atomic absorption spectroscopy

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1 Atomic Absorption Spectroscopy ( AAS )

Atomic Absorption Spectroscopy (AAS):

2 Introduction Elementary Theory Instrumentation Interferences Experimental preliminaries Applications Atomic Absorption Spectroscopy ( AAS )

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3 What is AAS ? A tomic Absorption Spectroscopy is a quantitative method of analysis that is applicable to many metals and a few nonmetals. I ntroduction

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4 What is AAS ? The technique was introduced in 1955 by Walsh in Australia The first commercial atomic absorption spectrometer was introduced in 1959

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5 What is AAS ? An atomic absorption spectrophotometer consists of a light source, a sample compartment and a detector. Light Source Detector Sample Compartment

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6 What is AAS ? A much larger number of the gaseous metal atoms will normally remain in the ground state. These ground state atoms are capable of absorbing radiant energy of their own specific resonance wavelength. If light of the resonance wavelength is passed through a flame containing the atoms , the part of the light will be absorbed. The extend of absorption will be proportional to the number of ground state atoms present in the flame.

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7 What is AAS ? the gaseous metal atoms specific resonance wavelength the extend of absorption vs the number of ground state atoms present in the flame. extend of absorption

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8 Characteristic wavelength Characters of the atomic absorption spectrum Δ E = E 1 – E 0 = hc /  E 1 - excited state E 0 – ground state h – Planck’s constant c – velocity of light  - wavelength Elementary Theory

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9 K 0 - maximal absorption coefficient Δ  - half width  0 - central wavelength Characters of the atomic absorption spectrum Profile of the absorption line

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10 I t = I 0ν e -Kνl The relationship between absorbance and the concentration of atoms A = log ( I 0ν / I t ) = 0.4343 K  l Beer ’ s law I t - intensity of the transmitted light I o – intensity of the incident light signal l – the path length through the flame (cm)

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11 Instrumentation Light source Monochromator Detector Read-out Nebulizer Schematic diagram of a AAS spectrophotomer Atomization

Hollow cathode lamp (HCL) :

12 Hollow cathode lamp (HCL) Cathode --- in the form of a cylinder, made of the element being studied in the flame Anode ---tungsten

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13 A hollow cathode lamp for Aluminum (Al)

SpectrAA - AAS:

14 SpectrAA - AAS motorized Mirror HCL

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15 Processes occurring during atomization Flame atomization

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16 Nebulizer - burner A typical premix burner Flame atomization

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17 Nebuliser - burner To convert the test solution to gaseous atoms Nebuliser --- to produce a mixture of sample or a test solution Burner head --- The flame path is about 10 –12 cm Vaporising chamber --- Fine mixture is mixed with the fuel gas and the carrier gas Larger droplets of liquid fall out from the gas stream and discharged to waste

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18 Fuel and oxidant flame b Air – acetylene Air - propane Air - hydrogen b Nitrous oxide – acetylene Auxiliary oxidant Fuel

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19 Common fuels and oxidants used in flame spectroscopy

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20 Disadvantages of flame atomization Only 5 – 15 % of the nebulized sample reaches the flame A minimum sample volume of 0.5 – 1.0 mL is needed to give a reliable reading Samples which are viscous require dilution with a solvent

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21 --- diffraction grating Monochromator

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22 Detector --- photomultiplier

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23 Read-out system --- meter --- chart recorder --- digital display

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24 Atomic absorption spectrophotometer

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25

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26 Interferences Spectral interferences Chemical interferences Physical interferences

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