1.3
Principles of operation
Flame photometry relies upon the fact that the compounds of the alkali and alkaline earth metals can be
thermally dissociated in a flame and that some of the atoms produced will be further excited to a higher
energy level. When these atoms return to the ground state they emit radiation which lies mainly in the
visible region of the spectrum. Each element will emit radiation at a wavelength specific for that element.
The table below gives details of the measurable atomic flame emissions of the alkali and alkaline earth
metals in terms of the emission wavelength and the colour produced.
Element
Emission Wavelength (nm)
Flame Colour
Sodium (Na)
589
Yellow
Potassium (K)
766
Violet
Barium (Ba)
554
Lime Green
Calcium (Ca)
622*
Orange
Lithium (Li)
670
Red
*Note: Calcium is measured by using the calcium hydroxide band emission at 622nm as the Calcium main
atomic emission occurs at 423nm.
Over certain ranges of concentration the intensity of the emission is directly proportional to the number of
atoms returning to the ground state. This is in turn proportional to the absolute quantity of the species
volatized in the flame, i.e. light emitted is proportional to sample concentration.
It can be seen that if the light emitted by the element at the characteristic wavelength is isolated by an
optical filter and the intensity of that light measured by a photo-detector, then an electrical signal can be
obtained proportional to sample concentration. Such an electrical signal can be processed and the readout
obtained in a digital form.
A simple flame photometer consists of the following basic components:
a) The burner: a flame that can be maintained in a constant form and at a constant temperature.
b) Nebuliser and mixing chamber: a means of transporting a homogeneous solution into the flame at a
steady rate.
c) Simple colour filters (interference type): a means of isolating light of the wavelength to be measured
from that of extraneous emissions.
d) Photo-detector: a means of measuring the intensity of radiation emitted by the flame.
500 795/Rev E/03-17
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