What Is a Spray Chamber?

The mist chamber is also called the atomization chamber or expansion chamber. It is one of the premixed atomizer components of the flame atomic absorption spectrophotometer. Its role is to uniformly mix the spray droplets emitted by the atomizer with the combustion gas and the auxiliary gas before entering the flame, and then enter the flame through diffusion and expansion. It is made of stainless steel and has cylindrical flying sphere, pear shape and cone shape. In order to improve the atomization efficiency, the inner cavity is provided with a liquid collision ball, a spoiler, etc., and the large mist particles can be crushed and removed. Some are heated by a red impact ball, a spoiler, a heating wire or a high-frequency heater to improve the atomization efficiency. ^[1]

The aerosol generated by the atomizer is mixed with the gas in the mist chamber before entering the flame. After the gas diffuses and expands in the mist chamber, the pressure decreases smoothly, so the mist chamber is also called a premixing chamber or an expansion chamber. In the fog room, the temperature is reduced due to the effect of the Joule-Thomson effect, and the heat is constantly transferred by the outside world to maintain balance. The larger-diameter droplets in the mist fall in the mist chamber, and collide with the walls of the colder mist chamber to form large droplets, which are discharged out of the mist chamber as waste liquid. It can be seen that the role and requirements of the fog chamber are: to have a fully mixed environment, the pressure change in the room must be smooth and stable, and play a buffering role (the indoor pressure is about 10-20mmHg above atmospheric pressure), and the indoor gas From turbulent flow to laminar flow, the gas flow is uniform and stable, no gas rotation noise is generated, the drainage of the fog chamber is smooth, and the memory effect is small. ^[2]

The material used to make the fog chamber is glass, stainless steel or Teflon plastic. The fog chamber has various shapes such as spherical, pear-shaped, and tapered. In the general instrument, the mist chamber is made into a cylindrical shape, and the inner wall has a taper of about three degrees to facilitate the smooth discharge of wastewater. The inner wall of the fog chamber generally has a good finish; the inner wall of the fog chamber made of polytetrafluoroethylene plastic is often engraved with drainage grooves. This is to prevent waste liquid from remaining in the interior of the fog chamber, which is conducive to draining waste liquid and reducing Memory effect. ^[2]

The design of the length and thickness of the fog chamber of various types of instruments is also very inconsistent, but the following aspects must be considered: it is beneficial to the change of air flow form (from turbulence to laminar flow), and the flame is stable and the noise is small ; To help shorten the time for the aerosol to pass through the mist chamber, so that the signal response is fast; try to avoid the loss of fine mist droplets during transportation, so that the sensitivity is not reduced due to the design of the mist chamber and so on. ^[2]

1. Change the structure of the fog chamber. A spherical spray chamber can achieve better delivery efficiency. This device allows the aerosol to reach a long distance before it reaches the burner, and makes the gas pressure stable. The large mist chamber can increase the evaporation and balance of the solvent, but the consumption of the test solution is large, and the memory effect is also increased. In addition, the sprayer mouth is sprayed against the lower wall of the mist chamber, which is conducive to the crushing of the droplets and causing large droplets to be discharged outdoors along the lower wall, improving the transportation efficiency and reducing the memory effect.

2. Heat the fog room. The heating method is used to accelerate the desolventization of the mist droplets, so that the "dry mist" with a higher concentration enters the flame, and the condensed solvent is eliminated, thereby improving the measurement sensitivity. Common heating methods: one uses an electric heating wire to heat the fog chamber shell, and the other uses infrared rays to illuminate the fog chamber. There are also high-frequency heaters. These can get better results. ^[2]

3. Convection spray chamber. A convection nozzle is installed in the opposite direction of the nozzle of the atomizer, and the fuel or the combustion-supporting gas is sprayed to the nozzle of the atomizer with a high-speed airflow, and collides with the droplets emitted from the nozzle to replace it. The impact of the ball, the large droplets are further dispersed into small droplets, reducing gas vortex and increasing atomization efficiency. During work, adjust the gas convection flow, nozzle position and distance. When the distance is 5-12mm, the two nozzles are on the same axis, and the velocity of the two air streams is equal, the atomization efficiency is the best. The highest sensitivity. ^[2]

4. Auxiliary gas. In order to maintain spray efficiency and flame stability, an auxiliary gas tube can be installed to make the air pressure in the fog chamber stable and stable.