Water Pollution Continuous Monitoring Technology in Japan
Principles of Analyzers for Water Pollution Continuous Monitoring

1. Analyzers for Water Pollution Continuous Monitoring

1.1 Atomic Absorption Spectrophotometer

Atomic absorption spectrophotometry utilizes the phenomenon that atoms absorb radiation of particular wavelength. By atomic absorption spectrophotometer, the metals in water (organic) sample can be analyzed. The basic structure of the machine is shown in Fig.4.1.1. It consists of 4 basic structural elements; a light source(hollow cathode lamp), an atomizer section for atomizinig the sample (burner for flame, graphite furnace for electrothermal atomization), a monochrometer for selecting the analysis wavelength of the target element, and a detector for converting the light into an electrical signal .

The hollow cathode lamp for the light source consists of a hollow cathode and an anode enclosed in a glass (quartz) tube and neon or argon gas is filled at around 10 Torr. in pressure in it. The cathode is made of the element to be measured or its alloy, so it emits the light its wavelength is equal to that absorbed by the atoms of the sample.

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

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

Fig.4.1.1 Basic Structure of Atomic Absorption Spectrophotometer

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