A nebulizer is a device that introduces a liquid sample into the AAS. The nebulizer chamber thoroughly mixes acetylene (the fuel) and oxidant (air or nitrous oxide), and by doing so, creates a negative pressure at the end of the small diameter, plastic nebulizer tube. This negative pressure acts to suck (“uptake”) liquid sample up the tube and into the nebulizer chamber, a process called aspiration. A small glass impact bead and/or a fixed impeller inside the chamber creates a heterogeneous mixture of gases (fuel + oxidant) and suspended aerosol (finely dispersed sample). This mixture flows immediately into the burner head where it burns as a smooth, laminar flame evenly distributed along a narrow slot in the well-machined metal burner head.
Liquid sample not flowing into the flame collects on the bottom of the nebulizer chamber and flows by gravity through a waste tube to a glass waste container (remember, this is still highly acidic).
For some elements that form refractory oxides (molecules hard to break down in the flame) nitrous oxide (N2O) needs to be used instead of air (78% N2 + 21% O2) for the oxidant. In that case, a slightly different burner head with a shorter burner slot length is used.
Functions of the nebulizer includes:
- Suck up liquid sample at a controlled rate
- Create a fine aerosol for introduction into the flame
- Mix the aerosol and fuel and oxidant thoroughly for introduction into the flame
Here, we will discuss the most common types: pneumatic and ultrasonic nebulizers.
A pneumatic nebulizer converts a sample solution into an aerosol of tiny droplets using a jet of compressed gas. The flow of inert gas carries the droplets to an atomizer. Several versions of pneumatic nebulizers are available and a few are shown below.
An ultrasonic nebulizer creates an aerosol of tiny droplets by pumping a sample solution onto the surface of a piezoelectric crystal that vibrates at a frequency of 20 kHz to several MHz. The vibrations convert the sample into a dense and more homogeneous aerosol than what a pneumatic nebulizer can achieve. However, viscous liquids and particulates lower its efficiency. The aerosol is then carried to an atomizer by an inert gas.
Posted by Adulhakeem Owolabi