When electromagnetic radiation interacts with matter, there are a lot of phenomena occur such as absorption, emission, luminescence, etc. Luminescence is a process of light emission which is not a result of heat. It is a spontaneous light emission that is a result of a chemical reaction or enzymatic reaction in a substance. 

Fluorescence is a form of luminescence caused by photons exciting a molecule, raising it to an electronically excited state. It is a result of the absorption of photons in the singlet ground state promoted to a singlet-excited state. As the excited molecule returns to the ground state, it emits a photon of lower energy, which corresponds to a longer wavelength, than the absorbed photon.

Atomic Fluorescence Spectrometry(AFS) is an analytical technique that is used to determine the concentration levels of different elements in a diverse range of samples. AFS monitors the fluorescence emission from the excited state.

The development of atomic fluorescence spectrometry as an analytical technique is credited to Wineforder and West who did the pioneering work in this direction. 

What is The Basis 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 Atomic Fluorescence Spectroscopy also follows Lambert-Beer’s law, which is as follows:

P = P₀ e⁻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 a low concentration of the element. At higher concentrations, when fluorescence emission is high, part of 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

Atomic fluorescence spectrometry 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, namely:

1. Radiation Source

In AFS the excitation of the sample can be achieved by both the continuous and single-line sources. The continuous sources include the tungsten halide lamp or the deuterium lamp. While 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. 

2. 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 having 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 made of graphite is a common non-flame cells. It is in the shape of a bowl, in which the solid sample can be vaporized by a high current pulse.

  • Cold Vapor Cell

These cells are specially designed to determine the concentrations of mercury 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 which 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. This is then carried to the atom cell with the help of an inert gas. 

2. Monochromator

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

3. Detector

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

4. Readout System

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

Interferences in Atomic Fluorescence Spectroscopy

When the sample is irradiated with the radiations, there is 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.

Advantages of Atomic Fluorescence Spectroscopy

  • It is a highly sensitive analytical technique.
  • Samples in any state can be analyzed by AFS.
  • The analysis period is rapid and quick.
  • For some elements like mercury, arsenic, selenium, etc. lower detection limits can be identified.

Disadvantages of Atomic Fluorescence Spectroscopy

  • Sample preparation is time-consuming.
  • Due to chemical reactions chemical interference is caused.
  • It is limited to metals and metalloids which produce fluorescence.

Applications of Atomic Fluorescence Spectroscopy

  • It can be used in the analysis of metallic poisons.
  • In forensic serology, the concentration of metals in biological fluids can be determined by AFS.
  • It is applicable for the determination of metals in hair and fiber samples.
  • Metallic constituents in petrochemical products can be easily determined by AFS.
  • Ink analysis and questioned document examination can be performed by using AFS.

Conclusion

AFS 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 in practice.

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Frequently Asked Questions

1. Who Invented Atomic Fluorescence Spectroscopy?

It was invented in 1964 by Winefordner and Vickers.

2. What Are The Types of Atomic Fluorescence Spectroscopy?

There are 5 types of AFS i.e., Resonance fluorescence, Stokes direct line fluorescence, Stepwise line fluorescence, Two-step excitation or double resonance, Sensitized fluorescence.

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