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Atomic Absorption Spectroscopy and its application

M. Sc. Environmental Engineering Firat University - Elazig - TurkeyPrepared by Shaimaa F. Alsadawee

Content 1- Introduction 2- Definition of AAS 3- the principle of AAS 4- The main parts of AAS 5- How the AAS instrument works ? 6- Applications of AAS 7- Advantage and disadvantage of AAS

Introduction

Atomic absorption spectroscopy was first used as an analytical technique in 19th century and then developed during the 1950s by Alan Walsh. AAS known as the most powerful instrumental method for quantitative determination of trace metals in liquids. It gives total metal content of the sample , independent on molecular form of the metal, very reliable and simple to use, can analyze over 62 elements, and also can measures the concentration of metals in the sample.

What is AAS ?

Atomic absorption spectrometry (AAS) is an analytical technique that measures the concentrations of elements. Atomic absorption is so sensitive that it can measure down to parts per million of a gram in a sample. The technique makes use of the wavelengths of light specifically absorbed by an element. They correspond to the energies needed to promote electrons from one energy level to another, higher energy level.

The principles of the Instrument - It requires standards with known analyte content to establish the relation between the measured absorbance and the analyte concentration and relies therefore on the Beer-Lambert Law.

-each wavelength corresponds to only one element, and the width of an absorption which gives the technique its elemental selectivity. The radiation flux in the atomizer is measured using a detector, and the absorbance is converted to analyte concentration or mass - basically the principle that free atoms (gas) generated in an atomizer that can absorb radiation at specific frequency.

- Atomic-absorption spectroscopy quantifies the absorption of ground state atoms in the gaseous state .

The main parts of AAS

1- hollow cathode lamp : The hollow cathode lamp (HCL) uses a cathode made of the element with a low pressure of an inert gas. A low electrical current is imposed in such a way that the metal is excited and emits a few spectral lines characteristic of that element. The light is emitted directionally through the lamp's window, a window made of a glass transparent in the UV and visible wavelengths. It Provide the analytical light line for the element and Provide a constant intense beam of that analytical line.

2- Neublizer and flame : The nebulizer chamber thoroughly mixes acetylene (the fuel) and oxidant (air or nitrous oxide), and by doing so, creates a negative pressure that acts to uptake the liquid sample up the tube and into the nebulizer chamber. A small glass impact or a fixed impeller inside the chamber creates a heterogeneous mixture of gases and suspended aerosol. This mixture flows immediately into the burner head where it burns as a smooth, the flame is evenly distributed. Liquid sample not flowing into the flame collects on the bottom of the nebulizer chamber and flows by gravity through a waste tube to a glass waste container. this flame can Destroy any analyte ions and breakdown complexes by oxidation and Create atoms (the elemental form) of the element of interest.

3- The Monochromator and the detector: the monochromator seeks to only allow the light not absorbed by the analyte atoms in the flame to reach the detector. That is, before an analyte is aspirated, a measured signal is generated by the detector as light from the HCL passes through the flame. When analyte atoms are present in the flame, while the sample is aspirated. some of that light is absorbed by those atoms. This last is true inside the linear range for that element using that slit and that analytical line. The signal is therefore a decrease in measure light: atomic absorption spectroscopy.

4- Read out: The data collected by the instrument is analog based, and needs to be converted into a digital format for the display. This is accomplished by atransducer. The transducer sends the digital energy to theprocessor, frequencies, and domains of the signal. This signal is put into a sequence of numbers or symbols that can be displayed on a readout. Several types of readout devices are used in modern instruments. These devices include Digital Meters, LCD panels, or Computer Displays

Calibration CurveA calibration curve is used to determine the unknown concentration of an element in a solution. The instrument is calibrated using several solutions of known concentrations. The absorbance of each known solution is measured and then a calibration curve is between concentration and absorbance. The sample solution is fed into the instrument, and the absorbance of the element in this solution is measured .The unknown concentration of the element is then calculated from the calibration curve

How the AAS worksAtoms of different elements absorb characteristic wavelengths of light. Analyzing a sample to see if it contains a particular element means using light from that element. For example with lead, a lamp containing lead emits light from excited lead atoms that produce the right mix of wavelengths to be absorbed by any lead atoms from the sample. In AAS, the sample is atomized which converted into ground state free atoms in the vapour state and a beam of electromagnetic radiation emitted from excited lead atoms is passed through the vaporized sample. Some of the radiation is absorbed by the lead atoms in the sample. The greater the number of atoms there is in the vapour, the more radiation is absorbed. The amount of light absorbed is proportional to the number of lead atoms. A calibration curve is constructed by running several samples of known lead concentration under the same conditions as the unknown. The amount the standard absorbs is compared with the calibration curve and this enables the calculation of the lead concentration in the unknown sample.

The application of Atomic Absorption Spectroscopy

- Clinical analysis : Analyzing metals in biological fluids such as blood and urine.

- Environmental analysis : Monitoring the pollutant metal in the environment . For example finding out the levels of various elements in rivers, seawater, drinking water, air, and petrol.

-Industry : AAS is widely used to check that the major elements are present and that toxic impurities are lower than specified.

Pharmaceuticals: In some pharmaceutical manufacturing processes, minute quantities of a catalyst amount by using AAS in the process can be determined.

- For Quantitative & Qualitative analysis

- To analysis the Trace element of cosmetics

-Used to determine the trace elements in food analysis

Advantage of AASDisadvantage of aas High selectivity and sensitivityFast and simple workingDoesnt need metals separation

Analysis doesnt simultaneous can not read the ions of the elements Limit types of cathode lamp (expensive)