The flame photometer is a relatively low-cost, rapid and accurate analytical device used to determine the presence and concentration of certain elements, especially some metals from Group I (lithium, sodium, potassium) and Group II (calcium, barium). It works by capturing the wavelengths and intensity of light emitted by excited atoms after a sample is sprayed into a flame and then comparing this pattern to known spectra.
If you want to know more about flame photometers then this is the right place to start. We take you through the key principles behind flame photometry, the features you can expect to find and how to analyse results.
The flame photometry process is underpinned by fundamental physical principles around excitation and relaxation of electrons in the outer orbital shells of atoms.
Carrying out a chemical analysis with flame photometry involves several stages.
0. Calibration: Before use, a flame photometer should be calibrated in line with any manufacturer instructions for the device. Calibration may be performed by measuring light emissions from a standardised reference solution.
1. Preparation: While the photometer’s operation is electrically powered, its flame is fuelled by propane, butane or natural gas. Each device will have its own minimum warm-up time. The sample for investigation must also be dissolved in an aqueous solution ready for use.
2. Aspiration and atomisation: A quantity of the solution is drawn in by the nebuliser / aspirator component of the photometer. It goes through a mixing chamber and into a flame where it is dehydrated and reduced to constituent atoms.
3.Excitation: Ionised atoms absorb heat energy from the flame, shifting electrons into higher energy orbitals and leaving the ions in an excited state.
4. Emission of light: As the excited atoms fall back into their energetic ground state, they release the energy they previously absorbed in the form of different colours (corresponding to wavelengths) of light. This light emission is detected and captured by photoelectric circuitry in the device.
5. Result reporting: Light emitted by the sample is compared to known spectra associated with individual elements. The wavelengths of light show which elements are present and the intensity of light released should indicate the concentrations. With modern flame photometers results data may be directly downloaded to computer via USB or other means. A device may also have its own integrated printer for quick paper readouts and graphs if needed.
How are flame photometry results analysed?
Individual elements produce their own characteristic emission spectrum when their atoms go through energetic excitation and return to ground state. This means that elements in a sample can be identified by the colour of the flame they produce.
Copper produces a blue flame, while barium glows green, and both lithium and strontium produce a red flame. Calcium’s flame is orange, sodium’s yellow and so on. Exact colour / wavelength will be detected by the photometer and matched to known emission spectra.
The intensity of light produced at each characteristic wavelength is directly proportional to the number of atoms emitting energy and therefore to the concentration of that element in the sample under analysis.
What are flame photometers used for?
Flame photometers are used in biology and medicine, as well as environmental and agricultural sciences, and general inorganic chemistry. In clinical laboratories these devices are a useful means of establishing levels of sodium, potassium, lithium and calcium ions in blood, urine or other body fluid samples. Other applications might include analysis of fertilisers and soil substrates, or production of synthetic fuels.
Which elements can flame photometers detect?
Not all chemical elements can be detected by flame photometry. For flame photometry to be a useful tool, the element must:
- ionise in an aqueous solution (carbon, oxygen, silicon etc.. are therefore unsuited),
- have sufficiently low activation energy for ions to become excited by exposure to a flame (noble gases etc.. have high activation energies and are therefore unsuited),
- produce an emission spectrum which is narrow and well-defined (some transition and heavier metals have electron structures which produce broad and less defined emission spectra and are therefore unsuited).
A final word…
Flame photometry is an excellent tool for detection and analysis of elements including lithium, sodium, potassium, calcium and barium. As well as being cost effective and easy to operate, flame photometry requires little training and can be used for high-volume rapid analyses.
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