FLAME PHOTOMETER

Typical Number in Hospital: 3

Cost Bands: 4

References: 5,6,9

These are used in the clinical chemistry department to determine the concentration of pure metals, particularly sodium and potassium, in plasma and urine. They can be used in the emission or absorption mode by which they emit or absorb light at characteristic wavelengths for the atoms involved.

The emission flame photometer injects droplets of a diluted solution of a body fluid into a flame of propane or natural gas and air, under which conditions some metal elements emit light at characteristic wavelengths. The characteristic wavelengths for sodium are detected by a photometer after passing through a monochromator (prism or diffraction grating), or set of filters. Since the power of the light will depend on the rate of solution uptake, quality of aerosol production, and flame characteristics, calibration may be made against a lithium salt. Lithium does not normally occur in the human body and so the sodium and potassium spectral intensity may be compared with that detected for the known concentration of lithium.

Atomic absorption flame photometers work by passing a light of known spectral content through a flame in which the test sample is being ionized. The light source is usually a hollow cathode lamp coated with the metal which is to be determined. Thus the light passing through the beam contains the necessary spectral lines. It is important to differentiate between radiation generated in the flame and that absorbed by it and to achieve this the light source is pulsed so that the change in detected light is proportional to that absorbed in the flame. This device is sometimes called an atomic absorption spectrometer.