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1. 1. Atomic Absorption Spectroscopy Presented by S.Saravanan M.Pharm (Pharmaceutics) Sri Ramachandra University
2. 2. Introduction Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and elements. It is so sensitive that it can measure down parts per billion of a gram in a sample . Measurement of absorption of light passing through gaseous ground state atoms forms the basis of AAS. 2
3. 3. COMMONLY ENCOUNTERED TOXIC HEAVY METALS  Arsenic  Lead  Mercury  Cadmium  Iron  Aluminum 3
4. 4. Principle: When a solution containing metallic species is introduced into a flame, the gaseous metallic atom will be formed .some fraction of the metal atoms may get the thermal energy and goes to an excided state, and emit the characteristic wavelength of radiation of the corresponding metal atom. This phenomenon involved in the FES. Whereas, a large percentage of the metal atom will be remain in the lower energy state(ground state). These ground state atom of a particular element are receptive of light radiation of their own specific resonance wavelength . Thus , when a light of this wavelength passed through a flame having atoms of metallic species, part of that light will be absorbed. This absorption intensity will be absorbed. This absorption intensity will be measured 4
5. 5. This absorption intensity will be proportional to the concentration of the atom in the ground state. Mathematically the total amount of light absorbed may be given by the following equation. At the lambda wavelength the total amount of light absorbed = ( e2 / mc).Nf Where, e= charge of electron M= mass of electron C= speed of light N=total number of atom that can absorb at specific wavelength. 5
6. 6. Flow chart of principle of atomic absorption spectroscopy Preparation of aqueous solution of sample Spraying of sample solution(Nebulization) Evaporation of solvent (Desolvation) Formation of fine residues of metallic samples (Residue formation) Formation of neutral atom (Atomization) Neutral atom absorb specific wavelength of radiation from hallow cathode lamp( Absoption & Excitation) Intensity of radiation absorbed is measured by photometric detector. Which is proportional to the concentration of sample 6
7. 7. : THE AAS INSTRUMENT 7
8. 8. 8
9. 9. THE SIMPLE DIAGRAM FOR THE AAS 1. We set the instrument at certain wavelength suitable for a certain element 2. The element in the sample will be atomized by heat 3. A beam of UV light will be focused on the sample 5. The monochromator isolates the line of interest 4. The element in the sample will absorb some of the light, thus reducing its intensity 6. The detector measures the change in intensity 7. A computer data system converts the change in intensity into an absorbance 9
10. 10. 10 Difference between AAS & FES FES AAS Measurement of emitted radiation forms the basis of FES. Measurement of intensity of absorbed radiation is basis of AAS. Intensity of emitted radiation is directly proportional to the number of atoms in excited state. Intensity of absorbed radiation is directly proportional to the number of atoms in ground state. Here excitation process and signal response is influenced by flame temperature. Here absorption intensity and signal response is independent to temperature. Relationship between emission intensity Vs concentration in not that much linear. Absorption intensity Vs concentration is very much linear.
11. 11. 11 Advantages of AAS  Highly specific in nature – atom of a particular element can only absorb radiation of their own characteristic wavelength other elements cannot be interfered in the study. Ex: light of a particular wavelength can easily be absorbed by specific element to which it is characteristic.  Variation in flame temperature shows relatively less effect in AAS than FES.  High sensitive.
12. 12. 12 Disadvantages of AAS  Need of separate lamp for each element to be determined is main limitation of AAS.  This technique cannot be used very successfully for the elements which produce oxides in the flame. Ex: Al, Ti, W, Mo, Si.  In aqueous solution, the predominant anion effect interfere the signal to a significant level.  AAS is applicable to analysis of metals only.  The one more major difficulty encountered with AAS is the presence of incompletely absorbed background emission from the source and scattered light from the optical system. As background becomes more intense relative to the absorption of the analyte, the precision of the measurement decreases dramatically..
13. 13. HOLLOW CATHODE LAMP (HCL) Cathode -- in the form of a cylinder, made of the element being studied in the flame Anode -- tungsten 13
14. 14. 1. A large voltage across the anode and cathode will cause the inert gas to ionize. 2. The inert gas ions will then be accelerated into the cathode, sputtering off atoms from the cathode. 3. Both the inert gas and the sputtered cathode atoms will in turn be excited by collisions with each other. 4. When these excited atoms decay to lower energy levels they emit a few spectral lines characteristic of the element of interest. 5. The light is emitted directionally through the lamp's window, a window made of a glass transparent in the UV and visible wavelengths. 6. The light can then be detected and a spectrum can be determined. 14
15. 15. Burners: 1. Mecker burner, 2. Total consumption burner, 3. Laminar flow burner. Total consumption burner 15
16. 16. Laminar flow burner Chopper: It is a rotating wheel lies between HCL and Atomizer. It is used to give Pulsating light. On rotation, it breaks the steady light from the lamp into an intermittent or pulsating light 16
17. 17. Nebulizer:
18. 18. 18 Common fuels and oxidants used in flame spectroscopy
19. 19.  The technique 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 ºC. Flame Atomic Absorption Spectroscopy: 19
20. 20. The process of lighting the AAS flame involves: turning on first the fuel then the oxidant and then lighting the flame with the instrument's auto ignition system. The flame breaks down the analyte's matrix  create the elemental form of the analyte atom. Ignition: 20
21. 21. 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. 21
22. 22. Atomizer Flame Graphite furnace 22 Flame atomizer
23. 23. 23 Graphite furnace technique process drying ashing atomization
24. 24. 24 Graphite furnace technique Advantages Small sample sizes ( as low as 0.5 uL) Very little or no sample preparation is needed Sensitivity is enhanced ( 10 -10 –10-13 g , 100- 1000 folds) Direct analysis of solid samples
25. 25. 25 Graphite furnace technique Disadvantages Background absorption effects Analyte may be lost at the ashing stage The sample may not be completely atomized The precision was poor than the flame method (5%-10% vs 1%) The analytical range is relatively narrow (less than two orders of magnitude)
26. 26. MONOCHROMATOR The light passes from the HCL through the element in the sample to the monochromator. It’s function is: It isolates the specific light of the element of interest from the other background lights and transfers it to the photomultiplier tube (detector). 26
27. 27. PHOTOMULTIPLIER TUBE (PMT) Before an analyte is aspirated, a measured signal is generated by the PMT as light from the HCL passes through the flame. When analyte atoms are present in the flame--while the sample is aspirated--some of that light is absorbed by those atoms. This causes a decrease in PMT signal that is proportional to the amount of analyte . PMT 27
28. 28. The PMT detects the amount of reduction of the light intensity due to absorption by the analyte, and this can be directly related to the amount of the element in the sample. The PMT converts the light signal into an electrical signal and a computer system translates it into absorbance. 28
29. 29. PhotomultiplierTube e- Light Dynode Dynode Dynode Photocathode Current Convert light energy to electrical energy 29
30. 30. Application:  Estimation of trace elements in biological fluids (Eg: Urine, blood etc…)  Estimation of elements like copper, Nickel and Zinc in food product.  Estimation of Magnesium, Zin etc in blood.  Estimation of Mercury in thiomersol solution.  Estimation of Lead in calcium carbonate, petrol etc.  Estimation of elements in soil samples, water supply, effluents, ceramics etc. 30
31. 31. Applications: 31 1. Qualitative Analysis: • Different HCL lamp is been used for each element to be tested. • As qualitative analysis involves the checking of one element at a time.It means that process is very laborious. 2. Quantitative Analysis: • This technique is based on the determination of the amount of radiation absorbed by the sample. • If the value of radiation absorbed is substituted in equation,the number of absorbing atoms in light path is been determined. Total amount of light absorbed= e2/mc.Nf Calibration Curve: The first job in quantitative analysis is preparation of calibration curve. • In order to prepare this curve, the read out device should be adjusted to 100% transmission with blank and 0% transmittance when no radiation energy is entering the monochromator slit. • A series of standard samples of that element which is to be determined quantitatively is aspirated into the burner and the percentage of absorption is measured. Absorbance(A)=Slope(m) x Concentration(c)
32. 32. 3. Simultaneous multi component analysis:  If a multi element emission source is available, one can do simultaneous multi-component analysis.  Previously, such determinations are not been made due to lack of such a multi-element hallow cathode.  The Mitchell(1973) described a multi element atomic absorption using a multi-element hallow cathode source and vidiocon detection system.  Using spectral region from 2320 to 3281A0.  Mitchell detect 8 elements(zn,cd,Ni,Co,Fe,Mu,Cu,Ag) simultaneously. 32
33. 33. 4. Determination of metallic elements in Biological material:  By this procedure, we extract the trace elements and go to estimation using 50% hydrogen peroxide.  It also has been used in combination with concentrated sulphuric acid and nitric acid. 5. Determination of metallic elements in Food Industry:  Copper,Zinc,Nickel are the most common toxic elements of interest to food analyst for solid food stuffs.  The most common procedure is to extract the trace metals by digestion with the dilute sulphuric acid or with nitric acid or with 50% hydrogen peroxide. 6. Determination of calcium,mg,Na,K in blood serum:  Dilute the sample serum 10,0 or 50 times in the presence of lanthanium chloride which overcome the possible under estimation of calcium due to phosphate suppression.  Then, the test solution are aspirated to the atomic absorption spectrum and the absorbance measured and compared with aqueous standard solution. 33