Atomic Emission Spectroscopy

The technique is also known as optical emission spectroscopy. This is the study of radiations which is emitted by the excited atoms or ions. Relaxation of atoms in the excited state results in the emission of light.

Atomic emission spectroscopy (AES) has long been a typical process for metal examination. Modern advancements in non-combustion plasma sources have increased the number of applications and produced new interest in AES.

In atomic emission spectroscopy, a small part of the sample is vaporized and thermally excited to the point of atomic emission. The energy needed for these methods is supplied by an electric arc or spark, or more recently by a laser or plasma constituted of the noble gas. The atomic spectrum emitted by a sample is used to identify the structure of elements. The wavelength at which the intensity measure is made identifies the element, whereas the intensity of the transmitted radiation identifies its concentration.

It produces the line spectra in the UV-Visible and the vacuum UV regions. It can be used for the qualitative identification of elements which is present in the sample. It is also used for quantitative examination from ppm levels to percent. It is a multi-element technique. Exciting atoms in the flame emit light that reaches the detector.

History

It was proposed by two groups of scientists A. Walsh from Australia and Alkamade in the year 1955 from Netherland.

Principle

• The procedure makes use of absorption spectrometry to estimate the concentration of an analyte in a sample.

• It needs measures with known analyte content to verify the relation between the measured absorbance and the analyte concentration and relies therefore on the beer-lambert law.

• Emission spectroscopy is a spectroscopic procedure that measures the wavelengths of photons transmitted by atoms or molecules during their transition from an excited energy level to a lower energy level.

• Each element transmits a specific set of discrete wavelengths according to its electronic structure, by following these wavelengths the elemental composition of the sample can be identified.

• The wavelength of atomic spectral lines gives the identity and the radiation of light are proportional to the atom.

• Flame is used as the light source in AES. In AES, atomization takes place step by step upon the initiation of the sample to the flame.

Beer-Lambert Law

This law states that the absorbance of a chromophore in a transparent solution varies linearly with both the sample cell, path length, and chromophore concentration. It is accurate for a variety of chromophores, solvents, and concentrations and it is widely utilized in quantitative spectroscopy. Absorption is measured in a spectrophotometer by passing a collimated beam of light at wavelength through a plane parallel slab which is normal to the beam.

For liquids, the solution is held in an optically flat and transparent container called a cuvette.

So, the absorbance is calculated from the ratio of light energy passing through a sample to the energy that is incident to the sample.

Instrumentation

• Burner assembly [Atomizer]
• Sample
• Monochromator
• Detector
• Recorder

Burner assembly [Atomizer]

• The most commonly used is the premix burner.
• It is the heart of the emission spectrometer.
• Nebulizer introduces sample aerosol into the base of the flame.
• Free atoms are formed and excited in flame.
• Exciting free atoms emit radiant energy.
• Dissolvation, vaporization, atomization, excitation, and emissions.
• These steps happen in this stage.

Sample insertion

Nebulizers commonly used Pneumatic and Cross-flow.

Monochromator

Select the desired wavelength and block unnecessary wavelength of emitted light and pass it to the detector.

Detector

A PMT is used as a detector.
Photo multiplier tube is generally used in spectroscopy. It consists of an evacuated tube containing one photocathode and 9 to 16 electrodes known as dynodes. Photocathode is negatively charged and then a photon hits the photocathode. Emission of electrodes takes place due to photovoltaic effect. When incident radiation falls on the metal surface of cathode and get emitted which are attracted towards the first dynode. These with are attracted by the second dynode and with are emitted by second dynode. Hence, the process is repeated at all the dynodes. At the end of dynode chain and anode is present which acts as collector of electrons. The current flowing from anode is directly proportional to photo electron flux generated by photo cathode.

Photographic Detection

Photographic materials consist of a light-sensitive emulsion coated on a glass plate or plastic film. The emulsion contains light sensitive crystals of silver halides suspended in gelatin. On exposure, the silver halide crystals that receive radiation form a latent image. It converts the silver halide crystals into a black deposit of silver at the site of the latent image. After development, the emulsion is fixed in a solution that dissolves the unexposed silver halides. Finally, the photographic material is washed thoroughly to remove the chemicals used in developing and fixing. The entire series of operations follows rigidly controlled conditions with respect to time, temperature, and chemicals.

Recorder

Readable form of result.

Application

• It can be used to determine metals, metalloids, and some non-metals simultaneously.

• It does not need a light source.

• It is helpful in analysis of ferrous and nonferrous alloys.

• It is also help in identification of metals in geological, environmental, and biological materials.

• Water analysis ICP instruments have been coupled with mass spectrometers to provide a powerful analytical technique.