ATOMIC ABSORPTION SPECTROSCOPY

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1 Atomic absorption spectroscopy BY EKTA PATEL M.PHARM 2 ND SEM(Q.A)

INTRODUCTION : 

INTRODUCTION Atomic absorption spectroscopy is a method of elemental analysis . It is particularly useful for determining trace metals in liquids and is most independent of molecular form of the metal in sample. For example we can determine total cadmium content of water sample-it does not matter whether the cadmium exists as a chloride, sulfate, or other salt form 2

definition: 

definition AAS is an absorption method where radiation is absorbed by non exited atoms in vapour form . 3

PRINCIPLE : 

PRINCIPLE When a solution containing metallic species is introduced into a flame, the vapour of metallic species will be obtained. These ground state atoms of a particular element are receive of light radiation of their own wavelength 4

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Part of that light will be absorbed by the particular element and the absorption will be proportional to the density of atoms in the flame. Thus, in AAS, one determines the amount of light absorbed. once this value of absorption is known, the concentration of the metallic element can be known. Mathematically the total amt of the light absorbed may be given by following equation 5

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Total amt of light absorbed =( ле 2 /mc) Nf Where, e = charge on the electron of mass C= speed of light N = Total number of atoms f= oscillator strength 6

INSTRUMENTATION : 

INSTRUMENTATION 7

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Light produced by special kind of lamp which is able to emit the spectral lines which is absorbed by the specific atoms .the sample solution is aspirated into the flame. This vapour will be dissociated into atoms of the element to be measured. Thus ,the flame possesses free unexcited atoms which are capable of absorbing radiation, from an external source when the radiation corresponds exactly to the energy required for a transition element from ground state to exited level 8

PARTS OF ATOMIC ABSOPTION SPECTROMETER: 

PARTS OF ATOMIC ABSOPTION SPECTROMETER RADIATION SOURCE CHOPPER ATOMISERS NEBULIZATION OF LIQUID SAMPLE MONOCHROMATORS DETECTORS AMPLIFIER READ OUT DEVICE 9

RADIATION SOURCE : 

RADIATION SOURCE HOLLOW CATHOD LAMP ELECTRODELESS DISHCHARGE LAMP 10

HOLLOW CATHOD LAMP : 

HOLLOW CATHOD LAMP 11

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The cathode consists of a hollow cup. In the cup is the element which is determined. The anode is tungsten wire. The two electrodes keep in a tube containing an inert gas. The lamp window is constructed of quartz, silica, or glass. 12

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When a potential is applied between the two electrodes, the inert gas is charged at the anode, and the charged gas is to the cathode. The impact with the cathode vapourizes some of the sodium atoms and give rise to the sodium emission spectrum 13

The neon or helium gas in the hollow cathode lamp performs three functions : 

The neon or helium gas in the hollow cathode lamp performs three functions It dislodges atoms from the surface of the cathode responsible for excitation of the ground state metal atoms It is main source of current carrying capacity in the HCL. 14

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The pressure maintained in the lamp is 1 to 5 torr. The spectral lines produced by the HCL are so narrow that they are completely absorbed by the atoms. Each hollow cathode lamp emits the spectrum of the metal which is used in the cathode , for example copper cathode emits copper spectrum which is absorbed by copper atoms. 15

ELECTRODELESS DISHCHARGE LAMP: 

ELECTRODELESS DISHCHARGE LAMP It is difficult to make stable hollow cathode lamp from certain elements particularly those that are volatile , such as arsenic, germanium. An alternative light has been developed in the EDL. It consists of an evacuated tube in which the metal of interest is placed. The tube is filled with argon at low pressure and sealed off. The sealed tube is then placed in microwave discharge cavity. Under these conditions the argon becomes a plasma and cause excitation of the metal sealed inside the tube. The emission from the metal is that of its spectrum . 16

CHOPPER : 

CHOPPER A rotating wheel is interposed between the HCL and the flame. This rotating wheel is known as chopper it is interposed to break the steady light from the lamp into a pulsating light. This gives pulsating current in photocell. There is also steady current caused by light which is emitted by flame. But only pulsating current is amplified and recorded. 17

ATOMISERS : 

ATOMISERS Atomization is separation of particles into individual molecules and breaking molecules into atoms .This is done by exposing the analyte to high temperatures in a flame or graphite furnace. FLAME ATOMISERS NON FLAME ATOMISERS 18

FLAME ATOMISERS : 

FLAME ATOMISERS A most common way is to use a flame which is used for converting liquid into gaseous state and also converting the molecular form into atomic vapour. There two types of burners Total consumption burner Premixed burner. 19

TOTAL CONSUMPTION BURNER : 

TOTAL CONSUMPTION BURNER 20

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In this burner, the sample solution, the fuel, and oxidizing gases are passed through separate passages to meet at the opening of the base of flame. Sample in liquid form then the flame breaks into the droplets which are evaporated and burns. Leaving the residue which is reduced to atoms. Total consumption burners do use oxygen with hydrogen or acetylene, gives very hot flames. Disadvantage is there it is noisy and hard to use. 21

PREMIXED BURNER : 

PREMIXED BURNER IN this burner, the sample is aspirated into a large chamber by means of fuel gas and oxidant under pressure. The fine droplets get carried out along with the fuel gas at outlet, the large drops of sample get collected in chamber and are drained out. 22

NEBULIZATION: : 

NEBULIZATION: Before the liquid sample enters the burner ,it is converted into droplets this method a formation of small droplets its called nebulization Common method of nebulization is by use of s gas moving at high velocity, called pneumatic nebulization. 23

MONOCHROMATORS : 

MONOCHROMATORS This is a very important part in an AA spectrometer. It is used to separate out all of the thousands of lines. Without a good monochromator, detection limits are severely compromised. A monochromator is used to select the specific wavelength of light which is absorbed by the sample, and to exclude other wavelengths. The selection of the specific light allows the determination of the selected element in the presence of others. 24

DETECTORS : 

DETECTORS 25

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As soon as photon strikes the photocathode an electrons is dislodged and photon is accelerated to dynode I., resulting in the liberation of two or more electrons from this dynode. Similarly, the electrons from this dynode I are accelerated to dynode 2, resulting in the liberation of move electrons. Thus, the current multiplied at each dynode and the resultant electron current is received by the anode which goes to external amplifier and read-out system. 26

AMPLIFIER : 

AMPLIFIER 27

READ OUT DEVICE : 

READ OUT DEVICE In the most of AAS measurement, chart recorders are used as read out device. A chart recorder is a potentiometer . 28

INTERFERENCES : 

INTERFERENCES Absorption of Source Radiation : Element other than the one of interest may absorb the wavelength being used. Ionization Interference: the formation of ions rather than atoms causes lower absorption of radiation .This problem is overcome by adding ionization suppressors. Back ground Absorption of Source Radiation : This is caused by the presence of a particle from incomplete atomization .This problem is overcome by increasing the flame temperature. 29

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Transport Interference: Rate of aspiration, nebulization, or transport of the sample (e g viscosity, surface tension, vapor pressure, and density). Cation cation interference: invariably decrease the signal intensity of the element present in the sample. These are neither spectral nor ionic in nature and mechanisms of their interactions are unknown. 30

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Anion cation Interference: the presence of certain anions in solution may effect the intensity of radiation emitted by an element and thus results in a serious analytical error. Oxide formation Interference: this type of Interference arises due to the formation of stable oxides with free metal atoms if oxygen is present in the flame. Thus, the emission intensity is lowered because a large percentage of free metal atoms have been removed from the flame. All of the alkaline earth element from oxides and are subject to this type of Interference. 31

application: 

application QUALITATIVE ANALYSIS : in atomic absorption spectroscopy, a different HCL is to be used for each element to be tested. It means that an element which Is used in in the construction of cathode of HCL, can be detected only. As QUALITATIVE ANALYSIS involves the checking of one element at a time, it means that the process is very laborious. There for , AAS is the seldom used in practice for the purpose. 32

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QUANTITATIVE ANALYSIS: it is used determination of the amt of radiation absorbed by the sample if the value of radiation absorbed is substituted in equation. 33

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Simultaneous malticomponent analysis is available, such determination using multicomponent HCL Determination of metallic elements in biological materials: Determination of metallic elements in food industry: copper, zinc, and nickel are the most common toxic elements of interests to food analysis Determination of calcium, magnesium, sodium and potassium in blood serum Determination of lead in petrol. 34