6. Atomization Method
Some atomization methods are described below.
- Flame method ... Atomizes with flame.
- Electric heating furnace method ... Atomizes with current.
- Hydride generation method ... Application of the frame method (uses a chemical reaction to vaporize the specific element)
- Reduction-vaporization method ... Atomizes using an element's easy-to-evaporate nature.
1. Flame Method
Function of burner
A specimen is introduced into the flame to atomize the elements contained in it.
Note that it is necessary to nebulize the specimen.
The flame will be extinguished without nebulization.
Nebulizer: Creates a fine mist of (nebulizes) the specimen.
Disperser: Supplies small particles of the nebulized specimen to the flame.
The whole is made of glass.
Burner head: Supplies a stable flame to atomize the elements in the specimen. In order to secure an optical path length, it has a structure to generate a long flame. Since it looks like the tail of a fish, it is called a fishtail burner.
There are two kinds of burner heads: a standard burner, and a high temperature burner.
The following table shows a comparison between the two.
|Standard burner||High temperature burner|
|Gas system||Air-acetylene: 2000°C
Argon hydrogen: 1600°C
|Dinitrogen oxide-acetylene: 2900°C|
|Tip size||0.5 mm x 100 mm||0.4 mm x 50 mm|
|Measured elements||Pb Cd Fe Cu Mn Cr Au K Ag Zn Na Ca Mg etc.||Al B Ba Be Ge Si Ti V W etc.|
Why a high temperature burner is necessary...
Some elements are not atomized at the temperature of a standard burner. In order to obtain a high-temperature flame, Dinitrogen oxide (nitrous oxide) - acetylene gas is necessary. In order to supply a stable flame, the following relationship must be satisfied;
[Gas exit velocity] > [Combustion wave velocity of mixed gas]
If this relationship fails, the flame may escape into the inside (backfire).
As for the gas system, since the combustion wave velocity of the dinitrogen oxide-acetylene combustion system is high, backfire will be generated with a standard burner head. So, a high temperature burner head with a narrow tip, increasing the gas exit velocity, is used.
2. Electric Heating Furnace Method
A large current is applied to a carbon tube to generate Joule heat for atomizing the elements in a specimen.
Energization and measurement are carried out in an inert-gas atmosphere of argon (Ar), etc in order to protect the carbon tube.
A high-capacity power supply is necessary (Single-phase 200V/30A).
Upon getting hot, the polar zone is water cooled.
Advantages compared with the flame method
- High sensitivity
- Requires no pretreatment of the specimen
- Capable of measuring a small amount of the specimen
Disadvantage compared with the flame method
- Long measurement time
- Poor reproducibility because of intermittent measurement.
3. Hydride Generation Method
High sensitivity analysis (at a level of several ppb) of arsenic (As), selenium (Se), and antimony (Sb) is possible.
Generate gas hydrides by reaction of hydrogen and metallic elements.
Introduce the generated gas into an atomization section. Atomize by heat decomposition.
The hydrogen used at this time causes an acid (hydrochloric acid etc.) and a reductant (sodium borohydride) to react. The generated hydride gas is carried to the atomization section using argon gas.
4. Reduction-vaporization Method
This method applies only to the measurement of mercury (Hg), which can be in the atomic state at room temperature.
When a specimen is added to the reaction chamber and a reagent (sulfuric acid and stannous chloride (6N HCl solution) is added, the mercury will evaporate and exit from the container (being reduced and exiting). The vaporized mercury is led to a mercury cell with an air pump to be measured (not requiring thermal decomposition). Commonly known as the flameless method.