SOIL AND PLANT ANALYSIS (1+1)

Functions

Flame atomic absorption spectroscopy (AAS) is the most used of atomic methods. The principle mentioned above can be applied to light absorption of "Free atoms". A "Free atom" means an atom not combined with other atoms. However, elements in the sample to be analyzed are not in the Free State, and are combined with other elements invariably to make a so-called molecule. The combination must be cut off by some means to free the atoms. This is called atomization. The most popular method of atomization is dissociation by heat - samples are heated to a high temperature so that molecules are converted into free atoms. This method is classified into the flame method, in which a chemical flame is used as the heat source; and an electrothermal atomization method, in which a very small electric furnace is used. The comparison of the flame atomization method and electrothermal atomization method is shown in the following table


Parameters	flame atomization	electrothermal atomization
Sensitivity	ppm level in the solution	ppb level in the solution
Sample Volume	about 1mL for one analysis	5 - 50 µL for one analysis
Atomizing efficiency	about 10%	More than 90%
Shape of signal	plateau shape	Peak shape
Repeatability	0.5 - 1.0% in R.S.D.	2.0 - 5.0% in R.S.D.
Matrix effect	Small	large
Time for analysis	10 - 30 sec. For one sample	2 - 5 min. for one sample

Working principle
Radiation from a line source is focused on the atomic vapor in a flame or an electrothermal atomizer. The attenuated source radiation then enters a monochromator, which isolates the line of interest. Next the radiant power from the source, attenuated by absorption, is measured by the photomultiplier tube (PMT). The signal is then processed and directed to a computer system for output.

For the light source, a hollow cathode lamp and a discharge lamp are mainly used. To the sample-atomizer, the flame type, electrothermal type, and the cold-vapor type are applied. The flame type is composed of a burner and a gas-flow regulator, the electrothermal type is composed of an electric furnace and a power source, and the cold-vapor type is composed of a mercury generator by chemical reduction-vaporization and thermal reduction-vaporization and an absorption cell.