Atomic Spectroscopy
Atomic spectroscopy is the study of the electromagnetic radiation
absorbed and emitted by atoms. It is an excellent analytical tool used for
the detection and measurement of elements in a sample with high precision
and confidence. It is typically based on the analysis of the electromagnetic
radiation emitted by the atoms in an element.
Principle:
The atoms (and ions) can absorb light at a specific, unique
wavelength. When this specific wavelength of light is provided, the
energy (light) is absorbed by the atom, then electrons in the atom
move from the ground state to an excited state.
Types: Atomic Spectroscopy is of 3 types.
1.Atomic absorption Spectroscopy
2.Atomic emission Spectroscopy
3.Atomic Fluoroscence Spectroscopy:
Atomic absorption spectroscopy (AAS) is an absorption
spectroscopic method that uses the absorption of light by free atoms
in a gaseous state to determine the quantitative composition of
chemical components. It is used to determine the concentration of
metals present in a sample to be analyze
Principle: If a solution containing metal salt (M+X–) is aspirated to
the flame, a vapor that contains atoms of metal may be formed. A
large number of the gaseous metal atom remains in the ground state,
and are capable of absorbing radiant energy of their specific
wavelength. If the light of resonance wavelength is passed through the
flame containing the atoms which are analyte, the part of the light will
be absorbed and the extent of absorption will be directly proportional
to the number of
ground state atoms present in the flame.
The process by which gaseous metal atoms are produced into
the flame can be illustrated as:
When a metal atom is changed into gas and light is passed from the
sources, the ground state of the atom gets excited by absorbing the
radiation of a particular wavelength. The absorbance is given
by Beer-Lambert’s law; the logarithmic ratio of the intensity of
incident light to the intensity of absorbing species.
Instrumentation:
1.Atomiser:
The atomizers most commonly used nowadays are flames and
electrothermal (graphite tube) atomizers.
*In Flame-AAS, the atomizer is a flame furnace, the heat of the
flame
produces the free atoms when sample introduced into atomiser.
*In Graphite-AAS, the atomizer is a graphite furnace and when
sample is introduced into the atomizer,the electrical energy in
the graphite furnace produces the free atoms.
* An atomizer is a device that produces a fine spray from a liquid.
Most combustors employ an atomizer in which fuel is forced under
high pressure through an orifice. The function of any atomizer is to
produce as homogeneous a spray as possible.
ATOMIC ABSORPTION SPECTROSOPY:
*Atomic absorption spectroscopy is based on the principle of
absorption of light by atoms in the gaseous state.
*Atoms absorbed light at a definite wavelength depending on the nature of
chemical elements.
Sodium is absorbed in 589 nm
Uranium is absorbed in 589 nm
Potassium is absorbed in 766.5 nm
*From the ground state of an atom is excited to a higher energy level by
absorption of energy. The electronic transition is specific to a particular
element.
*Elements with high excitation energy can be determined by atomic
absorption spectroscopy.
*Atomic spectra are identified by sharp lines which can be distinguished
from broadband spectra associated with molecules.
*The lines arising from the ground state are almost important in atomic
absorption spectroscopy. These are called resonance lines.
ATOMIC ABSORPTION SPECTROSCOPY FLAME:
We used fuel and oxidant to create an atomic absorption spectroscopy
flame.
Fuels used: natural gas, propane, butane, hydrogen, and acetylene
Oxidants: Air, oxygen, nitrous oxide, and a mixture of nitrous oxide and
acetylene are used as an oxidant for flame creation in atomic absorption
spectroscopy.
Atomic absorption spectroscopy flame
Fuel Air oxidant Oxygen oxidant Nitrous oxide oxidant
Hydrogen 2100 °C 2780 °C –
Acetylene 2200 °C 3050 °C 2955 °C
Propane 1950 °C 1950 °C –
Very high temperatures are required for the vapourisation of some metals
which are not essily atomised. It is attained by using an oxidant in the flame
along with fuel gas in atomic absorption spectroscopy.
*For example, we used oxyacetylene flame for the analysis
of aluminum, titanium, and rare earth elements in an AAS instrument.
Any atomic absorption spectroscopy instrumentation has the following
types of components,
Atomization
Hollow cathode lamp
Monochromator
Detector
Recorder
Atomization
Atomization can be carried out either by a flame or furnace. Heat
energy is utilized in atomic absorption spectroscopy to convert
metallic elements to atomic dissociated vapor. The temperature
should be controlled very carefully for the conversion of atomic
vapor. At too high temperatures, atoms can be ionized.
*Fuel and oxidant gases are fed into a mixing chamber which passes
through baffles to the burner. A ribbon flame is produced in the AAS
instrument. The sample is aspirated through the air into the mixing
chamber.
HOLLOW CATHODE LAMP:The hollow cathode lamp has
two electrodes, one is cup-shaped and made of a specific element.
Radiation from the hollow cathode lamp should not be continuous due to
spurious radiations. Therefore, we used a chopping wheel between the
radiation or pulsed potential.
The metal which is used in the cathode is the same as that metal that we
analyzed. The lamp is filled with noble gas at low pressure. The lamp forms
a glow of emission from the hollow cathode.
Multi-element hollow cathode Lamps: Such types of lamps facilitate
for determination of samples without change of lamps each time. These
types of lamps are widely used in atomic absorption spectroscopy
instruments.
Monochromator
A monochromator is an optical device that transmits a narrow band of
wavelengths of light or other radiation from a wider range of wavelengths.
The atoms in the AAS instrument accept the energy of excitation and emit
radiation.
A desired band of lines can be isolated with a monochromator by passing a
narrow band.
Detector
A detector can convert light coming from a monochromator to a simplified
electrical signal. Generally, we used a photomultiplier tube as a detector in
the atomic absorption spectroscopy instrument. A detector can be tuned to
respond by a specific wavelength or frequency.
Recorder
The recorder can receive electrical signals from the detector to convert
them into a readable response. In atomic absorption spectroscopy
instrumentation, today we used a computer system with
suitable software for recoding signals coming from the detector.
Application of atomic absorption spectroscopy
Today, the atomic absorption spectroscopy technique is the most powerful
tool in analytical chemistry, forensic science, environmental analysis, and
food industries. It is popular for analysts due to several advantages.
The most important advantage is the speed of analysis. It can
analyze various samples within a day.
Secondly, it is possible to determine all elements at
trace concentration.
Thirdly, it is not always essential to separate the element before
analysis because AAS can be used to determine one element in
presence of another.
The atomic absorption spectroscopy principle or instrumentation
can be used to analyze sixty-seven metals and several nonmetals
such as phosphorus and boron.
