Atomic Absorption Spectroscopy.pptx.....

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1. Spectroscopic Methods

2. Overview Electronic energy levels are widely spaced and spectral transitions between them are therefore observed towards the high energy end of electromagnetic spectrum , i.e. in the visible, ultraviolet and X-ray regions . The spectra are characterized by narrow lines and may be simple or complex depending on the number of excited states involved in the excitation process.

3. The wavelengths of the observed absorption and emission lines are characteristic of a particular element , and intensity of a given spectral line is proportional to the number of atoms undergoing the corresponding transition. Atomic Spectroscopic techniques thus provide means both for Qualitative identification of elements and for their Quantitative determination

4. Techniques Based on Atomic Spectroscopy/Spectrometry Arc/spark emission Plasma emission Atomic Absorption Atomic Fluorescence Flame Photometry (Flame atomic Emission) X-Ray Fluorescence

5. Atomic Structure and Spectra According to the Quantum Theory, the electrons of an atom occupy quantized energy levels or orbitals defined by a set of four Quantum numbers. The natural tendency of electrons, to occupy orbitals of the lowest possible energy

6. Atomic Absorption Spectroscopy

7. Introduction It was first introduced by a scientist Walsh in 1950. The atomic absorption spectrometer uses the principles of Quantum Chemistry to detect the presence of certain metals (Iron, Aluminium , Copper etc) and determines the concentrations of these metals in samples. It is one of the most powerful analytical tools for the Qualitative determination of trace metals in liquids.

8. Energy Transition All atoms and their components have energy. The energy level at which an atom exists is referred to as its state. Under normal conditions, atoms exist in their most stable states (Ground state). We cannot measure the precise energy state for an atom, so we measure changes to its energy relative to its ground states .

9. Principle When a solution of metallic salt is sprayed on to a flame, fine droplets are formed. Due to thermal energy of the flame, the solvents in the droplets evaporate, leaving behind the fine residue, which are converted to neutral atoms . These neutral atoms are converted to excited state atoms by the thermal energy of the flame As the excited state is not stable, these excited atoms return to ground state, with the emission of radiation of specific wavelength e.g. Sodium emits Y ellow Radiation (589nm).

10. The wavelength of radiation emitted is characteristic of the element and is used to identify the element (Qualitative analysis). The intensity of the radiation emitted depends upon the concentration of the element analysed (Quantitative analysis).

11. Components of Atomic Absorption Spectrophotometer Hollow Cathode Lamp Burner (with fuel and Oxidants) Monochromator (prisms, Gratings) Detector (Barrier Layer cell, Phototubes, Photo Multiplier Tube) Read out Device

12. 1- Hollow Cathode Lamp The lamp or source of light in AAS is a hollow cathode lamp. The cathode is made up of specific element or alloys of elements or coating of elements on cathode. When current is applied between anode and cathode, metal atoms emerge from hollow cup and colloid with filter gas, which is normally argon or neon. Due to these collisions, number of metal atoms are excited and emit their characteristic radiation.

13. This characreristic radiation is absorbed by neutral atoms of the same element in ground state, when sample solution is sprayed. The demerit in AAS is that, for determination of every element, separate hollow cathode lamp has to be used.

14. 2- Burner (with fuel and Oxidants) The temperature of flame is not critical but atomization of sample solution like in flame photometry is required. There are different burners available, which are used to spray the sample solution into fine droplets, mix the fuel and oxidant, so that homogeneous flame of stable intensity is obtained. e.g Mecker Burner, total consumption burner and Laminar flow burner

15. 3- Monochromator (prisms, Gratings) A monochromator which can provide good resolution of 1nm or less is required. Monochromators are better and more efficient than filters in converting a polychromatic light or heterochromatic light into a monochromatic light.

16. Monochromator units Enterance slit (to get narrow source) Grating or prism (To disperse radiation) Collimator (To reform the images of enterance slit) Exit slit (To fall on sample cell)

17. 4- Detector The radiations emitted by the elements is mostly in the visible region . Detectors used in UV/Visible spectrophotometers can be called as photometric detectors . When a radiation is passed through a sample cell, part of it is being absorbed by the sample solution and the rest is being transmitted. The transmitted radiation falls on the detector and the intensity of absorbed radiation can be displayed. The most commonly used detectors are, Barrier Layer cell, Phototube and Photo Multiplier Tube

18. Read out Device The read out device is capable of displaying the absorption spectrum as well as the absorbance at a specified wavelength (similar to UV spectroscopy). Beer,s Law is obeyed over a wide concentration range. Single beam & Double beam AAS

19. Applications This technique provides the total metal content of the sample and is almost independent of the molecular form of the metal in liquid. Its versatility is from the fact that 60-70 elements including most of the common rare earth metals have been determined by this technique in concentration ranging from trace to micro quantities.

20. The determination can be made in the presence of bivalent elements (so, it provides unnecessary separation of test elements from other elements in sample). This technique is not restricted to aqueous solutions , it can be applied to nonaqueous solution as well. This technique does not demand sample precipitate . Hence it is an ideal too for clinician who is interested only in the significance of results.

21. Estimation of trace elements in biological fluids (e.g. Blood, urine etc.) Estimation of elements like copper, nickel and zinc in food products Estimation of magnesium, zinc, etc. in blood Estimation of Zinc in Zinc insuline injection Estimation of lead in calcium carbonate, petrol etc. Estimation of elements in soil samples, water supply, effulents , ceramics etc.

22. Advantages of AAS The atomic absorption technique is specific because the atom of a particular elements can only absorb radiation of characteristic wavelength. AAS is independent of flame temperature

23. Disadvantages of AAS A separate lamp (Radiation source) for each element is required. This technique is unsuccessful for estimation of elements like Al, Mo and vanadium. https://www.youtube.com/watch?v=TB-Wt3p1UAU

24. Flame Photometry Flame photometry is a process wherein the emission of radiation by neutral atoms is measured. The neutral atoms are obtained by introduction of the sample into flame. Hence the name flame photometry . Since radiation is emitted, it is also called as flame emission spectroscopy .

25. Flame Photometry

26. https://www.youtube.com/watch?v=2LeUkiI1ow0

27. Fluorescence spectroscopy Fluorescence spectroscopy  (also known as  fluorimetry  or  spectrofluorometry ) It is a type of  electromagnetic spectroscopy   that analyzes  fluorescence  from a sample. It involves using a beam of light, usually  ultraviolet light , that excites the electrons in  molecules  of certain compounds and causes them to emit light. Devices that measure fluorescence are called  fluorometers

28. https://www.youtube.com/watch?v=sFVigyHyr_M https://www.youtube.com/watch?v=B22ZsUOFM98 https://www.youtube.com/watch?v=zGFQEbdeAF0 https://www.youtube.com/watch?v=awrN615hF8w