Atomic Absorption spectroscopy.pptx

AAS is an analytical technique used to determine the concentration of metal atoms/ions in a sample. Metals make up around 75% of the earth’s chemical elements. In some cases, metal content in a material is desirable, but metals can also be contaminants (poisons). 

1. Atomic Absorption Spectroscopy By Dr. Neetu Soni , SHUATS, Prayagraj , UP.

2. Alan Walsh, 1950’s introduced the atomic absorption spectroscopy. Walsh decided to measure absorption, not emission. AAS is an analytical technique used to determine the concentration of metal atoms/ions in a sample. Metals make up around 75% of the earth’s chemical elements. In some cases, metal content in a material is desirable, but metals can also be contaminants (poisons).  Atomic absorption spectroscopy (AAS) is based upon the principle that free atoms in the ground state can absorb light of a certain wavelength. Absorption for each element is specific, no other elements absorb this wavelength.

3. Atomic absorption spectroscopy has proved itself to be the most powerful instrumental technique for the quantitative determination of trace metals in liquids. The method provides a total metal content of the sample and is almost independent of the molecular form of the metal in the liquid for example one can determine the sodium content of water sample and in most of the cases it does not matter in what molecular form the sodium exits. Atomic absorption spectroscopy does not demand sample preparation. A disadvantage of the method is that only one element can be determined at a time, change in light source and a change of analytical wavelength are necessary to determine a second element.

4. Principle The absorption of energy by ground state atoms in the gaseous state forms the Atomic absorption spectroscopy. When a solution containing metallic species is introduced into a flame the vapor of metallic species will be obtained. Sum of the metal atoms may be raised to an energy level sufficiently high to emit the characteristic radiation of the metal- a phenomenon that is utilized in the familiar technique of emission flame photometry . But a large percentage of the metal atoms will remain in non- emitting ground state . These ground state atoms of the particular element are receptive of light radiation of their own specific resonance wavelength. Thus when a light wavelength is allowed to pass through a flame having atoms of metallic species, part of the light will be absorb and the absorption will be proportional to the density the of the atoms in the flame. Thus the atomic absorption spectroscopy, one determines the amount of light absorbed.

5. Disadvantages of Atomic absorption spectroscopy are as follows Separate lamp for each element to be determined is required This technique cannot be used very successful for emission of element like Al, Ti, W, Mo, si , etc because these element give rise to the oxide in the flame. In aqueous solution, the predominant anion effects the signal to a negotiable degree.

6. INSTRUMENTATION Radiation sources: The radiation source for Atomic absorption spectrophotometer should emit stable, intense radiation of the element to be determined, usually a resonance line of the element. Preferably the resonance spectral lines should be narrow as compared with the width of the absorption lines to be measured. These lines should not be interfered from other spectral lines which are not resolved by spectrophotometer. There should be no general background or other extraneous lines a emitting within the band pass of the monochromator . The problem of using such narrow spectral lines has been solved by adopting a Hollow Cathode Lamp as radiation source.

7. Instrumentation used to carry out atomic absorption spectrophotometry requires a source of light that matches the narrow bands of light that a particular atom absorbs (a hollow cathode lamp), a flame or graphite furnace to heat the sample, a monochromator to select the wavelength of light, and a photodetector .

8. Hollow cathode lamps (HCL) are discharge lamps designed for use in Atomic Absorption (AA) instruments. They consist of a cathode made from the element of interest, an anode and an inert filler gas contained in a glass envelope.

9. Electrodeless discharge lamp EDLs operate due to free electrons in the fill that are accelerated by the MW field energy. They collide with the gas atoms and ionize them to release more electrons .

10. Atomizer Flme atomizer : Total consumption burner and premix burner Non flame atomizer: Carbon atomiser and L‘vov Plateform   Background corrector: variety of background phenomena can interfere with the detector and influence the results. Known broadly as background absorption, issues like radiation scattering and molecular absorption can result in an incorrect measurement of the element content in a sample. Background correction is used to distinguish background absorption from elemental absorption, thus returning more accurate results.

11. Deuterium Background Correction Two separate lamps are used to determine levels of background absorption; a hollow cathode lamp (HCL) and a deuterium lamp. When the HCL is on and the deuterium lamp off, the total absorbance is measured. When the reverse is performed (deuterium on, HCL off), just the background absorption is recorded. The background absorption is then automatically subtracted from the total absorption to give the correct atomic result. Deuterium background correction does have its limitations, yet it remains the most frequently used and inexpensive method.

12. Zeeman-Effect Background Correction This correction technique is used mainly in graphite furnace atomic absorption. It is advantageous because it corrects high background levels and it requires only one light source, eliminating the issue of alignment. The Zeeman technique uses an alternating magnetic field to split the absorption line into three components. Like in the Deuterium method, total absorption is measured with the magnetic field off and background absorption with it on. The Zeeman technique is considered more accurate because it can detect any kind of background absorption. It is however much more expensive and there can be a loss in sensitivity.

13. Monochromator A monochromator is an optical device that transmits a narrow band of wavelengths of light or other radiation from a wider range of wavelengths. The atoms in the AAS instrument accept the energy of excitation and emit radiation. A desired band of lines can be isolated with a monochromator by passing a narrow band. The spectra through a monochromator can be shown by a curve.

14. Detector A detector can convert light coming from a monochromator to a simplified electrical signal. Generally, we used a photomultiplier tube as a detector in the atomic absorption spectroscopy instrument. A detector can be tuned to respond by a specific wavelength or frequency.

15. The recorder can receive electrical signals from the detector to convert them into a readable response. In atomic absorption spectroscopy instrumentation, today we used a computer system with suitable software for recoding signals coming from the detector. Recorder

16. Application of atomic absorption spectroscopy Today, the atomic absorption spectroscopy technique is the most powerful tool in analytical chemistry, forensic science environmental analysis, and food industries. It is popular for analysts due to several advantages. The most important advantage is the speed of analysis. It can analyze various samples within a day. Secondly, it is possible to determine all elements at trace  concentration . Thirdly, it is not always essential to separate the element before analysis because AAS can be used to determine one element in presence of another. The atomic absorption spectroscopy principle or instrumentation can be used to analyze sixty-seven metals and several nonmetals such as phosphorus and boron.