Info

A-Ac

N-Ac

The emission line in bold type indicates the method of choice. Key to gases: A, air; Ac. acetylene; N. nitrous oxide; H. hydrogen.

The emission line in bold type indicates the method of choice. Key to gases: A, air; Ac. acetylene; N. nitrous oxide; H. hydrogen.

Whereas flame emission photometry relies on the excitation of atoms and the subsequent emission of radiation, atomic absorption spectrophotometry relies on the absorption of radiation by non-excited atoms. Because the proportion of the latter is considerably greater than that of the excited atoms, the potential sensitivity of the technique is also much greater.

2.5.1 Flame emission photometry

A flame photometer (Figure 2.31) is designed to cause atomic excitation of the analyte and subsequently to measure the intensity of the emitted radiation. A monochromating system is essential to distinguish between the emission of the test element and other radiation from the flame.

The flame combines both the source of radiation and the atomized sample and hence must be very stable if steady readings are to be obtained. The flame temperature must be high enough to excite the atoms under investigation; the hotter the flame, the greater the proportion that will be excited (Table 2.5). If it is too hot, however, the atoms may be raised to higher energy levels and electrons may be removed altogether, resulting in ionization.

Figure 2.31 Components of a flame photometer.

Source of radiation

Monochromating system

Source of radiation

Monochromating system

> An atomizer is a device designed to pioauce an aerosol, i.e. a very fine liquid spray.

The sample is converted into an aerosol in an atomizer. It then passes through an expansion chamber to allow a fall in the gas pressure and the larger droplets to settle out before passing to the burner, where the solvent evaporates instantly, the atoms remaining as a finely distributed gas. Atoms in the sample that are bound in molecules should be decomposed at the flame temperature so rapidly that the same effect is achieved. In practice only a small proportion of the sample (approximately 5%) is effectively atomized because the drop size of the remaining 95% is so large that the water is never effectively stripped away. In low temperature flames, for instance, only one sodium atom in about 60000 is excited but despite this apparently low efficiency the technique is very sensitive.

Any of the monochromating systems described for absorptiometers may be used, although the cheaper models of flame photometer usually employ filter systems. In these cases interference from other elements at wave lengths near to the test wavelength may be a real problem but owing to the intensity of emission it is possible to reduce the bandwidth by using very dense filters. In many instruments, the use of interference filters improves the specificity of the analysis.

Table 2.5 Fuels for flame spectroscopic techniques

Fuel

Oxidant Remarks

Temperature <°C)

Propane Butane

Air Air

Acetylene Air

Acetylene Nitrous oxide

Low temperature flames suitable for easily excited atoms, such as Na, Li, K and Ca Medium temperature flame suitable for most emission analyses, e.g. Mg, Mn and Sr High temperature flame necessary for the more refractile elements, e.g. P

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