Spettroscopia a fluorescenza atómica

Atomic fluorescence spectroscopy (AFS) measures fluorescence radiation emitted by atomized samples after electronic excitation. Atomized samples are irradiated by electromagnetic radiation, causing electronic transitions. Excited atoms release energy as fluorescence at wavelengths specific to each atomic species, returning to their lower energy states. The fluorescence intensity is proportional to the target element's concentration.

An AFS instrument contains four essential components: a high-intensity light source, an atomizer, a wavelength selector, and a detector. The light source excites atomized analytes, while the wavelength selector isolates specific emission wavelengths. The detector, typically a photomultiplier tube, measures fluorescence intensity at a right angle to the incident beam, minimizing background noise and interference.

The detector is positioned at a right angle to the source beam because atoms fluoresce at their optimal absorption wavelength. This perpendicular geometry minimizes scattered excitation radiation from reaching the detector, reducing background noise and improving signal-to-noise ratio. This arrangement is critical for accurate fluorescence measurement and sensitivity.

Potential excitation sources for AFS include pulsed hollow-cathode lamps, electrodeless-discharge lamps, xenon or mercury arc lamps, and lasers. High-intensity sources are essential because fluorescence intensity is proportional to irradiation intensity. Continuum sources are rarely used due to their low power output, making these discrete-wavelength sources more practical for analytical applications.

AFS is particularly effective for identifying volatile-hydride-forming elements and mercury. Elements such as arsenic and selenium form volatile hydrides that are easily atomized and excited. Mercury is especially amenable to AFS analysis due to its favorable fluorescence characteristics, making this technique ideal for environmental and toxicological applications involving these elements.

Chemical and spectral interferences from atomization can be avoided by adding releasing or protective agents to the matrix. Releasing agents prevent analyte atoms from forming non-volatile compounds, while protective agents shield analytes from interfering substances. These additives enhance atomization efficiency and improve measurement accuracy by reducing unwanted interactions during the analysis process.

Fluorescence intensity is directly proportional to the target element's concentration and the irradiation intensity. This linear relationship enables quantitative analysis, where measured fluorescence signals can be converted to concentration values. High-intensity sources and minimal interfering radiation are essential to maintain this proportionality and ensure accurate quantitative results across a wide concentration range.