Introduction:

If a solution containing a metallic salt, for example sodium chloride is aspirated into a flame for example; acetylene burning in air, a vapour that contains atoms of the metal might be shaped. Several of such gaseous metal atoms might be raised to an energy level that is adequately high to permit the emission of radiation characteristics of the metal, for example the characteristics yellow colour imparted to flames via compounds of sodium. This is the basis of flame photometry. Though, a much larger number of the gaseous metal atoms will usually stay in an unexcited state or, in other words, in the ground state. Such grounds-state atoms are capable of absorbing radiant energy of their own precise resonance wavelength that in common is the wavelength of the radiation that the atoms would emit if excited from the ground state. Therefore, if light of the resonance wavelength is passed throughout a flame enclosing the atoms in question, then part of the light will be absorbed, and the extent of absorption will be proportional to the number of ground state atoms present in the flame. This is the underlying principle of atomic absorption spectroscopy (AAS).

Definition of Flame Spectroscopy

• Flame Emission Spectroscopy is a technique in that the emission of light via thermally excited atoms in a flame or furnace is used to measure the concentration of atoms, and Flame Atomic Absorption Spectroscopy is a technique in which the absorption of light via free gaseous atoms in flame or furnace is utilized to compute the concentration of atoms.

Differences between Flame Emission and Flame Atomic Absorption Spectroscopy.

Flame emission spectroscopy is basically the same as flame atomic absorption spectroscopy. The difference is that no light source is needed in flame emission. Some of the atoms in the flame are promoted to excited electronic states by collision with other atoms. The excited atoms emit their characteristic radiation as they return to their ground state. In flame emission spectroscopy, the emission intensity at a characteristic wavelength of an element is nearly proportional to the concentration of the component in the example. For both absorption and emission, standard waves are usually used to establish the relation between signal and concentration.

Working Principle of Flame Emission Spectrometry (FES)

The solution is introduced into the flame as a fine spray. The solvent evaporates leaving the dehydrated salt. The salt is disconnected into free gaseous atoms in the ground state. An indeed fraction of such atoms can absorb energy from the flame and be raised to an excited electronic state. The excited levels have a short lifetime and drop back to the ground state, emitting photons of trait wavelength. Such can be noticed through conventional monochromator-detector set up. The intensity of emission is straight proportional to the concentration of the analyte in solution being aspirated. A representation diagram of a flame emission spectrometer is specified below:

Fig: A schematic diagram of a flame emission spectrometer  

Working Principle of Flame Atomic Absorption Spectrophotometry (FAAS)

The example solution is aspirated into a flame as in flame emission spectrometry and the model element is transferred to atomic vapour. The flame then contains atoms of that element. Several are thermally excited via the flame, but must stay in the ground state. Such ground state atoms can absorb radiation of a scrupulous wavelength that is generated via a special source made from that element. The wavelength of radiation given off by the sources is the same as those absorbed by the atoms in the flame. The absorption follows Beer's Law, that is, the absorbance is straight proportional to the path length in the flame and to the concentration of atomic vapour in the flame. A schematic diagram of flame atomic absorption spectrophotometer is specified below:  

Fig: A schematic diagram of flame atomic absorption spectrophotometer

Interference:

By interference, we mean any consequence that transforms the signal when analyte concentration stays unchanged. In the measurement of atomic absorption or emission signals, interference is widespread and simple to overlook. If we are clever sufficient to discern that interference is occurring, it might be accurate through counteracting the source of interference. 

Types of interference:

• Spectral: refers to the overlap of analyte signal through signals due to other elements or molecules in the example or by signals due to flame or furnace. The best means of dealing via overlap between lines of dissimilar elements in the example is to choose an additional wavelength for analysis.
• Chemical: is caused via any component of the sample that reduces the extent of atomization of analyte. For example SO₄^2- and PO₄^3- hinder the atomization of Ca²⁺, perhaps by forming non volatile salts. This can be solved by adding releasing agents which are chemicals to a sample to decrease chemical interference, for example EDTA and 8 - hydroxyquinoline protect Ca²⁺ from the above interference.
• Ionization: In this, the ionization of analyte atoms decreases the concentration of neutral analyte atoms in the flame, which results in the decrease of the desired atomic signal. This is solved via adding an ionization suppressor to a sample so as to reduce the extent of ionization of analyte.

Application of the Techniques (FES and FAAS).

• They are utilized in many laboratories particularly whenever trace metal analyses are needed.
• Environmental models are analyzed for heavy-metals contamination.
• Pharmaceutical samples are analyzed for metal impurities using the atomic spectrometric techniques.
• The techniques are employed in the steel industry to find out minor components in addition to major ones.
• The techniques are employed in the measurement of sodium and potassium in serum and urine in diagnostic clinical analysis.

Sensitivity and Detection Limit in Atomic Absorption Analysis

Atomic absorption spectroscopy is a sensitive method in the examination of metals in trace concentration. Sensitivity is classified as that concentration of an element in aqueous solution that absorbs 1% of the incident radiation passing through a cloud of atoms being determined. Generally, a 1% absorbance corresponds to 99% transmittance or about 0.004 absorbance value. While detection limit is the concentration of an element in solution that provides a signal equal to twice the standard divergence of the series of measurements near blank level or the background signal.

As we know that both the sensitivity and detection limit fluctuate considerably by flame temperature and spectral bandwidth. For instance, the sensitivity of mercury is 2.2 mg/l, while the detection limit is 0.16 mg/l. Therefore, it is needed to specify the flame kind to be utilized in any determination.

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