AAS Principle

Atomic absorption spectrometry (AAS)
PRINCIPLE: The technique uses basically the principle that free atoms (gas)
generated in an atomizer can absorb radiation at specific frequency. Atomic-absorption
spectroscopy quantifies the absorption of ground state atoms in the gaseous state. The
atoms absorb ultraviolet or visible light and make transitions to higher electronic
energy levels. The analyte concentration is determined from the amount of absorption.
Atomizer: Elements to be analyzed needs to be in
atomic sate. 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.
The Nebulizer-Burner system/Flame

The purpose of the nebulizer-burner system is to convert
the test solution to gaseous atoms, and the success of
flame photometric methods is dependent upon the correct
functioning of the nebulizer-burner system.
As far as flame composition is concerned, it may be noted that an acetylene-air mixture is
suitable for the determination of some 30 metals, but a propane-air flame is to be
preferred for metals which are easily converted into an atomic vapour state. For metals
such as aluminium and titanium which form refractory oxides, the higher temperature of
the acetylene-nitrous oxide flame is essential, and the sensitivity is found to be enhanced
if the flame is fuel-rich.
Non Flame Techniques for Atomization
Instead of employing the high temperature of a flame to bring about the production of atoms from the
sample, it is possible in some cases to make use of either
(a) Non-flame methods involving the use of electrically heated graphite tube
(b) Cold vapour technique
Electrothermal Atomizer or Graphite tube Furnace: Samples of between 5–50 µL are injected into the
graphite tube through a small hole at the top of the tube. Atomization is achieved in three stages. In the first
stage the sample is dried to a solid residue using a current that raises the temperature of the graphite tube to
about 110oC. In the second stage, which is called ashing, the temperature is increased to between 350–
1200oC. At these temperatures any organic material in the sample is converted to CO2 and H2O, and volatile
inorganic materials are vaporized. These gases are removed by the inert gas flow. In the final stage the
sample is atomized by rapidly increasing the temperature to between 2000–3000oC.
Two additional commercially available atomizers are extensively used in
environmental and clinical analysis. They are the cold vapor AAS (CVAAS)
technique for determination of the element mercury (Hg) and the hydride
generation AAS (HGAAS) technique for several elements that form volatile
hydrides, including As, Se, and Sb.
These elements are toxic. So, their concentrations determination in drinking

water, wastewater, and air, at ppb concentrations is very important.
Analytical methods for the determination of mercury play an important role in

monitoring the safety of food and water supplies. One of the most useful methods
is based on the atomic absorption by mercury of 253.7 nm radiation.
Figure 28F-3 shows an apparatus that is used to determine mercury by atomic

absorption at room temperature.
A sample suspected of
containing mercury is
decomposed in a hot mixture
of nitric acid and sulfuric acid,
which converts the mercury to
the +2 state. The resulting
Hg2+ and any remaining
compounds are reduced to the
metal with a mixture of
hydroxylamine sulfate, and
tin(II) sulfate. Air is then
pumped through the solution
to carry the resulting mercury-
containing vapor through the
drying tube and into the
observation cell.
Hydride generation methods
Elements such as arsenic, antimony, and selenium are difficult to analyze by flame AAS because
it is difficult to reduce compounds of these elements (especially those in the higher oxidation
states) to the gaseous atomic state.
Although electrothermal atomization methods can be applied to the determination of arsenic,

antimony, and selenium, the alternative approach of hydride generation is often preferred.
Compounds of the above three elements may be converted to their volatile hydrides by the use of
sodium borohydride as reducing agent. The hydride can then be dissociated into an atomic vapour
by the relatively moderate temperatures of an argon-hydrogen flame.
The reaction sequence, in the case or arsenic, may be represented as follows:
It should be noted that the hydride generation method may also be applied to the determination of
other elements forming volatile covalent hydrides that are easily thermally dissociated. Thus, the
hydride generation method has also been used for the determination of lead, bismuth, tin, and
germanium.
Radiation 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). Each element has its own unique lamp which
must be used for that analysis. When a potential is applied across the electrodes, the filler gas is
ionized. The positively charged ions collide with the negatively charged cathode, dislodging, or
“sputtering,” atoms from the cathode’s surface. Some of the sputtered atoms are in the excited state
and emit radiation characteristic of the metal from which the cathode was manufactured. By
fashioning the cathode from the metallic analyte, a hollow cathode lamp provides emission lines
that correspond to the analyte’s absorption spectrum. Hollow cathode lamps are available from
several manufacturers either as single or multiple elements lamps.
Wave length selector/ 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.
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.
Flame Versus Electrothermal Atomization
The choice of atomization method is determined primarily by the analyte’s

concentration in the samples being analyzed. Because of its greater sensitivity,
detection limits for most elements are significantly lower when using electrothermal
atomization.
A better precision when using flame atomization makes it the method of choice
when the analyte’s concentration is significantly greater than the detection limit for
flame atomization. In addition, flame atomization is subject to fewer interferences,
allows for a greater throughput of samples, and requires less expertise from the
operator.
Electrothermal atomization is the method of choice when the analyte’s concentration

is lower than the detection limit for flame atomization. Electrothermal atomization is
also useful when the volume of sample is limited.
Interferences in AAS
An interference is a phenomenon that affects the measurement or the population of ground state atoms of an
analyte element.
These factors may be broadly classified as

(a) Spectral interferences
(b) Chemical interferences
(a) Spectral interferences in AAS arise mainly from overlap between the frequencies of a selected
resonance line with lines emitted by some other element.
Chemical interferences
The quantitative analysis of some elements is complicated by chemical interferences
occurring during atomization.
The two most common chemical interferences are the formation of nonvolatile
compounds containing the analyte and ionization of the analyte.
One example of a chemical interference due to the formation of nonvolatile

compounds is observed when PO43– or Al3+ is added to solutions of Ca2+.
These interferences were attributed to the formation of refractory particles of

Ca3(PO4)2 and an Al–Ca–O oxide.
The formation of nonvolatile compounds often can be minimized by increasing the
temperature of the flame, either by changing the fuel-to-oxidant ratio or by
switching to a different combination of fuel and oxidant.
Another approach is to add a releasing agent or protecting agent to solutions

containing the analyte.
A releasing agent is a species whose reaction with the interferent is more favorable
than that of the analyte.
Adding Sr2+ or La3+ to solutions of Ca2+, minimizes the effect of PO43– and Al3+ by
reacting in place of the analyte.
Protecting agents react with the analyte to form a stable volatile complex.

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AAS Principle

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Atomic absorption spectrometry (AAS)

The Nebulizer-Burner system/Flame

These elements are toxic. So, their concentrations determination in drinking

Analytical methods for the determination of mercury play an important role in

Figure 28F-3 shows an apparatus that is used to determine mercury by atomic

Although electrothermal atomization methods can be applied to the determination of arsenic,

Detector: The light selected by the monochromator is directed onto a detector

The choice of atomization method is determined primarily by the analyte’s

Electrothermal atomization is the method of choice when the analyte’s concentration

These factors may be broadly classified as

One example of a chemical interference due to the formation of nonvolatile

These interferences were attributed to the formation of refractory particles of

Another approach is to add a releasing agent or protecting agent to solutions

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