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The atoms of certain metals when heated emit radiation and methods of analysis have been developed which use the wavelength of emission for qualitative analysis and the intensity of emission in quantitative work (Table 2.4). Emission spectroscopy is most frequently encountered as flame emission photometry, which is almost entirely restricted to the visible region of the spectrum and to those elements that are easily excited at the temperature of a flame. Alternatively, excitation may be achieved by means of a high voltage arc struck between two electrodes. More recently, temperatures as high as 10000 °C, produced by inductively coupled plasma (ICP) discharges, have been used to cause the excitation of atoms and offer improved levels of sensitivity. Atomic fluorescence is a variant of flame photometry in which atoms are excited by the absorption of radiation rather than thermal energy. The instruments used for such measurements require optical systems similar to those used in molecular fluorimetry. However, despite the great potential increase in sensitivity of such a method, it is limited to a few elements for which the necessary intense excitation sources of radiation are available.

Table 2.4 Emission lines of various elements

Element

Wavelength most suitable for:

Other lines

Flames suitable for:

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