One instrument that can be used to determine an element in a sample is atomic absorption spectrophotometry (AAS).
The following will explain the principles, equations and parts.
- Principles of Spectrophotometry
The principle of spectrophotometry is the interaction between energy and matter. In atomic absorption spectroscopy occurs the absorption of energy by the atom so that the atom experiences an electronic transition from the ground state to the excited state. In this method, the analysis is based on measuring the intensity of the light absorbed by the atom so that excitation occurs.
To be able to occur the process of absorption of atoms required a monochromatic radiation source and a device to evaporate the sample so that the atom is obtained in a ground state of the desired element. Atomic Absorbtion Spectroscopy (AAS) is a spectroscopy that is based on the absorption of light by atoms. Atoms absorb light at certain wavelengths, depending on the nature of the elements.
Light at these wavelengths has enough energy to change the electronic level.
The AAS (Atomic Absorption Spectrophotometry) method is based on the absorption of light by atoms, the atoms absorbing the light at specific wavelengths, depending on the nature of the elements. For example Sodium absorbs at 589 nm, uranium at 358.5 nm while potassium at 766.5 nm.
The light in this wave has enough energy to change the electronic energy level of an atom. With energy absorption, means getting more energy, an atom in the ground state is raised to an excitation level.
If light with a certain wavelength is passed to a cell that contains the relevant free atoms then some of the light will be absorbed and the intensity of absorption will be directly proportional to the number of metal free atoms in the cell.
(Also read: Practice Questions on Spectrophotometry )
- AAS Equations (Lambert-Beer Law)
The relationship between absorbance and concentration is derived from:
Lambert’s Law: if a monchromatic ray source passes through a transparent medium, the intensity of the transmitted beam decreases with increasing thickness of the absorbing medium.
Beer’s Law: The intensity of the transmitted beam decreases exponentially with increasing concentration of the species that absorbs the beam.
From the two laws obtained an equation:
A = Ebc
E = intensity of the light source
= continued light intensity = molar absortivity
b = medium length
c = concentration of atoms absorbing light
A = absorbance
From the equation above, it can be concluded that the absorbance of light is directly proportional to the concentration of atoms (Day & Underwood, 1989).
(Also read: Getting to know the Spectrophotometer )
- Parts of AAS
- Cathode Lamp
Cathode lamps are a source of light in AAS. Cathode lamps have a lifetime or a lifetime of 1000 hours. The cathode lamp for each element to be tested varies depending on the element to be tested, such as the cathode lamp Cu, can only be used for measurement of the Cu element. Cathode lamps are divided into two kinds, namely:
• Cathode Lamps Monologam: Used to measure one element
• Cathode Lamps Multilogam: Used for measurements of some metals at the same time, it’s just more expensive.
- Gas cylinders
The gas cylinder used in AAS is a gas cylinder containing acetylene gas. The acetylene gas in AAS has a temperature range of ± 20,000K, and there is also a gas cylinder containing N2O gas which is hotter than acetylene gas, with a temperature range of ± 30,000K.
Ducting is part of the chimney to suck smoke or residual combustion in AAS, which is directly connected to the outer chimney on the roof of the building, so that the smoke produced by AAS, is not harmful to the surrounding environment. Smoke produced from combustion in AAS, is processed in such a way as in ducting, so that the pollution produced is not dangerous.
The compressor is a separate device from the main unit, because this tool serves to supply the air needs to be used by AAS, when burning atoms.
The burner is the most important part in the main unit, because the burner serves as a place for mixing acetylene gas, and aquabides, so that it is mixed evenly, and can burn on the lighters properly and evenly.
- Exhaust in AAS
Disposal on AAS is stored in the drigen and placed separately on the AAS.
It functions to isolate one of the resonant lines or radiation from the many spectrums produced by the hollow cathode lamp or to convert polychromatic rays into monochromatic rays as required by the measurement.
Two types of detectors are known, namely the photon detector and the heat detector. Heat detectors are used to measure infrared radiation, including thermocouples and bolometers. The detector functions to measure the intensity of the radiation that is transmitted and has been converted into electrical energy by the photomultiplier. The results of the detector measurements are strengthened and recorded by a recording device in the form of a printer and a number observer. There are two types of detectors as follows:
• Light Detector or Photon Detector Photon
detectors work based on the photoelectric effect, in which each photon will free electrons (one photon one electron) from materials that are sensitive to light. Photon material can be Si / Ga, Ga / As, Cs / Na.
• Infrared Detector and Heat Detector
A common infrared detector is a thermocouple. A thermoelectric effect will occur if two metals which have different temperatures are joined together.