Flame photometers read atomic emissions to detect the presence of metal salts, principally sodium (Na), potassium (K), lithium (Li) and calcium (CA). They measure the flame emission of solutions containing these metal salts and perform quantitative analysis of the species. The process of flame photometry uses a hot flame to evaporate a solvent, which atomizes the metal and excites a valence electron to an upper state. Light is emitted at characteristic wavelengths for each metal as the electron returns to the ground state. Flame photometers use optical filters to monitor for the select emission wavelength produced by the analyte species. Comparison of emission intensities of unknowns to either that of standard solutions, or to those of an internal standard, allows quantitative analysis of the analyte metal in the sample solution.
Flame photometers provide high sample recognition through a simple and rather inexpensive process when compared with other excitation methods such as arcing and sparking. Additionally, unlike gas plasma detectors, they are not limited to only a few easily ionized metals. For these reasons, flame photometers are used widely for biological sample testing, clinical research, and environmental analysis.
However, flame photometers are susceptible to a number of strategic disadvantages due to the low temperatures used to excite metal salts. Most of these disadvantages are related to interference and instability of aspiration conditions. A number of problems can affect these readings including aspiration rates, the viscosity of the solutions being measured, the flow rates and purities of the fuels and oxidants being used to stimulate the process, and the purity of the samples being tested. For these reasons, it is extremely important to measure the emissions of unknown or untested solutions under the exact same conditions as a control or standard solution to make sure that all readings are legitimate and balanced.
The two most important specifications to consider when selecting between flame photometers are the ranges of spectral wavelengths monitored by the device and its resolution. Spectral and wavelength range is a determination of the dispersion of the grating across the linear array. This reading may also be expressed as the "size" of the spectra on the array. Resolution is the width of the analytical peak at half its height expressed, generally expressed in nanometers. This is a measure of the flame photometers ability to separate two overlapping peaks. At the point of resolution, the two peaks are of equal height, and are said to be resolved if a dip between the tops of the two peaks can be measured.