Flame photometry Determination of Na level in vegetables

1.1 Instrument description
The PFP7 and PFP7/C are low temperature, single channel emission flame photometers
designed for the routine determination of sodium (Na) and potassium (K). Additional filters are
available for the determination of lithium (Li), calcium (Ca) and barium (Ba). Both versions are
fitted with automatic flame failure detection for user safety, making them ideal for use in clinical,
industrial or educational applications. The model PFP7/C is specifically designed for use in
clinical applications. The in-built lineariser circuitry enables readings of both Na and K, at normal
clinical serum concentrations to be displayed directly in mmol/l. Serum samples must be diluted
200:1 or 100:1 prior to presentation to the flame photometer. Jenway are able to offer a diluter
to enable this to be carried out efficiently and accurately.
1.2 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 an analogue or 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.