Elemental Brilliance: Your Guide to Atomic Fluorescence Spectroscopy

Atomic Fluorescence Spectroscopy (AFS) is a powerful tool in the realm of analytical chemistry. AFS helps us dive deep into the world of elements, allowing scientists to detect even tiny amounts with remarkable precision.

Think of this technique as a cosmic dance between light and atoms. By building on the principles of atomic absorption spectroscopy, AFS goes a step further, tapping into the enchanting glow of fluorescence. This technique is like a spotlight for trace elements, making it vital in fields like environmental science, biochemistry, materials science, and forensics.

In this blog post, we’ll break down the basics of Atomic Fluorescence Spectroscopy, exploring how atoms respond to light and emit unique signals that reveal their identity and concentration. Whether you’re a student starting a scientific journey or just curious about the magic happening at the atomic level, join us as we uncover the science behind AFS and its role in deciphering the elements that shape our world.

Principle of Atomic Fluorescence Spectroscopy

Atomic Fluorescence Spectroscopy is based on optical emission from gas-phase atoms that have been excited to higher energy levels by absorption of radiation. It involves both the absorption and emission aspects of the radiation. 

Each element has its own characteristic atomic fluorescence spectrum, where the location of the fluorescence emission signal indicates the identity of the analyte whereas the intensity is a measure of its concentration. This means that besides the concentration, the fluorescence intensity is related to the exciting light source and the radiating intensity. 

It concludes that the AFS also follows Lambert-Beer’s law, which is as follows:

P = P₀e⁻ɛᵇᶜ
P(abs) =  P₀ – P =  P₀(1- e⁻ɛᵇᶜ)
P(f) ∝ P(abs) = P(abs)ϕ
P(f) =  P₀(1- e⁻ɛᵇᶜ)ϕ

P(f) =  P₀ .2 303εcb× ϕ where,

P₀ = intensity of incident radiation
P = intensity of transmitted radiation
P(abs) = amount of radiation absorbed
P(f) = intensity of fluorescence radiation
ɛ = molar absorptivity
b = thickness of the sample cell
c = concentration of sample
ϕ = fraction of excited atoms that undergo fluorescence when εcb is small

The above equation shows that the fluorescence intensity is directly proportional to the concentration of the sample. However, it is valid only at the low concentration of the element.

At higher concentrations, when fluorescence emission is high, part of the emitted light will be absorbed by the atoms in the ground state, which is called self-absorption. This will lower the intensity of the emitted radiation and the proportionality is lost.

Instrumentation of Atomic Fluorescence Spectroscopy

AFS concerns the measurement of fluorescence emission of the atomic species that have been excited with the help of suitable electromagnetic radiation. The analyte is brought into an atom reservoir (flame, furnace, etc.) and excited by absorbing monochromatic radiation emitted by a primary source.

The atomic fluorescence radiation emitted by the excited atoms is then suitably dispersed and detected by monochromators and photomultiplier tubes and sent to appropriate readout devices. 

The basic instrument of AFS is composed of the following components:

I. Radiation Source

In AFS, the excitation of the sample can be achieved by both continuous and single-line sources. The continuous sources include the tungsten halide lamp or the deuterium lamp. The single-line sources are the Hollow cathode lamp and the electrodeless lamp. Continuous sources are easy to operate but they have low radiance. Therefore, single-line sources are used due to their high radiating power. 

II. Atom Reservoir

In AFS, the analyte sample is converted into atom vapor in the ground state before being excited by suitable radiation. The container or cell with these vaporized atoms is called an atom reservoir or atom cell. There are different types of atom reservoirs employed in the AFS instrument, such as,

  • Flame Atom Cell: Hydrogen diffusion flame is the most common flame cell used in AFS. The hottest parts of this flame are only around 1000°C while the bulk of the flame is at about 350 – 400°C. This permits excellent detection limits to be obtained because of the very low background. A combination of acetylene/nitrous oxide and hydrogen/oxygen/argon using a rectangular flame with a premix laminar flow burner is also used extensively. 
  • Non-Flame Cell: The reservoirs are made of graphite a common non-flame cell. It is in the shape of a bowl, in which a high current pulse can vaporize the solid sample.
  • Cold Vapor Cell: These cells are specially designed to determine mercury concentrations in the sample. The dissolved mercury is converted into elemental mercury by reacting it with SnCl2. The elemental mercury obtained is then transported into a quartz cell with the help of gas flow.
  • Hydride Generating Cell: This cell is used for the elements that form the hydrides such as antimony, arsenic, selenium, and tellurium. The sample is treated with sodium borohydride and hydrochloric acid to generate a volatile hydride of the analyte. It is then carried to the atom cell with the help of an inert gas. 

III. Monochromator

A monochromator selects the desired radiation from a polychromatic light for analyzing a sample. Usually, the AFS uses diffraction gratings as monochromators which can maintain a high resolution over a range of wavelengths.

IV. Detector

The common detectors used in AFS are the photomultiplier tubes, like those in UV-visible spectroscopy.

V. Readout System

The output from the detector is suitably amplified and displayed on a readout device like a meter or digital display. 

Interference of Atomic Fluorescence Spectroscopy

When the sample is irradiated with the radiations, there is an interference which is produced by the non-analyte molecules present in the sample. This can be because of the chemical reactions(chemical interference) between the molecules in the analyte or because of the radiations other than fluorescence(spectral interference).

Chemical interference reduces the number or percentage of gaseous atoms in the analyte while spectral interference decreases the resolution of the spectrum.

Conclusion

Atomic Fluorescence Spectroscopy is a quantitative technique that is based on the fluorescence produced by the sample to be analyzed. It is a type of spectroscopy that involves the absorption and emission of electromagnetic radiation.

In some cases, it is preferred over atomic absorption and emission spectroscopy due to its high sensitivity. It is a reliable and reproducible analytical technique but is only limited to metals and metalloids. Hence it is not very popular.

Suksham Gupta

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