Fotometria a fiamma: panoramica

Flame photometry, also called flame emission spectrometry, uses a flame to excite a sample solution and measure emitted radiation. A nebulizer converts the liquid sample into a fine mist or aerosol, which enters the flame where thermal energy excites atoms. As excited atoms relax, they emit characteristic radiation detected by a photocell or photomultiplier tube, enabling qualitative and quantitative elemental analysis.

Propane-air flames are typically used because they provide sufficient energy for atomization, low emission and absorption at relevant wavelengths, and operate efficiently at low gas velocities. Propane-air flames reach temperatures of 1900-2000 °C, making them more sensitive than butane-air or natural gas-air flames, which operate at lower temperatures and produce less reliable results.

Pre-mix or laminar flow burners generate a fine mist in a vaporizing chamber, then mix it with fuel and oxidant gases before reaching the burner head. Total consumption or turbulent flow burners use three concentric tubes carrying sample solution, fuel gas, and oxidant gas separately, allowing them to mix only at the burner tip for different mixing characteristics.

A nebulizer passes a high-pressure gas stream over a capillary tube containing the sample, aspirating it into a spray chamber. This process converts the liquid sample into a fine mist or aerosol. The aerosol then passes to the burner, where flame heat desolvates it, forming dry particles that volatilize and produce free atoms for analysis.

Flame photometry is particularly effective for measuring sodium, potassium, lithium, and calcium. These elements emit characteristic radiation when excited in the flame, making them ideal candidates for this technique. However, flame photometry has limitations for other elements that can be overcome using higher temperatures and more reducing flames with advanced detection methods.

Early instruments developed in the 1860s used quartz prism spectrographs and photographic recording to capture atomic emission lines. The 1929 introduction of the nebulizer by Lundegardh enabled reproducible sample introduction. Modern advancements replaced these components with optical filters and electrical photodetectors, improving precision and convenience while maintaining the fundamental flame excitation principle.

Flame photometry has limitations that can be addressed using higher temperatures and more reducing flames like air-acetylene with higher-resolution detection. However, these approaches are not cost-competitive compared to atomic absorption spectroscopy or inductively coupled plasma atomic emission spectroscopy, which offer broader applicability and better performance for diverse elemental analysis.