Flame Photometry, Emission

a type of emission spectrochemical analysis that is used primarily for the quantitative determination of small amounts of numerous metals and rare-earth elements in solutions on the basis of the spectral lines or bands of the elements. The spectra are excited by a flame fueled by illuminating gas, hydrogen, acetylene, or cyanogen. The solution to be analyzed is injected into the flame as an aerosol in a stream of oxygen or air. The most widely used type of flame is an oxygen-hydrogen flame, which has a sufficiently high temperature (2900°K) and a low flame radiation intensity and contains no solid particles when the combustion is incomplete.

The radiation to be measured is isolated by means of a narrow-band filter or a monochromator in which a prism or diffraction grating is used as the dispersion element. Owing to the relative simplicity of the flame spectra and the high stability of the flame radiation, the spectral line intensities are measured almost exclusively by photoelectric means. A photocell or multiplier phototube serves as the radiation detector, and a galvanometer or chart recorder is used as the recording device. The spectral lines or bands are usually plotted on a chart recorder by scanning. The plot obtained gives the radiation intensity as a function of wavelength. The intensity of an element’s spectral line is the measure of the concentration of the element. The dependence of line intensities on concentration is determined from the results of photometry of the spectra of standard solutions.

The advantages of emission flame photometry are accuracy, speed, and high sensitivity. For example, the sensitivity limits are as low as 0.01 microgram per milliliter (μg/ml) for alkali metals and as low as 0.1 μg/ml for alkaline-earth metals. Spectrophotometers that automatically record the spectra and display the results are used for analysis by emission flame photometry.