Flame Atomic Absorption Spectroscopy
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:
- Nebuliser
- Spray chamber
- Burner Head
Each of the
components of the atomisation assembly are discussed in detail below:
Nebuliser
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.
Spray
Chamber
Spray
Chamber
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
Head
Burner Head
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.
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