Page 32 - AAS & AES & FES 01082016_Neat

EMISSIVE SPECTROMETRY
Emission of ultraviolet-visible radiation from atoms and monoatomic Ions occurs when

the atom or the ion in an excited electron state loses energy by changing to a lower energy
electron state. The emitted radiation can be monitored and used for either qualitative or

quantitative analysis. If the initial excitation of the atom or ion is caused by a process other than
the absorption of ultraviolet visible radiation and if emitted radiation is monitored, the analytical

technique is atomic emissive spectrometry (AES).

AES is divided into categories according to the source of the energy that causes the initial
excitation. In AES the cell is required to atomize or ionize the sample, to excite the atoms or

ions, and to contain the sample for a sufficient time to obtain the analytical measurement. Flame
emissive spectrometry (FES) uses a flame as the cell. The heat from the flame provides the

energy that is required for the excitation. Other forms of AES use plasma, electricity, or some
non flame heat source to excite the atoms.

FLAME EMISSIVE SPECTROMETRY
Flame emissive spectrometry uses a flame in a dual role to atomize the sample and to

excite the atomized analyte. The radiation that is emitted when the excited atoms relax is

measured and used for the assay. A schematic diagram of the apparatus is sown in fig.

In that case the radiative source is turned off and a mechanical chopper is placed between
the flame and the monochromator. The chopper is required with most instruments that were

designed for use with AAS because the detectors in those instruments only respond to an AC

signal. In instruments that are designed solely for FES, a chopper is usually not required
because the detectors in those instruments are designed to respond to a DC signal.

In most instruments the wavelength selector is a monochromator. In instruments that are
designed solely to assay sodium or potassium by FES, however, filters are commonly used as the

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