Flame photometry, also known as flame emission spectrometry, is a technique used for the qualitative and quantitative analysis of elements present in a sample using a flame as the source of excitation energy. The concept of flame photometry was realized in the early 1860s by Kirchhoff and Bunsen, who discovered that specific elements emit characteristic radiation when excited in flames. The first instrument developed for this purpose was used to measure sodium (Na) in plant ash using a Bunsen flame. However, the challenge lay in finding the most effective way to introduce the sample into the flame. It wasn't until 1929, when Lundegardh introduced a nebulizer, that a significant breakthrough was made, allowing a reproducible introduction of the sample into the flame.
In flame emission spectrometry, the nebulizer converts a liquid sample into a fine mist or aerosol. This is achieved by passing a high-pressure gas stream over the end of a capillary tube containing the sample and aspirating it into a spray chamber. The aerosol produced is then passed to the burner, where flame heat desolvates it, forming dry particles that volatilize and produce free atoms for analysis.
Early instruments utilized quartz prism spectrographs and photographic recording to disperse and capture atomic emission lines. However, advancements in optical filters and electrical photodetectors replaced these components, improving precision and convenience.
Flame photometry involves introducing the sample solution into a nebulizer, which converts it into a fine mist or aerosol. The atomized sample then enters the flame, along with air or oxygen and a fuel gas such as propane or acetylene. The flame provides the thermal excitation necessary to energize the atoms in the sample. As these excited atoms relax, their emitted radiation is detected by a photocell or photomultiplier.
Flame photometry is particularly effective in measuring sodium, potassium, lithium, and calcium elements. The flame used in flame photometry is typically a propane-air flame with a temperature range of 1900-2000 °C, although alternative flames like butane-air or natural gas-air can also be used. Flame photometry has some limitations that can be overcome by using higher temperatures and more reducing flames, such as air-acetylene, along with higher-resolution spectrometric detection. However, these approaches are not cost-competitive compared to the more broadly applicable flame atomic absorption spectrometry or AAS technique.
Flame photometry, or flame emission spectrometry, uses a flame to excite a sample solution.
Flames should possess sufficient energy for atomization, low emission and absorption at relevant wavelengths, and the ability to operate at low gas velocities.
Propane-air flames are typically used, as butane-air and natural gas-air flames are less sensitive due to lower temperatures.
Flame photometers use a nebulizer-burner system to transform sample solution into a mist or aerosol. There are two main types of burners.
Pre-mix or laminar flow burners generate a fine mist in a vaporizing chamber, which is then mixed with fuel and oxidant gases before reaching the burner head.
Total consumption or turbulent flow burners consist of three concentric tubes that carry the sample solution, fuel gas, and oxidant gas separately, allowing them to mix only at the burner tip.
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