Atomic Absorption Spectroscopy lecture.pptx

Atomic absorption spectroscopy
Basic principles and working methodology

1. Atomic Absorption Spectroscopy

2. A life-saving technique Canada: AAS was used to determine unsafe levels of lead in children who lived nearby a lead smelter. Japan: From 1932 to 1968, AAS was used to identify the reason why over 3,000 residents who lives near the Minimata Bay started showing neurogical problems and pregnant women starts giving birth to impaired children. Scientist starts taking samples and performing AAS process; AAS results shows a very high concentration of mercury in their blood. This result on stopping the company, Chisso corporation who dumped approximately 27 tones of mercury in the bay.

3. Discovery of AAS 1952, the Australian scientist Alan Walsh was working on the measurement of small concentrations of metals at the CSIRO using atomic emission spectroscopy. The idea of AAS came into his mind as he was gardening at his Melbourne home. On the normal Sunday morning he had the idea about looking at the light absorbed by the atoms except than looking at the light they emits . Alan Walsh did not just discover a process that has the ability to save lives but also proven that atoms will only absorb light that has the EXACT value requires to promote their electron to a higher level.

4. Elements detectable by atomic absorption are highlighted in pink in this periodic table                                                                                                Most of these are METALS

5. ‘Excited’ atom Generally, atoms are in their ‘ground state’ but when an atom receives enough input of energy that their electrons requires to be promoted to a higher energy level. They will then turn to their ‘excited state’. Ground state: sodium atom Excited state: sodium atom Since, an atom’s excited state is very unstable it rapidly ‘jump’ back down to its ground state. This ‘jump’ then causes the atom to release the energy it absorbed in the form of photons of light. !Note: Take note that the electron can go back down to its ‘original’ place in more than one jump.

6. Atomic Absorption Spectroscopy Studies of the interaction of radiations with matter in atomic state The specific photons are absorbed by the atoms and the absorbance is measured The source is specially designed one to give Single λ The absorbance is proportional to the number of atoms in ground state (N ) 6

7. The ratio of population in ground and excited states g e and g are the statistical weights of the excited and ground states, E is energy, k is Boltzmann constant (1.3805 x 10 -16 erg K -1 ), T is absolute Temperature The population is ALWAYS greater in ground state 7

8. The Elements having less excitation potential is measured by AAS The basic reason is the preparation of the SOURCE for AAS The wavelength of the radiation being absorbed and emitted by source is EXACTLY same Beer’s law is followed Absorbance is directly proportional to path length in flame and conc of atomic vapors in flame Path length is kept constant Abs is proportional to the conc of anlayte in solution 8

9. The Atomic Absorption Spectrometer Atomic absorption spectrometers have 4 principal components 1 - A light source ( usually a hollow cathode lamp ) 2 – An atom cell ( atomizer ) 3 - A monochromator 4 - A detector , and read out device .

10. Light source (hollow cathode Lamp ) atomizer monochromator Detector and readout device

11. Atomic Absorption Spectrophotometer

12. Source Sharp, narrow absorption line width, high intensity, exact wavelength Usually a Hollow Cathode Lamp (HCL) is used It contains a tungsten anode and a hollow cylindrical cathode made of the element to be determined. These are sealed in a glass tube filled with an inert gas (neon or argon). With a quartz window (output is often in UV region) Each element has its own unique lamp which must be used for that analysis .

13. Hollow Cathode Lamp Quartz window Pyrex body Anode Cathode

14. How it works Potential Difference causes the gas atoms to ionize Cations move towards cathode and attack its surface SPUTTERING causes dislodgment of some of atoms Collision causes these atoms to excite De-excitation results in the emission of specific wavelength Since atoms of different elements absorb characteristic wavelengths of light. Analyzing a sample to see if it contains a particular element means using light from that element.

15. Photons are absorbed by the atoms The most strongly absorbed is related with the most probable transition – (Resonance Line) Lines are specific, so only specific atoms absorb Qualitative as well as quantitative Sometimes an Alloy of several atoms are used as cathode (Multi-element HCL) Lifetime is shorted than Single element

16. Atomizer or Burners Elements to be analyzed needs to be in atomic sate Atomization is conversion of matter into atomic state This is done by exposing the analyte to high temperatures in a flame or graphite furnace . The role of the atom cell (atomizer) is to primarily de-solvate a liquid sample, vaporize the solid into free gaseous ground state form. Atoms will be available to absorb radiation (from HCL) generating a measurable signal proportional to concentration . There are two types of atomization : Flame and Graphite furnace atomization .

17. Flame Sources Flame AA can only analyze solutions , where it uses a slot type burner to increase the path length and the total absorbance . Sample solutions are usually introduced into a nebulizer by venturi effect . In the nebulizer, the sample is dispersed into tiny droplets (aerosols), readily broken down in the flame. Used in all Atomic Spectroscopic techniques Converts analyte into free atoms in the form of vapor phase free atoms Heat is required Routes for sample introduction

18. Various flame atomization techniques

19. Types of Flames Used in Atomic Spectroscopy

20. Processes that take place in flame

21. Burners Premix Chamber Burner 22 Fuel in Oxidant in Burner Flame Aspirator air Mixing baffles Drain Sample

22. Total Combustion Burner 23 Fuel Oxidant Sample Oxidant Fuel

23. Monochromator The wavelength selector Filters (not fit for AAS) Prisms (less accurate, economic) Work on the Dispersion of Light Gratings (accurate, costly) Work on the principle of Diffraction 24

24. Prism Monochromator Dispersion of light Shorter the wavelength, greater the dispersion Slits are there to pass a particular wavelength 25

25. Gratings Diffraction of Radiations Single wavelength selection Accuracy Cost In AAS, Gratings are used To separate the transmitted radiations from the rest 26

26. Detectors The determination of the intensity of the absorption Convert the light signals into electrical signals Transducers Sensitive, rapid response, smaller dead times, good resolution, minimum interferences, less cost, accurate 27

27. Photomultiplier tubes (PMT) A phototube with amplifier inside A photoemissive material (cathode) emits electron when photons strike the surface (photoelectric effect) Dynodes accelerate electrons and emit 2° electrons If the above process is repeated several times, so more than 10 6 electrons are finally collected for each photon striking the first cathode. 28 photochathode high voltage dynodes light

28. Read Out Systems A display Usually computer attached 29

29. Calibration Curve A calibration curve is used to determine the unknown concentration of an element in a solution. The instrument is calibrated using several solutions of known concentrations. The absorbance of each known solution is measured and then a calibration curve of concentration vs absorbance is plotted. The sample solution is fed into the instrument, and the absorbance of the element in this solution is measured .The unknown concentration of the element is then calculated from the calibration curve

30. Calibration Curve A 1.0 - b 0.9 - S 0.8 - . o 0.7 - . r 0.6 - . b 0.5 - . . a 0.4 - . n 0.3 - . c 0.2 - e 0.1 - 10 20 30 40 50 60 70 80 90 100 Concentration ( g/ml )

31. Determining concentration from Calibration Curve A 1.0 - absorbance measured b 0.9 - S 0.8 - . o 0.7 - . r 0.6 - . b 0.5 - . . a 0.4 - . n 0.3 - . concentration calculated c 0.2 - e 0.1 - 10 20 30 40 50 60 70 80 90 100 Concentration ( mg/l )

32. Next is Interferences Graphite Furnace as a Non-Flame Atomization Source 33