ATOMIC ABSORPTION SPECTROSCOPY (AAS)

Atomization: It is the conversion of molecules to their component atoms in gaseous state using a source of heat (flame).Atoms in vapor state absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The intensity of the absorbed light is proportional to the concentration of the element in the flame.In AAS, atoms in the vapor state are subjected to external source of radiation which produces one line or beam of monochromatic light with single wavelength. This wavelength is the resonance wavelength that will be absorbed by the atoms.

The absorbance takes place following Beer’s lawA = log I0 / I = 0.434 k C b where K is a coefficient that depends on the

nature of atoms and the radiation. C is the concentration of atoms which depends

on the sample concentration. b is the light path length which depends on the

dimensions of the flame (standard and constant for each instruments).

INSTRUMENT FOR ATOMIC ABSORPTION

• Atomic absorption spectrometers (AAS) consist of 5 main parts:

 1- A light source (usually a hollow cathode lamp)2- Chopper2– An atom cell (atomizer)3- A monochromator 4- A detector5- A read out device.

1-Source of radiation (Hollow cathode lamp):

Analysis of each element requires the use of its own unique lamp.

2-The chopper

In front of the lamp there is the light chopper which is a fan-shaped object.

Its function is to fluctuate the source output.

3- Atomizer:There are two types of atomizers a- Flame atomizer: It serves two functions - Atomization of the sample.- Source of thermal energy to excite the atoms.b- Flameless atomizer: a graphite furnace heated electrically up to

6000oC . Upon heating the furnace, sample is dried,

ashen, and then vaporized by the action of heat.

Advantages of non flame atomizer: Solutions, slurries and solid samples can be analysed. High sensitivity. Smaller quantities of sample (typically 5 – 50 μL). Heat distribution is uniform and temperature is

steady. No flame noise.

 Disadvantages Expensive. Low precision. Requires high level of operator skill.

4- Detector: Photomultiplier.

 5- Readout meter:Absorbance or transmission output.

Quantitative Application Calibration curve method: 1- Choose a suitable lamp.2- Prepare a series of standard solutions of the metal

to be determined, in the solvent to be used in the sample. The concentration range has to be in ppm.

3-Adjust zero absorbance while spraying a blank sample (solvent).

4- Measure the absorbance while spraying each of the standard solutions.

5- Plot a graph representing the concentration vs absorbance.

6- Spray the sample and measure the absorbance and determine the concentration from the graph.

Standard addition method:1-Prepare a standard solution of exact and known

concentration e.g. 100 ppm.2- Prepare a spiked sample by addition of a volume of the

standard solution to the sample solution (unknown concentration). For example: add a volume of the standard equivalent to 5 ppm of the standard.

3- Measure the absorbance while spraying the sample A1 and the spiked sample solution A2.

  A1 = K CX

A2 = K (CX + CS)

where Cx is the sample concentration and CS is the concentration of standard added (spiked sample).

  A1 CS

CX = __________

(A2 - A1)

Applications for atomic absorption spectrophotometry:

Clinical analysis: blood samples: whole blood, plasma and

serum; Ca, Mg, Li, Na, K, Fe.

Environmental analysis: finding out the levels of various

elements in rivers, seawater, soil, air and petrol.

Water analysis: (e.g. Ca, Mg, Fe, Si, Al, Ba content).

Food analysis and animal feed (e.g. Mn, Fe, Cu, Cr, Zn).

Analysis of additives in lubricating oils and greases (Ba, Ca, Na,

Li, Zn, Mg).

Pharmaceuticals: In some pharmaceutical manufacturing

processes, minute quantities of a catalyst used in the process

(usually a metal) are sometimes present in the final product. By

using AAS the amount of catalyst present can be determined.