Atomic absorption spectroscopy

detail chapter of AAS

1. Atomic absorption spectroscopy By Saniya Shaikh

2. Atomic absorption spectroscopy ( AAS ) is a spectroanalytical procedure for the quantitative determination of chemical elements using the absorption of optical radiation (light) by free atoms in the gaseous state. In analytical chemistry the technique is used for determining the concentration of a particular element (the analyte) in a sample to be analyzed. AAS can be used to determine over 70 different elements in solution or directly in solid samples

3. When a solution containing metallic species in introduced into a flame, the vp of the metallic species is obtained. Energy Aq sample molecules atoms Excited atoms* Desirable in AAS Desirable in AES

4. When light of a particular wavelength is allowed to pass through the flame (having atoms of sample), Part of the light will be absorbed . The absorption will be proportional to the density of the atoms ion the flame. In AAS the amount of light absorbed in determined

5. At , υ the total amount of light absorbed = ( π e 2 /mc)* Nf Where, e = charge on electron. m = mass of electron. c = speed of light. N = total number of atoms that can absorb frequency ν in the light path. f = the oscillator strength or ability for each atom to absorb at frequency ν. As ,e, m, c are constant equation becomes Total amount of light absorbed = constant * N * f

6. Sodium Na 11

7. Grotrian diagram AAS for sodium

8. Absorption of a photon is accompanied by the excitation of an electron from a lower-energy atomic orbital to an orbital of higher energy. Not all possible transitions between atomic orbitals are allowed. For sodium the only allowed transitions are those in which there is a change of ±1 in the orbital quantum number ( l ); thus transitions from s → p orbitals are allowed, and transitions from s → d orbitals are forbidden.

9. The spectrum consists of a small number of discrete absorption lines corresponding to transitions between the ground state (the 3 s atomic orbital) and the 3 p and 4 p atomic orbitals . Absorption from excited states, such as the 3 p → 4 s and the 3 p → 3 d transitions included in Figure are too weak to detect. Because an excited state's lifetime is short Features of AAS spectra

10. Instrumentation

11. Atomic absorption spectroscopy is based on the same principle as the flame test used in qualitative analysis.

12. Light Source Hollow-cathode lamp (HCL): The cathode contains the element that is is to be analysed .

13. Hollow cathode sourse lamp for Na Hollow-cathode lamps are a type of discharge lamp that produce narrow emission from atomic species. They get their name from the cup-shaped cathode, which is made from the element of interest. The electric discharge ionizes rare gas atoms, which are accelerated into the cathode and sputter metal atoms into the gas phase. Collisions with gas atoms or electrons excite the metal atoms to higher energy levels, which decay to lower levels by emitting light. Windows -- Two window materials, quartz and borosilicate glass , are available on standard types. Special window materials have been developed, such as MgF 2 , and LiF . Gas Fills -- Two gas fills, argon and neon, are available on standard types. In general, neon will give the highest spectral output and lamps containing it are recommended

14. Electrodeless discharge lamp The internal electrodeless lamp or induction light is a gas discharge lamp in which the power required to generate light is transferred from outside the lamp envelope to the gas inside via an electric or magnetic field , in contrast with a typical gas discharge lamp that uses internal electrodes connected to the power supply by conductors that pass through the lamp envelope. There are two advantages to elimination of the internal electrodes: Extended lamp life higher efficiency Used for volatile elements such as As , Cd , Pb

15. In case of emission measurements, there will always be a positive error since emission from flame is an additive value to the actual sample emission. It is therefore obvious that we should get rid of this interference from emission in flames. 2. Chopper (source modulation correction ) Excluding the emission signal from flames can easily be done by an addition of a chopper to the instrumental design. The chopper is a motor driven device that has open and solid (mirrors in some cases)

16. Light from the source is modulated electronically or chopped mechanically by rotating chopper. This helps isolate and remove sample cell emissions from light emitted by the source. The specific wavelength isolated by monochromator is led to the detector and the electrical signal generated is proportional to the elemental concentration in the sample Single beam AAS

17. Light beam from source is split into two beams by the chopper. One beam passes directly through the flame and the other beam passes round the flame. Detector response represents the ratio of sample and reference beams. Fluctuations in light intensity are eliminated electronically to get greater reliability of results. Double beam AAS

18. Sample introduction: Atomizers1) FLAME ATOMIZEER 2) NON-FLAME ATOMIZER

19. Burner system The burner assembly comprises of nebulizer to reduce the liquid sample to a fine aerosol, a spray chamber and a burner head which is used to generate a flame to produce atoms of the same elements that are present in the sample. TOTAL CONSUMPTION BURNER PREMIXED BURNER Burner system TOTAL CONSUMPTION BURNER

20. PREMIXED BURNER

21. The sample is nebulized and mixed with the fuel and oxidant prior to introduction into the flame, with the use of a series of baffles. the sample is drawn from the sample container via the vacuum created by the rushing fuel and oxidant (aspiration). A drain line is required in this design in order to remove sample solution droplets that do not make it all the way to the flame. The fuel, oxidant and sample all meet at the base of the flame and is premixed prior to entering the flame.  Pre-mixed burner

22. Graphite furnace atomizer With the graphite atomizer, a discreet volume of sample solution is vaporized and wastage is virtually eliminated.