Presented bySruthi P. Vijayaraghavan(26)

ATOMIC ABSORPTION SPECTROSCOPY

In the later 1800, scientists studied the composition of the sun based on the various atomic emissions at different wavelengths

• Atomic absorption spectroscopy dates to the nineteenth century

• Modern form was largely developed during the 1950s by a team of Australian chemists led by Alan Walsh at the CSIRO (Commonwealth Science and Industry Research Organisation) Division of Chemical Physics in Melbourne, Australia.

Kirchhoff summarized the law, "Matter absorbs light at the same wavelength at which it emits light“. It is under this law that atomic absorption spectroscopy works.

AAS is of four types depending on method of volatilization:• Flame atomization: Sample sprayed into

flame where it undergoes atomization and irradiation

• Electro thermal oven: Sample placed in an oven where solvent is evaporated & sample vaporized into irradiation area

• Ion bombardment(tanatalum boat/carbon rod analyser):Sample placed on cathode and bombarded with + ions (Ar+), sputtering process dislodges them from cathode & directs them to irradiation region

• Electric arc/spark: Sample subjected to high current or high potential A.C. spark

• technique to determine the concentration of a specific metal element(gas phase atoms) in a sample in the solution phase• technique is based on the fact that ground state metals absorb light at specific wavelengths & the atoms enter excited state in a process called the atomic absorption

Elements detectable by atomic absorption are highlighted in pink in this

periodic table

The Beer-Lambert law,

Operation principle of an atomic absorption spectrometer

atomisation

• Atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms by means of a flame; which absorb light at characteristic wavelengths FAAS

• Each wavelength corresponds to only one element which gives the technique its elemental selectivity

• Due to unique configuration of electrons in its outer shell

• The quantity of energy (the power) put into flame is known & quantity remaining at the other side (at the detector) can be measured

• It is possible, from Beer-Lambert law, to calculate how many of these transitions took place & thus get a signal that is proportional to the concentration of the element being measured

• Light of the appropriate wavelength is supplied and the amount of light absorbed can be measured against a standard curve

• Simple, speedy and reliable technique

Interferences: Since the concentration of the analyte element is considered to be proportional to the ground state atom population in the flame, any factor that affects the ground state population of the analyte element can be classified as interference

A) Spectral interferences- due to radiation overlapping that of the light source

B) Formation of compounds that do not dissociate in the flame

C) Ionization of the analyte reduces the signal

D) Matrix interferences due to differences between surface tension & viscosity of test solutions

E) Broadening of a spectral line, which can occur due to a number of factors:

• atoms can have different components of velocity along line of observation

• concentration of foreign atoms present in the environment of emitting/absorbing atoms

• collision can occur in flames due to the presence of foreign gas molecules with vibrational levels very close to the excited state of the resonance line

• The characteristic wavelengths are element specific and accurate to 0.01-0.1nm.

• To provide element specific wavelengths, a light beam from a lamp whose cathode is made of the element being determined is passed through the flame.In 1955, Walsh suggested the use of cathode

lamps to provide an emission of appropriate wavelength; and the use of a flame to produce neutral atoms that would absorb the emission

as they crossed its path

∆E- change in energy in an atom during excitation from E0 to E1,i.e.the energy of emitted photonh- Plank’s constant, 6.626068 × 10-34 m2 kg / s

∆E=h/λ

• This relationship determines the presence of a specific element based on absorption in a specific wavelength

• An electron can change energy only by going from one energy level to another-transition

• Explains how and why line spectrum is formed

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