The flame of the atomic absorption spectrometer is comparable to the absorption cell in an optical spectrometer. Atoms in the ground state serve as the light absorbing species. The absorption signal is dependent on flame length and number of absorbing ground state atoms in the light path. Role of AAS Burner has been briefly discussed in the earlier article Flame Atomic Absorption Spectroscopy.
Important considerations for selecting Burner Head design
Selection of Atomic Absorption Spectroscopy burner head design is based on analysis requirements. Before going into the selection parameters you should be familiar with the sample premixing before it gains entry into the burner head.
The sample is premixed with oxidant and fuel gas in the required proportion in the mixing chamber prior to reaching the Atomic Absorption Spectroscopy burner. It is introduced into the spray chamber through a nebulizer which produces a fine sample aerosol. The larger particles are led to the waste drain and only very fine droplets are permitted to reach the flame.
- Flame path length
The flame path length in common use is 10 cm in comparison to 1cm path length of UV-VIS absorption cell Longer path length is necessary as only a small fraction (< 5%) of sample enters the flame and the residence time is also short \( (10^-^3 sec)\). Longer path length increases residence time of the atoms in the flame and maximizes the absorption signal strength.
The Atomic Absorption Spectroscopy burner head needs to be made of an inert material such as titanium which can withstand the operational temperatures of flames and also attack of corrosive acids.
- Choice of gas
Alkali metals and alkaline earths such as Na,K,Li,Ca,Ba and some transition metals can be analysed using air – acetylene flames whereas refractory elements such as Al,Ti.Si, Zr,etc which form stable oxides require higher temperatures of nitrous oxide – acetylene flames.
- Burning velocity of gas mixture
The burning velocity of a gas mixture is dependent on the oxidizing nature of oxidant gas. Strong oxidants or even oxygen cannot be used at it as it can cause flashback inside the burner slots resulting in a loud explosion which can damage the spray chamber. Present AAS systems have safety interlocks that shut down the flame in such an event.
- Slit widths
Gas flow is dependent on slit width of burner slot. Narrow widths lead to faster flow velocities. The flame propagation velocities can be controlled by reducing slit width but narrowing down of slits increases the risk of clogging by suspended solids in sample matrices.
Burner top Geometries
Flat top Atomic Absorption Spectroscopy burner heads often lead to localized overheating. Such a situation is overcome by using triangular top design. This design also reduces warm up time after flame ignition to about 10 min prior to sample or standard introduction
Burner geometries that are in common use are:
- –10-cm single slot burner head used for air-acetylene flames
- – 5 –cm single slot burner used for nitrous oxide – acetylene flames
- – 3 – parallel slot burner. The central flame is shielded by outer flames as cooling by entrained air is minimized. This flame geometry is beneficial for some elements which are difficult to atomize.
Every burner has provision of making adjustments of height, forward-backward movement and angular rotation by means of knobs at the base for getting optimum sensitivity under given set of operational conditions.
The article has briefly covered the benefits of using different Atomic Absorption Spectroscopy burners. Please feel free to share your experiences and offer your valuable comments.