The flame may be produced by burning various gas mixtures, some of which
are listed in Table 2.
The structure of the flame comprises an inner cone, which is the primary
reaction zone for combustion, and the outer cone or mantle where secondary
reactions occur. For the best results, the optical axis is arranged to pass through
the flame at the junction of the inner and outer cones. The supply of fuel and
oxidant is adjusted to give an optimum burning velocity.
The processes that occur to transfer the sample to the flame may be summa-
rized as follows:(i) production of an aerosol from solution (nebulization)
(ii) removal of solvent MA(aq) ÆMA (solid)
(iii) vaporization of sample MA(solid) ÆMA(vapour)
(iv) atomization MA ÆM• +A•
(v) excitation M• ÆM*
(vi) emission M* ÆM•Ionization may also occur to give the M+ion.
These stages each depend on the experimental parameters used in the
instrument. For example:● the viscosity of the solvent, which affects the aerosol production;
● the nature of the solvent, which may affect the vaporization;
● the rate of fuel flow, which can change the nebulization and the time the
atoms spend in the flame;
● the flame temperature, which controls the evaporation, the atomization and
the extent of ionization; and
● the nature of the flame.Because of the chemical reactions taking place in the flame, various species such
as OH radicals, CO, water and other combustion products are present, and may
give a background emission throughout the UV-visible range. Compensation for
this background must be made.Instrumentation Flame atomic emission spectrometers have similar optical systems to those of
UV-visible spectrometers, but the source of radiation is provided by the sample
itself. A flame photometeris a simpler instrument employing narrow bandpass
optical filters in place of a monochromator (Fig. 1). The sample is prepared as a
solution, which is drawn into a nebulizer by the effect of the flowing oxidant
and fuel gases. The fine droplets produced pass into the flame where sample
atoms are progressively excited. The emitted radiation passes through the
monochromator or filter and is detected by a photocell or photomultiplier tube.
E4 – Flame atomic emission spectrometry 207
Table 2. Gas mixtures used in flame atomic emission spectrometry
Fuel Oxidant Maximum flame temperature (K)
Natural gas Air 1800
Propane Air 1900
Hydrogen Air 2000
Acetylene Air 2450
Acetylene Nitrous oxide 2950
Acetylene Oxygen 3100