Atomic absorption spectroscopy

atomic absorption spectroscopy

1. IN 504 Analytical Instruments Module 3 1 Presented by; Anju Sunny CUSAT Reference Text: R S Khandpur “Handbook of Analytical Instrumentation”

2. ATOMIC ABSORPTION SPECTROMETRY (AAS) 2

3. Introduction UV/Vis, IR or Raman Spectrometry gives molecular functional group information, but no elemental information. Atomic Absorption Spectrometry provides information on elemental composition of sample or compound. It is based on the breakdown of a sample into atoms, followed by the measurement of the atom’s absorption of light.

4. Principle of AAS A much larger number of the gaseous metal atoms will normally remain in the ground state. These ground state atoms are capable of absorbing radiant energy of their own specific resonance wavelength. If light of the resonance wavelength is passed through a flame containing the atoms of the sample, then part of the light will be absorbed The extend of absorption will be proportional to the number of ground state atoms present in the flame.

5. Schematic diagram of an AAS

6. Schematic diagram of an AAS Detector Grating Monochromator

7. Instrumentation - Source of light in AAS 1) Hollow cathode lamp 2) Elecrodeless discharge lamp 3) Laser Hollow cathode lamp (HCL) Anode : tungsten Cathode : in the form of a cylinder, made of the element being studied in the flame.

8. Instrumentation - Source of light in AAS The source of light is a lamp whose cathode is composed of the element being measured. Each analyzed element requires a different lamp. The cathode lamps are stored in a compartment inside the AAS. The specific lamp needed for a given metal analysis is rotated into position for a specific experiment.

9. Instrumentation – Fuel & Atomizer The sample is prepared in aqueous solution. A flame is created, usually using ethyne & oxygen (fuel ) & the solution is subjected into the flame by nebulizer . The flame gases flowing into the burner create a suction that pulls the liquid into the small tube from the sample container. This liquid is transferred to the flame where the sample is atomized [mixing the sample with air to create fine droplets]. The metal atoms then absorb light from the source (cathode lamp). 9

10. Sample is vaporized in the flame. Aspirator tube sucks the sample into the flame in the sample compartment. Light beam Instrumentation – Fuel & Atomizer

11. Instrumentation – Monochromator , Detector, Read out The light passes through a Grating monochromater (a device used to select a particular wavelength of light for observation) The intensity of the light is fairly low, so a photomultiplier tube (PMT) is used to boost the signal intensity and the purpose of detection. The read out specified by the user is displayed on the computer screen for each sample measured. 11

12. Advantages High sensitivity Good accuracy High selectivity Widely used Disadvantage A resonance line source is required for each element to be determined 12