Atomic absorption spectroscopy

BY- K. Sai Manogna (MSIWM014)

Atomic absorption spectroscopy (AAS) is a method in analytical chemistry for determining the concentration of a specific metal element in a sample. The process can be used in a solution to analyze the concentration of over 70 different metals. While atomic absorption spectroscopy dates from the nineteenth century, a team of Australian chemists primarily developed the modern form during the 1950s. They were headed by Alan Walsh and served in the Chemical Physics Division of the CSIRO (Commonwealth Science and Industry Research Organisation) in Melbourne, Australia. 

By applying characteristic wavelengths of electromagnetic radiation from a light source, atomic absorption spectrometry detects elements in either liquid or solid samples. Wavelengths can be absorbed differently by individual components, and these absorbances are calculated against expectations. In effect, AAS takes advantage of the various wavelengths of radiation that different atoms absorb. In AAS, analytes are first atomized so that their characteristic wavelengths are emitted and registered. When those atoms consume particular energy during excitation, electrons go up one energy level in their respective atoms. 

These atoms emit energy in the form of light as electrons return to their original energy state. There is a wavelength of this light that is characteristic of the element. According to the light wavelength and intensity, relevant elements can be detected, and their concentrations determined according to the light wavelength. 

Principle: 

The approach uses absorption spectrometry to determine an analyte’s concentration in a sample. Thus it relies heavily on the Beer-Lambert rule. In short, by consuming a given amount of energy, the atoms’ electrons in the atomizer can be promoted to higher orbitals for a short period. This quantity of energy is unique to a specific transformation of electrons in a particular element, and each wavelength corresponds to only one element in general. This gives its elemental selectivity to the process. 

Since the amount of energy placed into the flame is known and it is possible to calculate the amount remaining on the other side of the detector, it is possible to estimate from the Beer-Lambert law how many of these transitions have occurred and thus obtain a signal proportional to the concentration of the measured product.

The Instrumentation 

There are four components of the standard AAS instrument: the sample introduction region, the source of light (radiation), the monochromator or polychromator, and the detector. 

It needs to be atomized in order to test a sample for its atomic constituents. The light could then illuminate the sample. Finally, the light emitted is measured through a detector. A spectrometer is usually used between the atomizer and the detector to minimize the effect of the atomizer’s emission (e.g., black body radiation) or from the atmosphere. 

Types of Atomizer:

Usually, the method uses a flame to atomize the sample, but other atomizers are also used, such as a graphite furnace or plasmas, particularly inductively coupled plasmas. 

It is side-long (usually 10 cm) and not deep when a flame is used. The flame’s height above the burner head can be adjusted by changing the fuel mixture’s flow. At its longest axis (the lateral axis), a ray of light passes through this flame and reaches a detector. 

Liquid analysis 

A liquid sample is usually converted in three stages into an atomic gas: 

1. The liquid solvent is evaporated (Drying), and the dry sample remains 

2. Vaporization (Ashing)-the solid specimen vaporizes into a gas 

3. Atomization is divided into free atoms by the compounds that make up the sample. 

Sources of Radiation 

The chosen radiation source has a narrower spectral range than that of the atomic transitions. 

Cathode Hollow Lamps 

The most common source of radiation in atomic absorption spectroscopy is hollow cathode lamps. A cylindrical metal cathode holding the metal for excitation and an anode is inside the lamp, filled with argon or neon gas. Gas particles are ionized when a high voltage is applied to the anode and cathode. Gaseous ions gain sufficient energy to eject metal atoms from the cathode as the voltage increases. Some of these atoms are excited, releasing light with the characteristic frequency of the metal. Various modern hollow cathode lamps are selective for several metals. 

Lasers with diodes 

Lasers, especially diode lasers because of their strong properties for laser absorption spectrometry, can also conduct atomic absorption spectroscopy. The method is then either referred to as diode laser atomic absorption spectrometry (DLAAS or DLAS) or, since wavelength modulation is most commonly used, spectrometry of absorption of wavelength modulation. 

Context Methods of Correction:

The spectral overlap is unusual due to the limited bandwidth of hollow cathode lamps. That is, an absorption line from one element is unlikely to overlap with another. Molecular emissions are much larger, so a specific molecular absorption band is more likely to overlap with an atomic line. This can lead to artificially high absorption and an improperly high measurement of the solution concentration. In order to correct this, three methods are usually used: 

Zeeman correction: A magnetic field is used to break the atomic line into two sidebands. To still overlap with molecular bands, these sidebands are close enough to the initial wavelength, but far enough, they do not overlap with the atomic bonds. It is possible to equate the absorption in the presence and absence of a magnetic field, the difference being the absorption of interest atomically. 

Correction to Smith-Hieftje: This was invented by Stanley B. Smith and Gary M. Hieftje. The high current pulses the hollow cathode lamp, creating more significant atoms and self-absorption population during the pulses. This self-absorption allows the line to be broadened, and the line intensity decreases at the original wavelength. 

Deuterium lamp correction: In this case, for the calculation of background emissions, a different source known as a broad-emission deuterium lamp is used. The use of a specific lamp makes this method the least reliable, but this method is most widely used because of its relative simplicity and the fact that it is the oldest of the three.

Advantages of AAS are given below: 

1. Strong throughput of samples 
2. Simple to make use of 
3. High accuracy 
4. Inexpensive methodology 

Disadvantages/drawbacks of AAS are as follows: 

1. It is only possible to evaluate solutions. 
2. Less sensitivity compared to the furnace with graphite 
3. Relatively large quantities of samples are needed (1-3 ml) 
4. Difficulties with refractory components