Flame Analysis

Introduction:

We shall discuss here few techniques using flame for different analysis.

- In atomic absorption spectrometry the flame is used for atomisation, i.e. to convert the analyte in a sample solution to atomic vapour by desovation, volatilization and dissociation.
- In flame emission spectrometry the flame is used for excitation i.e. an electron of the outermost shell of the atom is raised to an energy enriched orbit by gaining energy.  The atom is said to enter an excited state.

A number of fuel combination with air or oxygen can be used in order to obtain optimal flame temperature including butane, propane, acetylene and hydrogen.

The flame temperature for atomic absorption shall be chosen to give as many atoms in the ground state as possible and for the flame emission as many excited atoms as possible.  To maintain sturdiest flame of both fuel and oxidant are correctly regulated.  The flame can be replaced by other devices such as an electric furnace and electric arc or spark.

Atomic Absorption spectrometry:

Atomic absorption spectrometry utilized the property of unexcited atoms in the gaseous state to absorb exactly defined amounts of energy i.e. characteristic wave lengths of light and thereby become excited.

In atomic fluorescence spectrometry, the re-emitted energy is measured at an angle to the incident light.  The emission is at the same wavelength as the one absorbed.
In Electrorhermal Atomisation, electrical heating of a graphite rod or tube containing a sample removes the water and causes ashing with pyrolysis of protein and finally results in the element being votalised as a dense monoatomic vapour into the radiation beam.

The presence of a greater concentration of ground state atoms in a radiation beam than is possible with flame atomisation means that small sample volumes can be used and also very low concentrations of particular elements can be measured.  The method is also applicable to solid samples such as hair in which cadmium and lead have been analyzed.  The disadvantages are that precision may not be as good and the operation time may be longer than when using flame atomisation.

Instruments:

The measurement is essentially identical to measurements with any absorption photometer- Beer's law applies in its conventional form.

The spectral line light source is a hollow cathode lamp.  The cup shaped cathode is made of the same metal as that being analysed.  the anode is of some other suitable material such as tungsten and the lamp is filled with an inert gas at low pressure.  The electrode are surrounded by a glass tube, except the front window which is made of quartz.

When the lamp has reached working temperature the potential across the electrodes produces a flow discharge with most of the emission coming from the cathode.  The cup shape of the cathode concentrates the radiation and reduces redeposition of the metal on other parts of the tube.  The warm up time required causes delays if analyses of more than one element are required.
This can be over come by use of multi-element lamps.

The flame emits a background light, due to the excitation of its fuel molecules, and a line spectrum due to atoms in the sample.  the monochromator removes radiation at wave lengths other than the spectral line being measured.

Flame Emission Spectrometry:

When an atom is excited in a flame, an outer shell electron will be raised to a higher orbit or energy level.  Almost instantly it returns to a lower energy level emitting light.  For each transition from one energy level to the other a specific wavelength is emitted.  The transitions are specific for each element which therefore has its own unique line spectrum.

Instruments:

The monochromator or filter arrangement, photo detector and recording devices used in flame emission spectrometry are similar to those found in absorption spectrometers.

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