Atomic emission spectroscopy PPT

Atomic emission spectroscopy PPT

1. Introduction to principle and instrumentation of atomic emission spectroscopy BY Humna Mehmood BS Chemistry (2017-2021) 1

2. Introduction to atomic emission spectroscopy : Atomic spectroscopy is thought to be the oldest instrumental method for the determination of elements . Atomic emission spectroscopy uses quantitative measurement of the optical  emission  from excited atoms to determine analyte composition. It is used to identify elements and determine their concentration in analyte. 2

3. Principle : The working principle involves the examination of the wavelengths of photons discharged by atoms and molecules as they transit from a high energy state to a low energy state . 3

4. A characteristic set of wavelengths is emitted by each element or substance which depends on its electronic structure. The atomic spectral line wavelength identifies the element. The intensity of light emitted is proportional to the atom count of the element . The sample solution is nebulized and introduced into the excitation source (flame). Atoms are raised to excited states. Upon their return to a lower or ground electronic state, the excited atoms emit radiations. The emitted radiations passes through a wavelength selector that isolates the specific wavelength . A photo detector measures the radiant power of radiation, which is then amplified and sent to a readout device . 4

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6. Instrumentation : 6

7. Pretreatment of Sample Solid sample must be dissolved in solvent. Substances in sample that interfere with the emission measurement must be removed or masked Reagents used to dissolve samples must not contain substances that lead to interference problems. 7

8. Nebulizer This is a component of sample delivery system, which breaks up the bigger liquid droplet to smaller liquid droplets The process of conversion of sample to a fined mist of finely divided droplets using a jet of compressed gas is known as nebulization . Types of nebulizer Ultrasonic nebulizer Electro-thermal vaporizer Pneumatic nebulizer 8

9. Ultrasonic nebulizer The sample is pumped onto the surface of vibrating piezoelectric crystals. The resulting mist is denser and more homogeneous than pneumatic nebulizer. 9

10. Electro-thermal vaporizers It is an electro thermal vaporizer It contains an evaporator in a closed chamber through which an inert gas carries the vaporized sample into atomizer. 10

11. Pneumatic nebulizer It is of four types: Cross flow Concentric tubes Fitted or porous disc Babington 11

12. Cross flow The stream jet flows right angle to capillary tip. It uses a high speed stream of gas perpendicular to the tip of the sample capillary. 12

13. Concentric tubes The liquid sample is sucked through a capillary tube by a high pressure jet of gas flowing around the tip of the capillary tube. The high velocity breaks the sample into mist and carries it to atomization region. 13

14. Babington The jet is pumped through a small orifice in a sphere on which a thin film of sample flows In this type of nebulizer sample solution flows freely over small aperture rather than passing through a fine capillary tube 14

15. Fritted or porous disk The sample is pumped into a fritted disk through which the gas jet is flowing and this gives fine aerosol then others High efficiency can be obtained by introducing the sample at predetermined location of fritted surface. 15

16. Source of flame A Burner is the source of flame. It is maintained at a constant temperature. Temperature vary for different fuel-oxidant mixtures as shown table. Fuel + Oxidant mixture Temperature ( ) Natural gas + Air 1700 Propane + Air 1800 Hydrogen + Oxygen 2000 Hydrogen + Oxygen 2650 Acetylene + Air 2300 Acetylene + Oxygen 3200 Acetylene + Nitrous oxide 2700 Cyanogen + Oxygen 4800 Fuel + Oxidant mixture Natural gas + Air 1700 Propane + Air 1800 Hydrogen + Oxygen 2000 Hydrogen + Oxygen 2650 Acetylene + Air 2300 Acetylene + Oxygen 3200 Acetylene + Nitrous oxide 2700 Cyanogen + Oxygen 4800 16

17. Zones of flame 17

18. Processes occurring in the flame : Desolvation : The metal particles are dehydrated and the solvent is evaporated. Vaporization : This also led to the evaporation of the solvent. Atomization : Reduction of metal ions in the solvent to metal atoms by the flame heat. Excitation : The atoms jump to the exited energy state. Emission process : A toms jump back to the stable low energy state with the emission of energy in the form of radiation of characteristic wavelength. 18

19. The emitted radiations generally lie in the visible region of the spectrum so flame shows color. Each of the alkali and alkaline earth metals has a specific wavelength . Element Emitted wavelength Flame color Sodium 589 nm Yellow Potassium 766 nm Violet Barium 554 nm Lime green Calcium 622 nm Orange Lithium 670 nm Red 19

20. Types of burner 20

21. Mecker burner This burner was used earlier and employed natural gas and oxygen. Produces relatively low temperature and low excitation energies. This is best for alkali metals only. Now a days it is not used. 21

22. Total consumption burner In this burner fuel and oxidant are hydrogen and oxygen gases. Sample solution is aspirated through a capillary by high pressure of fuel and oxidant and burnt at tip of burner. Entire sample is consumed. 22

23. Premix or laminar burner In this type of burner aspirated sample fuel and oxidant is mixed thoroughly before reaching the burner opening and then entering in the flame. There is high loss of sample (95%) as large droplets are drained out . 23

24. Shielded burner This flame is shielded from ambient atmosphere by a stream of inert gas. Shielding is done to get better analytical sensitivity and quieter flame. 24

25. Nitrous oxide -acetylene flame This flame is superior to other flames for effectively producing free atoms. The drawback of it is its high temperature reduces its usefulness for the determination of alkali metals as they are easily ionized and intense background emission, which makes the measurement of metal emission very difficult. 25

26. Lundergraph burner In this burner sample and air is mixed in a chamber, this mixed composition is send to fuel nozzle where it is atomized . Here sample reaches flame only about 5%. 26

27. Mirror, Lens and Wavelength selector Mirror The radiation from flame are emitted in all the directions in the space. Mirror is located behind the burner to reflect radiations back to slit of monochromator. The reflecting surface of mirror is front faced . Lens: It helps to focus the light on a point or slit. Wavelength selector:   The radiations from the mirror pass through the slit and reach the filters. Filters will isolate the light of different wavelengths. 27

28. Photodetector, Amplifier and Readout device Photo-detector:   The intensity of radiation emitted by the flame is measured by photo detector. Here the emitted radiation is converted to an electrical signal with the help of photo detector. These electrical signals are directly proportional to the intensity of light. Amplifier: It amplify electrical signals. Readout device: LCD display is used as readout device. 28