Atomic Absorption
Atomic Absorption
Atomic Absorption
Spectrophotometry (AAS)
concentration .
- 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.
- Concentration measurements are usually
determined from a working curve after
calibrating the instrument with standards of
known concentration.
Most interference that occurs have been well studied and documented.
The signal from the detector is transferred to the computer, and the output
registers on the monitor in a manner specified by the user.
Relationship Between Atomic Absorption and
Flame Emission Spectroscopy
Flame Emission ->
it measures the radiation emitted by the excited atoms that is related to concentration.
Atomic absorption depends only upon the number of unexcited atoms, the absorption
intensity is not directly affected by the temperature of the flame.
The flame emission intensity in contrast, being dependent upon the number of excited
atoms, is greatly influenced by temperature variations.
The source of light is a lamp whose cathode is composed of the element being measured.
Lasers are also used in research instruments. Since lasers are intense enough to excite
atoms to higher energy levels, they allow AAS and atomic fluorescence measurements
in a single instrument. The disadvantage of these narrow-band light sources is that
only one element is measurable at a time.
Hollow Cathod
Lamps (HCL):
The cathode is coated
with the metal of interest.
Inert filler gas (Ne or Ar) is
ionized by an electric
current and these ions are
then attracted to the
cathode. The ions
bombard the cathode and
excite the metal ions
coated on it. This
excitation of the metal
produces the emission of
EMR with wavelengths
characteristic of the
analyte.
Hollow Cathode Lamps (HCL)
Hollow-cathode lamps are a type of discharge lamp that produce narrow
emission from atomic species.
They get their name from the cup-shaped cathode, which is made from the
element of interest.
The electric discharge ionizes rare gas atoms, which are accelerated into the
cathode and sputter metal atoms into the gas phase. Collisions with gas atoms
or electrons excite the metal atoms to higher energy levels, which decay to
lower levels by emitting light. Hollow cathode lamps are available from several
manufacturers either as single or multiple elements lamps.
Electrodeless Discharge Lamps
(EDLs)
For most elements, the hollow cathode lamp is a source for atomic
absorption. The primary cases involve the more volatile
elements, where low intensity and short lamp life are a problem.
Flame AAS can only analyze solutions, while graphite furnace AA can accept
solutions, slurries, or solid samples.
Flame AAS uses a slot type burner to increase the path length, and therefore to
increase the total absorbance. Sample solutions are usually aspirated with the gas
flow into a nebulizing/mixing chamber to form small droplets before entering the
flame.
Basic
Principle
The technique of flame atomic
absorption spectroscopy (FAAS)
requires a liquid sample to be
aspirated, aerosolized, and
mixed with combustible gases,
such as acetylene and air or
acetylene and nitrous oxide.
The mixture is ignited in a flame
whose temperature ranges from
2100 to 2800 O C. During
combustion, atoms of the
element of interest in the
sample are reduced to free,
unexcited ground state atoms,
which absorb light at
characteristic wavelengths
AAS Nebulization
Chamber A solution of the sample was sprayed as an
aerosol through a nebulizer into a flame.
Some elements can only be converted to atoms at high temperatures. Even at high
temperatures, if excess oxygen is present, some metals form oxides that do not
redissociate into atoms.
Carbon rod analyser: This device can be used to convert a powdered sample into atomic
vapour. A current is applied to a very thin, heated carbon rod that contains the solid
sample in order to vaporise it.
Tantalum boat analyser: This is another technique that produces an atomic vapour from
a solid sample. A Tantalum boat is electrically heated in a manner similar to the carbon
rod system, within an inert atmosphere
Graphite furnace
The graphite furnace has several advantages over a flame.
Samples are placed directly in the graphite furnace and the furnace is electrically
heated in several steps to dry the sample, ash organic matter, and vaporize the analyte
atoms.
Three types of high-temperature
plasmas
The direct current plasma (DCP).
The main purpose of the monochromator is to isolate the absorption line from
background light due to interferences. Simple dedicated AAS instruments often replace
the monochromator with a bandpass interference filter.
Metallurgical and inorganic analysis for determination of alloys as Co, Cr, Mg, Mn,
Pb, and Zn.
Pollution analysis
Analysis of Fe in Haematonics
Analysis of co in radiopharmaceuticals