Flame photometer (Atomic Emission Spectroscopy) Flame emission spectroscopy

Principle, Instrumentation, Types of nebulizers, flames and burners, application,

1. FLAME EMISSION SPECTROSCOPY/ FLAM PHOTOMETRY M Asif Shaheen 1

2. Introduction Flame photometry more accurately called Flame Atomic Emission Spectrometry A flame photometer is an instrument used to determine the concentration of certain metal ions among them sodium, potassium, calcium and lithium. Flame Photometry is based on measurement of intensity of the light emitted when a metal is introduced into flame . 2

3. Principle When a solution of metallic salt is sprayed as fine droplets into a flame. Due to heat of the flame, the droplets dry leaving a fine residue of salt. This fine residue converts into neutral atoms. Due to the thermal energy of the flame, the atoms get excited and there after return to ground state. In this process of return to ground state, exited atoms emit radiation of specific wavelength. This wavelength of radiation emitted is specific for every element 3

4. – The wavelength of colour tells what the element is (qualitative) – The colour's intensity tells us how much of the element present (quantitative) 4

5. 5

6. 7

7. 8

8. 9

9. 10

10. CONTENTS 11

11. NEBULIZER This is the component of sample delivery system. which breaks up the bigger liquid droplet to smaller liquid droplets. The process of conversion of sample to a fine mist of finely divided droplets using a jet of compressed gas is known as Nebulization. 12

12. Types of Nebulizers 13

13. 14

14. 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. The high velocity breaks the sample into a mist and carries it to the atomization region. 15

15. CROSS FLOW The jet stream flows right angles to the capillary tip. It uses a high speed stream of gas perpendicular to the tip of the sample capillary 16

16. 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 the sample solution flows freely over small aperture, rather than passing through a fine capillary 17

17. FRITTED or porous DISK The sample is pumped into a fritted disk through which the gas jet is flowing and this gives fine aerosol than others High efficiencies can be obtained by introducing the sample at predetermined location of the fritted surface 18

18. ELECTRO-THERMAL VAPORIZERS It is an electro thermal vaporizer contains an evaporator in a closed chamber through which an inert gas carries the vaporized sample into the atomizer 19

19. ULTRASONIC NEBULIZER The sample is pumped onto the surface of a vibrating piezoelectric crystal. The resulting mist is denser and more homogeneous than pneumatic nebulizers 20

20. ZONES OF FLAME 21

21. REQUIREMENTS OF FLAME It should have proper temperature Temperature should remain constant throughout the operation There should not be any fluctuation during burning FUNCTIONS OF FLAME To convert the analyte of the liquid sample into vapour state To decompose the analyte into atoms and simple molecules To excite the formed atoms/free atoms/simple molecules to emit radiant energy 22

22. BURNERS 23

23. MECKER BURNER This burner was used earlier and employed natural gas and oxygen. Produces relatively low temp. and low excitation energies. This are best used for ALKALI metals only. Now-a-days it is not used. 24

24. 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 the tip of burner. Entire sample is consumed. 25

25. Premix OR laminar flow burner In this type of the burner, aspirated sample, fuel and oxidant are thoroughly mixed before reaching the burner opening and then entering the flame. There is high loss of sample(95%) as large droplets are drained out. 26

26. LUNDERGRAPH BURNES In this sample and air is mixed in a chamber, this mixed composition is send to fuel nozzle where it is atomized. Here the sample reaches the flame is only about 5% 27

27. SHIELDED BURNERS In this flame was shielded from the ambient atmosphere by a stream of inert gas. Shielding is done to get better analytical sensitivity and quieter flame 28

28. Nitrous oxide-acetylene flame These flames were superior to other flames for effectively producing free atoms. The drawback of it is the 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 NITROUS OXIDE ACETYLENE FLAME 29

29. Fuel Oxidant Temperature C Natural gas Air 1700-1900 Natural gas Oxygen 2700-2800 Hydrogen Air 2000-2100 Hydrogen Oxygen 2550-2700 Acetylene Air 2100-2400 Acetylene Oxygen 3050-3150 Acetylene Nitrous oxide 2600-2800 30

30. 31 Flame Photometry Non Flame Atomizers For example: Heated Gravite Furnace Sample evaporation → time and temp. controlled drying and ashing Advantages small samples are analysed 2. 1000-fold more sensitive than flame 3. Oven is adaptable to determination of solid samples Disadvantages Low accuracy 2. Low precision More ionic interferences due to very high temp.

31. MIRRORS The radiation from the flame is emitted in all the directions in space. Much of the radiation is lost and loss of signal results. A mirror is located behind the burner to reflect the radiation back to the entrance slit of the monochromator. The reflecting surface of the mirror is front-faced. 32

32. SLITS The entrance and exit slits are used before and after the dispersion elements. The entrance slit cuts off most if radiation from the surroundings and allows only the radiation from the flame and the mirror reflection of flame to enter the optical system. The exit slit is placed after the monochromator and allows only the selected wavelength range to pass through the detector 33

33. Photocell 34 Measures the amount of light passing through the sample. Usually works by converting light signal into electrical signal The least expensive of the devices is known as a barrier-layer cell, or photocell. The photocell i s composed of a film of light-sensitive material, frequently selenium, on a plate of iron. Over the light-sensitive material is a thin, transparent layer of silver. When exposed to light, electrons in the light-sensitive material are excited and released to flow to the highly conductive silver in comparison with the silver, a moderate resistance opposes the electron flow toward the iron, forming a hypothetical barrier to flow in that direction. Consequently, this cell generates its own electromotive force, which can be measured. The produced current is proportional to incident radiation.

34. 35

35. Phototube The third major type of light detector is the photomultiplier (PM) tube, which detects and amplifies radiant energy. incident light strikes the coated cathode, emitting electrons. The electrons are attracted to a series of anodes, known as dynodes, each having a successively higher positive voltage These dynodes are of a material that gives off many secondary electrons when hit by single electrons. Initial electron emission at the cathode triggers a multiple cascade of electrons within the PM tube itself. Because of this amplification, the PM tube is 200 times more sensitive than the phototube 36

36. PM tubes are used in instruments designed to be extremely sensitive to very low light levels and light flashes of very short duration. The accumulation of electrons striking the anode produces a current signal, measured in amperes, that is proportional to the initial intensity of the light. The analog signal is converted first to a voltage and then to a digital signal through the use of an analog to- digital (A/D) converter. Digital signals are processed electronically to produce absorbance readings

37. Readout device. In the past nearly all spectrophotometer   used ammeters or galvanometers. Newer digital devices and printers have now replaced these, and many instruments relay their electrical output directly to computer circuits where calculations are performed, allowing direct reporting of sample concentration. Microprocessor and recorders

38. 39

39. Limitations Limited number of elements that can be analyzed. The sample requires to be introduced as solution into fine droplets. Many metallic salts, soil, plant and other compounds are insoluble in common solvents. Hence, they can’t be analyzed by this method. Since sample is volatilized, if small amount of sample is present, it is tough to analyze by this method. As some of it gets wasted by vaporization. Further during solubilisation with solvents, other impurities might mix up with sample and may lead to errors in the spectra observed. 40

40. 41

41. REFERENCES Chatwal & Anand; Instrumental M ethods of Chemical Analysis, 5/e 2013, page no- 2.370 to 2.375, Himalaya Publishing House. B.K Sharma; Instrumental Methods of Chemical Analysis, 26/e 2007, page no- 430 to 437, GOEL P ublishing House. 42

42. 43