Atomic Absorption

Topics: Spectroscopy, Analytical chemistry, Photon Pages: 19 (5711 words) Published: April 10, 2011
INTRODUCTION: In atomic spectroscopy the samples are vaporized at 2000-8000k. The process of this involves two steps , one is atomization of sample and the other is absorption of light radiation from light source. The atoms in the vapour are measured by absorption or emission at characteristic wave lengths. The main usage of this was to detect one element from another in the complex sample which has taken ie., it has ability to perform multiple element analyses. Absorbance should obey Beer’s law ie., Analyte is measured in diluted concentrations from parts per million level to parts per trillion level and the monochromatic light radiation. There are types of spectroscopy according to the measurements we do they are. 1. Atomic absorption: Atoms from ground state to the excited state are measured. 2. Atomic emission: Atoms from excited state fall to the ground state are measured. 3. Atomic flourescence : atoms from excited electronic state fluoresce to ground state. In atomic spectroscopy the analyte is analyzed by using flame, graphite furnace and a plasma. In this experiment we are dealing with flame and plasma to measure absorption and emission. Atomic absorption spectroscopy: This is done by using Flames and Hollow cathode lamp. The method is to introduce the sample in to the flame as a fine spray by adjusting the flow of fuel and oxidant. The sample is vaporized in to atoms and these atoms absorb the light from the Hollow cathode lamp. The wavelengths are adjusted to particular elements which we analyze from the complex sample, so there is no interference with measurement and this gives us both sensitivity and selectivity of elements. To reduce the back ground interference the wavelength of interest is isolated by the monochromator placed between the sample and detector. Now let us go in to detail about Flames and Hollow cathode lamp how it works.

Flames: Most of the atomic absorption spectrometers use burner for the absorption to take place. The burner is designed as follows in which fuel, oxidant and sample are mixed before introduction in to the flame. Sample solution is drawn in to the nebulizer by the controlled flow of oxidant( commonly air) and then reaches the tip of the capillary. Liquid then breaks into fine mist and it leaves the capillary. The spray is then passed against glass bead which is located inside break down the droplets of the whole mist from capillary into smaller particles. This formation of smaller particles called as Nebulization. A fine suspension of liquid particles in a gas called as Aerosol. The Nebulizer which is present form aerosol from liquid sample. The mist, oxidant and fuel pass trough Baffles and helps in further mixing and block large droplets of liquid. Excess liquid collects at the bottom of the spary chamber and flow out to a drain. Aerosol reaching the flame contains only about 5% of the initial sample. The most common fuel-oxidizer combination is acetylene and air which creates the temperature of flame at about 2400-2700k. High boiling elements such as refractory elements are detected by high boiling temperature so hotter flame is required and acetylene-nitrous oxide combination of fuel-oxidant is used.

Hollow cathode Lamp: This is a type of discharge lamp that produce narrow emission from atomic species. They get their name from cup shaped cathode. The lamp is filled with Ne or Ar at a pressure of 130-700Pa. The cathode is made of the element which emission lines we want. When voltage is applied between anode and cathode, gas is ionized and positive ions are moved towards the cathode. After the ionization occurs the lamp is maintained at constant current by a lower voltage. Cations that strike the cathode with sufficient energy to sputter metal atoms from the cathode into gas phase. Collisions with gas atoms or electrons...

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