ATOMIC ABSORPTION SPECTROSCOPY

TARANPREET SINGH

MS12044Chem. Major

INTRODUCTIONATOMIC ABSORPTION SPECTROSCOPYIT IS A QUANTITATIVE METHOD FOR DETERMINING THE CHEMICAL ELEMENTS IN SAMPLE AND ITS CONCENTRATION .IT IS APPLICABLE TO MANY METALS AND SOME NON METALS

The technique was introduced by Alan walsh in 1955 At (CSIRO) Australia .

The first commercial spectrometer was introduced in 1959

Atomic Absorption Spectrophotometer

• Atomic absorption spectrophotometer consist of Radiation source , Atomizers Monochromator , Detector, Datadisplay or Computer

Schematic diagram of Atomic absorption spectrometer

LINE SOURCE ATOMIZATION MONOCHROMATOR DETECTOR

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NEBULIZER

Principle of Atomic Absorption SpectrophotometerAtomic absorption spectrophotometer works on the principle of absorption during atomization most of gaseous elements remains in ground state each elements absorb radiant energy of their own specific wavelength . Atoms after absorbing light make transition to higher energy states then eventually get back to its ground state by emitting light .during absorption the light that get transmitted from atoms will reach detector . It has linear relationship that more the concentration of ground state atoms more will be the absorption.

Light absorption process of atoms

Elements that are highlighted in green are detectable by AAS

RADIATION SOURCE (HCL - Hollow Cathode Lamp) (EDL – Electrodeless Discharge Lamp)

Hollow Cathode Lamp (HCL)

• CATHODE – Made of metal that is being studied In the flame• ANODE - TungstenDISADVANTAGE• FOR EVERY ELEMENT SEPARATE LAMP MUST BE USED• WORKS ONLY FOR VISIBLE AND U.V REGION

EDL – ELECTRODE LESS DISCHARGE LAMP

• Edl are commercially Available for Sb, As, Bi, Cd, Cs, Pb, Hg, K, Rb, Sn, Te, etc.

ATOMIZATION TECHNIQUES • FLAME ATOMIZATION• ELECTROTHERMAL ATOMIZATION• HYDRIDE ATOMIZATION• COLD-VAPOUR ATOMIZATION

LAMINAR FLOW BURNER

Nebulizer• To convert the analyte solution to gaseous atoms• Nebulizer – To produce a mist or aerosol of the analyte

solution In vaporizing chamber Fine mist is mixed with fuel gas and oxidant Larger droplets of sample falls out from gas stream and discharge into waste• Flame path – The path of flame is about 10-12 cm

Common fuel and oxidant

DISADVANTAGE OF FLAME ATOMIZATION

• Low efficiency and low sensitivity• REASONS OF LOW SAMPLING EFFICIENCY Large portion of the sample flows down in the waste The relative time of individual atoms in the optical path in the flame is brief (≈10-4 s ）• 5-15% of the nebulized sample reaches the flame• A minimum sample volume of (0.5-1 mL) required for

reliable reading

Graphite Furnace techniques

Graphite furnace techniques • Step involved in it(Process)

Drying Ashing Atomization

HYDRIDE GENERATION METHOD• FOR METALS LIKE ARSENIC(AS) , ANTIMONY (Te),SELENIUM(Se),BISMUTH(Sb)Volatile hydride usually be generated by adding acidified aqeous solution of sample to a small volume of 1% sodium borohydride in a glass vessel. Rxn given below , the volatile hydride then swept into atomization chamber usually quartz absorption tube that heated to several hundred degree in a flame or tube where decomposition of the hydride takes place leading to the formation of atoms of the analyte. The concentration of analyte is then measured by absorption or emission .

NaBH4 HeatAs (V) AsH3 As0

(gas) + H2

[H+] In Flame

Hydride Generation methods

COLD VAPOR TECHNIQUE• Cold vapor technique is an atomization method for determination of

mercury because it is the only metallic element that has appericiable vapor pressure at room temperature the determination of mercury In various sample is important because of toxicity of organic mercury compound . Determination of the mercury is done by cold vapourisation followed by atomic absorption spectroscopy.so first mercury is converted to its +2 oxidation state by treatment of sample with an oxidizing mixture of nitric and sulfuric acid followed by reduction of hg2+ to the metal with sncl2. the elemental mercury is swept into a long –passs absorption tube by bubling a stream of inert gas through the reaction mixture.

• The determination is completed by measuring the absorbance at 253.7mm

Cold-vapour technique

MONOCHROMATOR• Monochromator used to disperse the radiation according

to their wavelength.• Two types of monochromator 1. Prism 2. Diffraction grating • Monochromator consist of entrance slit , collimator lens ,

prism or diffraction grating ,focusing lens , exit slit

Bunsen Prism Monochromator

Diffraction Grating

MONOCHROMATORS

Detector• The role of detector is to convert light signal to electrical

signal that can be displayed on computer. Generally in most of the instruments PMT is usedThe detector contains a photoemissive cathode and a series of dynodes. The number of electrons emitted from the cathode is directly proportional to the intensity of the light beam. Electrons emitted from the cathode are accelerated to the first dynode by a 90 volt potential where the electron impact dislodges several additional electrons which are accelerated to the next dynode by an additional 90V potential. After nine dynodes (each one at +90V with respect to the one before it), the number of electrons finally reaching the anode is in the order of ten million for each incident photon. The current measured at the anode collector is still proportional to the intensity of the light but it has been amplified over a million times.

Detector(photomultiplier tube)

Applications• PHARMACEUTICALS• PETRO-CHEMICAL• ENVIRONMENTAL SCIENCE• NANOMATERIALS• GEOCHEMICAL• FORENSIC SCIENCE• CLINICAL ANALYSIS• FOOD INDUSTRY

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