Flame Atomic Absorption Spectroscopy
“Formal education will make you a living, self education will make you a fortune”
— Jin Rohn
Sample atomisation produces ground state atoms that are necessary for atomic absorption to take place. This involves application of thermal energy to break the bonds that hold the atoms together.
The complete atomisation assembly comprises of:
- Spray chamber
- Burner Head
Each of the components of the atomisation assembly are discussed in detail below:
Nebuliser converts the liquid sample into a fine spray or aerosol. In order to provide efficient nebulisation for different sample solutions (aqueous or organic, acids or bases, etc) the nebuliser should be adjustable and corrosion resistant. Stainless steel is commonly used but for corrosive solutions other corrosion resistant materials such as inert plastic, Pt/Ir or Pt/Rh alloy are also used. High sensitivity in combination with inert ceramic bead can be used to enhance nebulisation efficiency for lowest detection limits.
Aerosol from the nebuliser is led to the mixing or spray chamber. In this chamber the aerosol is mixed with fuel and oxidant gases and carried to the burner head. Only a fraction of the sample introduced by the nebuliser is used for analysis. An impact device prevents larger droplets from reaching the burner as these would delay sample vaporisation and atomisation through short transit through the flame. Only fine sized droplets are carried to the burner head
An impact device such as a flow spoiler or an impact bead is aligned at the exit of the aerosol stream of the nebulizer. A flow spoiler is more efficient at removing large droplets whereas the impact bead removes fewer large droplets and exhibits better sensitivity since more sample is led to the burner. However, the increased number of large droplets may have undesirable effects and increase interference.
Glass and ceramic impact beads can cause memory and contamination problems compared to the chemically inert flow spoiler and for this reason flow spoiler is preferred for routine work and for greater sensitivity impact bead is preferable. The excess sample is removed from the pre-mix chamber through a drain. The drain uses a liquid trap to prevent combustion gases from escaping through the drain line. The inside of the spray chamber is coated with wettable plastic material to provide free drainage of excess sample and prevent burner chamber memory. A freely draining burner chamber rapidly reaches equilibrium typically in less than two seconds for response to sample changes.
Burner heads are constructed of titanium to provide extreme resistance to heat and corrosion. A 10 cm single slot burner is recommended for air- acetylene flames. Its long length provides best sensitivity. A special 5 cm burner head is recommended for nitrous oxide –acetylene flame applications. The flame can be rotated to provide reduced sensitivity
Single slot 5 cm air-acetylene burner head is available when reduced sensitivity is required. It can be rotated to provide further sensitivity redaction and it has a wide slot to prevent clogging
A 3- slot burner head is designed for analysis of samples having high concentration of dissolved solids.
Majority of elements can be an analysed using air – acetylene flames which have high temperature range of 2150° C – 2300° C. Nitrous oxide – acetylene flames attain temperatures of 2600°C- 2800° C and can be used for analysing refractory elements which form stable oxides at lower temperatures.