What is an aerosol and how is it used in flame photometry?
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas.
Aerosols occur naturally and can also be produced via different mechanical methods.
Here, we break down the properties of an aerosol and how these properties make it ‘stable’.
What is the most important property of aerosols in flame photometry?
Particle size distribution is the most important property of aerosols in flame photometry.
Ideally, it would be as small as possible and the diameter variance between different particles would be zero, meaning they are all the same size.
In the real world, however, when the sample is pulled through a nebuliser (a device for turning solution into a fine spray) and turned into an aerosol, the size of the particles released into the aerosol will vary.
The size of the particles of water in the aerosol are the medium that brings the ions dissolved in the solution from the liquid sample to the burner head for detection.
Therefore, the surface tension of the liquid medium is of vital importance to the size of aerosol particles.
What happens when surface tension increases?
The particles of water that break off from the liquid solution would be larger, as the binding force of the sphere is much greater.
This requires more energy which is supplied in the form of kinetic energy from the pressure forcing through the orifice of the nebuliser.
Particle size distributions when recorded graphically (‘X’ axis being size and ‘Y’ being quantity of particles) are usually a few peaks that form a classical bell curve.
The size of the particles would seem to average out over the curve of each peak, meaning that the width of the bell curve would be the range of particles emitted from a single site, and the area under the peaks would be the total amount of particles emitted from the nebuliser.
What happens when the particle size is not sufficiently small enough?
When this happens, the aerosol can begin to revert back into a sitting liquid.
The particles would collide with the walls of the mixing chamber and, if hit with sufficient momentum (based upon their speed as well as mass), it could break the surface tension of the particle and cause it to stick to the wall.
Other particles may then collide onto this droplet, causing it to expand and then eventually could break off back into the aerosol mixture.
This could then be pulled up into the burner head causing a huge spike in concentration read by the photodiode array.