Atomization is separation of
particles into
individual molecules and breaking molecules into atoms. This is
done by exposing the
analyte to high temperatures in a flame or graphite furnace.
Flame Atomizer: The
oldest and most commonly used atomizers in AAS are flames. To
create
flame, we need to mix an oxidant gas and a fuel gas. In most of the
cases, air-acetylene
flame or nitrous oxide-acetylene flame is used. A
flexible
capillary tube connects the solution to the nebulizer. At the tip of
the
capillary, the solution is ‘nebulised’ which is broken into small drops.
The larger
drops fall out and drain off while smaller ones vaporise in the
flame. Only
1% of the sample is nebulised.
Graphite
Tube Atomizer: 25?l of sample 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 second at150? to evaporate the
solvent,
30 seconds at 600? to drive off any volatile organic
material
and char the sample to ash, and with a very fast heating
rate to
2000-2500? for 5-10 seconds to vaporise and atomise
elements.
Finally heating the tube to a still higher temperature
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.
Monochromator: This is a
very important part in an AA spectrometer. It is used to separate
out all of
the thousands of lines. A monochromator is used to select the
specific
wavelength of light 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.
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. 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
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 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.
Detector: The light
selected by the monochromator is directed onto a detector that is typically
a photomultiplier tube,
whose function is to convert the light signal into an
electrical signal
proportional to the light intensity. The processing of electrical
signal is fulfilled by a
signal amplifier. The signal could be displayed for readout,
or further fed into a
data station for printout by the requested format.
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 as
well
as that of the sample. One way of measuring and correcting this
background
absorption is to use two light source, 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 cathode lamp.
By alternating the measurements of the two
light
sources generally at 50-100Hz, the total absorption 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 Curve: A
calibration curve is used to determine the unknown concentration of an
element in
a solution. The instrument is calibrated using several solutions
of known
concentrations. The absorbance of each known solution is
measured
and then a calibration curve of concentration vs absorbance is
plotted. The sample solution
is fed into the instrument, and the absorbance
of the
element in this solution is measured. The unknown concentration of
the
element is then calculated from the calibration curve.
Specific Uses
Agriculture – analysing
soil and plants for minerals necessary for growth
Trace metals are
essential for plant growth. Atomic spectroscopy facilitates precise soil
analysis to ensure that metals are not at levels that could unduly affect the food
source (livestock and/or crops). Plants may be sampled to monitor nutrient
uptake efficiency and also to check for toxic metal accumulation for health
reasons.
Industrial and Chemical –
analysing raw chemicals as well as fine chemicals
From the analysis of raw
materials and components to finished product testing and quality control,
industrial and chemical manufacturers require accurate analytical techniques to
ensure the safety and performance of their products. 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.
Environmental Study –
determination of heavy metals in water, soil, and air
In the environment we
live in, understanding heavy-metal contamination is critical. The accurate
measurement of concentrations of these metals is imperative to maintain clean
air, water and soil for a safer world. AAS is used to monitor 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.
Food Industry – quality
assurance and testing for contamination
Accurate analysis of food
for nutritional content, contamination or authenticity – the exact geographic
source of the product – is critical for regulatory and quality assurance.
Forensic Science –
substance identification
AAS functions in
determination of trace elements, mode of poisoning, and ammunition
manufacturers, elemental profiles of biological samples, trace elements in
artificial fibres, hair analysis for heavy metal poisons and discrimination of
objects/elements.
Mining – testing the
concentration of valuable substances in potential mining areas
Atomic spectroscopy
offers a fast, accurate solution for broad geological surveys as well as an
invaluable means of testing potential mining areas before incurring the high costs
associated with digging. 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
Nuclear Energy –
monitoring potentially hazardous elements in water and waste output
Operating under constant
scrutiny, the nuclear field is required to monitor and measure the levels of a
variety of elements to an exacting degree. Atomic spectroscopy is commonly used
to determine trace elements in everything from process water to low-level
waste.
Petrochemical – analysing
products for metals and other substances that can have adverse effects such as
oil and gas
From petroleum refining
to a broad spectrum of applications using lubricants and oils, many industries
require the determination of metals – particularly analytes that can lead to degradation
and contamination – to ensure conformity as well as monitor and control
processes.
Pharmaceutical – many
applications from quality control to detecting impurities in drugs
Drug research, development
and production is dependent on elemental analysis, starting with the testing of
individual ingredients and continuing through production to final quality control,
as impurities can affect drug efficacy and metabolism. Most Pharmaceutical
Companies these days develop drugs which are targeted at specific cells in the
body. These drugs must be tested for correct activity but more importantly for
the absence of any adverse side reactions. 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.
ADVANTAGES
Accuracy.
Atomic absorption spectroscope is a great way of producing
accurate results, with a rate of 0.5-5%, the result can be even better rate if
appropriate standards are used.
Sensitivity.
AAS is a sensitive method
of detection, it can measure down to parts per billion of a gram (µg dm–3 ) in
a sample. As such, it has many uses in
different areas of chemistry. For example, in medicine, it can be used
to detect trace toxin levels of atmosphere or medication. Similarly, in
pharmaceuticals, 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. In industry, AAS is
used to check that the major element are present and the toxic impurities are
lower than specified.
Cost
AAS uses less argon than other methods, thus
its running costs are often lower compare to other methods.
Accessibility.
Since the process relying upon radiation and light
absorption, it can reach previously inaccessible places. For example, miners
can now use AAS to determine if a rock contains enough elements of gold or
other precious metals to be worthwhile mining.
DISADVANTAGES
Lack of Versatility
Sample must be in solution or at least volatile. This
is because the substances have to be vaporised before it can be analysed.
Liquids lend themselves to this much more than solids. Besides, the technique
that allow for solid-substance testing can not be used on non-metals. This
technique has also not proved very successful for the estimation of elements
like V, Si, Mo, Ti and A1 because these elements give oxides in the flame.
Low precision
Other chemicals that are found in the sample or in the
surrounding atmosphere can have an interfering and distorting effect on the
results of the study.
Weakness
It is a destructive
analysis method and therefore scarce sample should be analysed by a
non-destructive method such as raman. Due to the sample preparation is time
consuming, it cannot give information as detailed as other techniques such as Nuclear
Magnetic Resonance (NMR). It is also qualitative rather than quantitative and
there are lots of compounds which are not infrared (IR) active and therefore
cannot be detected.
