Atomic Absorption Spectrophotometry. Presentation by, Mrs. Sangita J. Chandratre Department of Microbiology M. J. college, Jalgaon

Transcription

1 Atomic Absorption Spectrophotometry Presentation by, Mrs. Sangita J. Chandratre Department of Microbiology M. J. college, Jalgaon

2 Defination In analytical chemistry, Atomic absorption spectroscopy is a technique for determining the concentration of a particular metal element in a sample. Atomic absorption spectroscopy can be used to analyze the concentration of over 62 different metals in a solution

3 Atomic Absorption Spectrophotometry is designed to determine the amount(concentration) of an object element in a sample, utilizing the phenomenon that the atom in the ground state absorb the light of characteristic wavelength passing atoms. Apparatus Usually the apparatus consists of a light source, a sampleatomizer, a spectroscope, and a photometer, and a recording system. Some are equipped with a background compensation system. For the light source, a hollow cathode lamp and a discharge lamp are mainly used. To the sample-atomizer- the flame type, electrothermal type, and the cold-vapor type are applied.

4 The flame type composed Of regulator, a burner and a gas-flow The electro thermal type is composed of a electric furnace and a power source, and The cold-vapor type is composed of a mercury generator by chemical reduction-vaporization and thermal reductionvaporization and an absorption cell. The cold-vapor flameless type is categorized as the two methods. Reduction vaporizing method and heat vaporizing method. For the spectroscope, a grating for light diffraction or an interference filter prism is used.

5 Specifications of AAS Absorption by atoms than molecular species To observe optical properties of free atom the sample must be in gaseous state, requires volatilization of liquids or solids followed by dissociation of molecule to give free atoms To this free atoms measure, require a particular resonance wavelength, so in AAS hollow cathode lamps used Such cathode lamps produce monochromatic radiation characteristics of the element analyzed

6 The photometer mainly consists of a detector and a signal treatment system. A recording system is composed of a display and a recording device. A background compensation system is employed for the correction of matrix effects on the measuring system. Several principles can be utilized for background compensation, using the continuous spectrum sources, the Zeeman split spectrum, the non resonance spectrum, or the self-inversion phenomena.

7 The flame is arranged such that it is laterally long (usually 10cm) and not deep. The height of the flame must also be monitored by controlling the flow of the fuel mixture. A beam of light passes through this flame at its longest axis (the lateral axis) and hits a detector.

8 principle Atomic Absorption Spectroscopy The absorption of light by atoms in the ground state is measured when the sample is irradiated by an appropriate source. Solid samples and some liquids require pretreatment to destroy organic matter followed by dissolution of the inorganic matter in acid solution. The digestion process is carried out in strong acids (nitric, sulphuric or per chloric acids) and over the last decade microwave digestion has predominated This is then aspirated into a flame or injected into a graphite furnace.. The greatest advantage of AAS is its specificity.

9 Flame AAS (FAAS) The sample is drawn into the nebulizer by compressed air that propels the solution into a spray chamber where it forms an aerosol, mixes with fuel and then into the burner. The hollow cathode lamp provides narrow line width radiation: A potential of 100V across the lamp electrodes ionises the argon or neon fill gas which then strikes the cathode made of the metal being analyzed.

10 This releases atoms of the element and after excitation with the fill gas ions, a narrow line emission spectrum of the analyte is produced and shows up in the flame. Flame AAS requires at least 5 ml of solution and viscous solutions must be diluted. About 90% of the sample is rejected as large droplets are filtered out before reaching the flame.

11 How concentration measered? This amount of energy is specific to a particular electron transition in a particular element. As the quantity of energy put into the flame is known, and the quantity remaining at the other side (at the detector) can be measured, it is possible to calculate how many of these transitions took place, and thus get a signal that is proportional to the concentration of the element being measured.

12 When a solution of a metallic salt is sprayed into a flame, the solvent evaporates and fine salt particles are formed and are vaporised. Many of the molecular species are broken down and free atoms are released. Some of these atoms undergo chemical reactions or remain as ground state atoms. Particularly in the case of the alkali metals, a significant portion of free atoms have outer shell electrons thermally excited

13 from the ground state to a higher energy level. When these excited state electrons fall back to lower energies, light is emitted. The energies associated with these transitions correspond to wavelengths in the visible region of the spectrum and So color the flame, as in the case of sodium and potassium

14 The principal stages in the process are summarized below: Nebulisation M+ + A- (Solution) Desolvation M+ + A- (Aerosol) Liquefaction MA (Solid) Vaporisation MA (Liquid) Atomisation MA (Gas) Excitation M + A (Gas) Emission M* (Gas) M (Gas)

15 Diagram of flame AAS

16 Applications : Technique Property measured Applications Atomic absorption spectrometry Atomic fluorescence spectrometry Flame emission spectrometry Atomic absorption of the light from the lamp after atomization by flame or electrothermal means Atomic fluorescence emission after flame excitation Atomic emission after flame excitation Determination of trace metals and some non-metals at ppm levels Determination of mercury and arsenic at trace levels Determination of alkali and alkali earth metals

17 Other Applications Determination of trace elements in foods, plants and tobacco Elemental profiles of biological samples to monitor health. Trace elements in artificial fibres Determination of the mode of poisoning in food stuffs and drugs Hair analysis for heavy metal poisons Determinations of ammunition manufacturers through the measurement of trace elements in bullets and cartridges Discrimination of objects made of steel, aluminium, glass and precious metals.

18 Cleaning Procedure: Ensure that the instrument is in OFF position. Clean the Hollow Cathode Lamp with use of tissue paper. Clean Nebulizer with use of nebulizer clean wire. Clean Burner. Operating Procedure: Switch ON the main power supply. Switch ON the instrument by using ON / OFF switch, which is provided at the right side of the instrument. Switch ON the computer and air compressor. Open the Acetylene gas cylinder Outlet valve open half turn only and Regulator knob up to 1 bar pressure. Click the Data Station software logo to display the Database setup window.

19 Calibration Procedure: Prepare Cu standard (2, 4, 6, 8 ppm). Switch on the instrument. Put Cu lamp into lamp holder. Check lamp current 4mA, wavelength 324.8nm, slit width 0.5nm. Focus the lamp correctly by adjusting the circular patch of lamp exactly at the centre of lens (on monochromator side). This can be done by adjusting vertical and horizontal movement of the lamp. Take reading and plot a graph. If linear graph is found instrument is calibrated.

20 AAS -model 201 Make-Chemito Cost-4,25,000 Rs. Specifications-software + basic unit,compressor,hollow cathode lamp(multi lamp also avilable-cu,co,fe,mn,ni,pb, Zn,Hg, As,Cr) Wavelegthrange-190 nm to 900 nm Fuel-acetelene

21