By Nidhi DhullReviewed by Lexie CornerUpdated on Jul 31 2024Atomic fluorescence spectroscopy (AFS) is a recently developed analytical method for determining the concentration of various elements in a wide range of samples.1,2 It involves exciting atomized gaseous elements to higher energy levels using electromagnetic radiation and measuring the resulting fluorescence emission from these excited atoms.2
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In contrast to conventional absorption-based techniques, AFS has significantly low background noise, enabling high detection sensitivity with minimal interference in the fluorescence signal.1,2 Additionally, vapor-generation atomization with element-specific photon sources allows for the simultaneous detection of multiple elements.1 Thus, AFS is widely employed in medical, agricultural, geological, and environmental applications.2
AFS: The Basics
Atoms consist of quantized energy levels, and in AFS, atoms in the ground state get excited by absorbing radiation from an intense electromagnetic source and subsequently relax back to the ground state through atomic fluorescence emission. This principle forms the basis of AFS.2
AFS involves bringing analyte atoms into a reservoir and exciting them with a monochromatic electromagnetic beam. The fluorescence emission due to the relaxation of these excited atoms is recorded. The atomic reservoir can be a flame, plasma, glow discharge, or furnace, while xenon lamps or line sources are commonly used as radiation sources.2
The radiative excitation and de-excitation processes in AFS lie in the ultraviolet-visible range.2 The radiation source is mounted perpendicular to the direction of the light detector to avoid interference with the fluorescence process. For multielement analysis, multiple lamps are positioned around the atomization/excitation source.1
The recorded radiation intensity, plotted as a function of wavelength, gives the atomic fluorescence spectrum of the analyte, which is unique for each element.2 The position of the fluorescence emission signal helps identify the analyte, while the intensity indicates its concentration. The fluorescence intensity increases with increasing analyte concentration.1
Components of an Atomic Fluorescence Spectrometer
The essential components of an atomic fluorescence spectrometer include an atom reservoir, radiation source, monochromator, detection system, and readout system.2
Firstly, the analyte atoms are de-solvated, vaporized, and atomized at low temperatures using a heat pipe, quartz cell, flame, or graphite furnace to create an atom reservoir.1,2 The graphite furnace with a laser radiation source can help achieve AFS detection limits up to the attogram.2
A hydrogen diffusion flame is a widely used atom reservoir due to its low background level. Alternatively, cold vapor cells are used for mercury detection, in which dissolved mercury is converted into elemental mercury by reacting it with SnCl2.2
Continuous and monochromatic radiation sources can be used to excite atoms electromagnetically. Continuous sources such as tungsten halide or deuterium lamps allow multielement determinations but have low radiant densities. Alternatively, line sources such as hollow cathode lamps and electrodeless discharge lamps provide much higher radiance but are unsuitable for multielement analysis. Modern AFS instruments use lasers for their high radiant power.2
Diffraction gratings are commonly employed as monochromators to isolate fluorescence emission lines from other lines. This emission radiation, dispersed by monochromators or filters, is sent to a photomultiplier tube detector. Finally, the amplified detector output is displayed on a readout device.2
Applications of AFS
Commercially available AFS systems are successfully used for analyzing over sixty elements in various samples, including water, soil, plants, pharmaceuticals, food, biologicals, and petrochemicals.1,2
In clinical analysis, AFS helps in the accurate detection of metals, such as Pb, Hg, Se, and As, in blood, urine, tissue, nail, and hair. These are also detected in active pharmaceutical ingredients and agricultural products such as dairy, wine, and meat.2
AFS can detect low amounts of Hg, As, Se, Sb, and Te in the air, aiding environmental monitoring.2 Water and soil contamination can also be identified with AFS. Electrothermal vaporization combined with AFS helps directly analyze soil samples, simultaneously detecting Cd and Hg with low detection limits. Alternatively, ultra-trace determination of Cd in water and rice has been achieved by combining chemical vapor generation (CVG) with AFS.3
AFS has become an important tool for monitoring and controlling heavy metal pollution in geological samples due to its high sensitivity and precision. Amounts of As, Sb, Se, and Hg can be determined simultaneously using CVG-AFS.4
Metallurgical and petrochemical industries also employ AFS to analyze ores, rocks, and mineral deposits. In combination with chromatography, AFS provides qualitative and quantitative data regarding metal speciation and compositions of alloys. Additionally, elements like Pb, Hg, Cd, As, Sn, and Zn are quantified in fuels, lubricants, and crude oil using AFS.2
AFS: Advantages and Disadvantages
The primary advantages of AFS compared to other absorption/emission-based detection methods are a high signal-to-noise ratio and greater sensitivity. Fluorescence signals face low interference from radiation sources, making AFS ideal for identifying and quantifying certain metals and metalloids such as Pb, Cd, Tl, Hg, As, Sb, Se, and Te.
The detection limits of AFS range in the attogram for certain elements like Cd and Pb with favorable atomization and radiation source combinations. Moreover, AFS-based linear calibration curves generally extend over a wide range for samples in solid, liquid, or gaseous states, allowing trace elemental analysis when combined with other analytical techniques like chromatography.2
Despite several advantages, AFS has some limitations. The sample preparation process, especially for solids, is tedious and time-consuming. The analyte may also undergo chemical reactions with other components in the sample during analysis, requiring matrix separation for accurate quantification.1,2
The actual performance of AFS depends on multiple variables, such as the laboratory environment, sample type, reagents, and instrumentation. Additionally, the total operational cost of AFS is greater than that of plasma spectrochemical techniques with comparable sensitivity.1 Irrespective of several advancements, the commercial application of AFS is currently limited to detecting metals and metalloids.2
Future Outlooks
Significant advancements are being made in AFS technology to overcome the current limitations. For instance, a recent study in Talanta demonstrated an automated, on-site-applicable atomic fluorescence spectrometer for determining dissolved inorganic arsenic in estuarine and coastal waters. The proposed AFS setup was successfully applied in different coastal waters of China with varying salinities.5
Another recent study in Talanta proposed non-chromatographic direct detection of As using AFS. Cost-effective electro-synthetic sample introduction technology using iron-modified nickel foam electrodes with nano-flower structure was employed to enhance the sensitivity and selectivity of AFS.6
Further advancements in instrumentation and analyte introduction methods will enable the expansion of AFS to new application fields, enhancing its utility across various industries.
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References and Further Reading
1. Thomas, R. (2023). Practical Guide to ICP-MS and Other Atomic Spectroscopy Techniques: A Tutorial for Beginners. CRC Press. DOI: 10.1201/9781003187639
2. IGNOU. (2018). Atomic Fluorescence Spectrometry. IGNOU. Available at: http://egyankosh.ac.in//handle/123456789/43287
3. Bacon, JR., et al. (2024). Atomic spectrometry update – a review of advances in environmental analysis. Journal of Analytical Atomic Spectrometry. DOI: 10.1039/D3JA90044D
4. Zhao, W.-Z., Liu, J.-F., Zhang, B.-B., Zhang, Y., Lu, B. (2024). Chemical Vapor Generation Non-Dispersive Atomic Fluorescence Spectrometry Technique for the Determination of Arsenic, Antimony, Selenium, and Mercury in Geological Samples by One-Time Digestion. Advances in Environmental Analysis for Today's Spectroscopists. DOI: 10.56530/spectroscopy.ty8867r2
5. Bo, G., Fang, T., Chen, L., Gong, Z., Ma, J. (2024). Shipboard determination of arsenite and total dissolved inorganic arsenic in estuarine and coastal waters with an automated on-site-applicable atomic fluorescence spectrometer. Talanta. DOI: 10.1016/j.talanta.2023.125082
6. Gao, Y.-Y., Yang, X.-A., Zhang, W.-B. (2024). High sensitivity atomic fluorescence spectroscopy for the detection of AsIII by selective electrolysis of arsenic on nanoflowers-like Fe/NFE. Talanta. DOI: 10.1016/j.talanta.2024.126127
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