Atomic Absorption Spectrophotometry - AAS
Before we understand the instrument CVASS which is widely used to detect mercury, we must first understand AAS. This is because CVAAS originated from the principle of AAS.Component of AAS
• Hollow cathode lamp : A light source which emits a stable and intense light of a particular wave length which will be absorb by the sample (253.7nm for mercury lamp)
• A burning unit
• Monochromator : A prism to disperse and isolate emission line
• A photodetector which is fixed together with an appropriate amplifier
Basic working principle of AAS
Each element can readily absorb a specific wavelength. Therefore the decrease in intensity of the light source can be use to determine the concentration of the element present in the sample. The graph of the absorbance against the concentration (standard graph) could be plot out with standard solution, thus the reading of the machine can be converted to a result.
Standard graph ( Meant for calibration)
Calibration is done by using a series of standards solution of known concentration in the same manner as the samples. A straight line would be obtained when the absorbance is plotted against concentration in standard concentration
Steps of AAS:
1.The intensity of light is first measure without the sample
2.Pre-treated sample containing mercury is being absorb into the machine
3.It is mix with a flammable gas (acetylene) and oxidising air (oxygen) in the mixing chamber
4.The mixed gas is then transported to the burner so that it could be excited and reduce to its elements status
5.Some of the light is absorb by the mercury while the rest proceed on to the photodetector
6.The reading are thus compared with standard graph to obtain the concentration of mercury