Page 3 ATOMIC ABSORPTION SPECTROSCOPY (AAS) concerns the absorption of radiation by the atomised analyte element in the ground state. Only applicable for the detection of trace metals.
Page 4 ATOMIC EMISSION SPECTROSCOPY (AES) In atomic emission spectrometry, atoms are thermally excited so that they emit light and the radiation emitted is measured. Only applicable to determination of alkali and alkaline earth metals.
Page 6 ATOMIC SPECTRA In an atom, electrons have specific and discrete energies in which electron are arranged in definite energy levels. When an electronic transitions (‘jumps’) from one energy level to another (by an electric arc,temperature or flame), it emits or absorbs light – a photon – with a discrete, specific wavelength, the collection of all these specific wavelengths ( spectral lines) form the spectrum of the atom and it will be the characteristic of particular atom…so atomic spectra are the spectra of atoms.
Page 9 ATOMIC ABSORPTION SPECTRA When an electron is excited to a higher energy level it must absorbed energy. The energy absorbed as an electron jump from an orbit of low energy to one of the higher energy is characteristic of that transition. This mean that the excitation of electron in a particular element result in energy absorption at specific wavelength it will be the characteristic of particular atom thus in addition to emission spectrum every atom possess a characteristic absorption spectrum.
Page 10 For example In gaseous medium, sodium atoms are capable of absorbing radiation of wavelength characteristic of electronic transition from 3s state to higher excited states, sharp absorption peaks are observed experimentally.
Page 11 ATOMIC EMISSION SPECTRA
Page 12 For example When a sodium salt is heated in a flame the outer electron in the volatilized atoms are excited and returned to ground state with emission of energy, which appears as a yellow light (wavelength 589.5) The major line in the sodium emission spectrum is due to an electron falling from 3p excited state to 3s ground state. Common atom which give their bands in the emission spectrum are Ca, Ba, Na, Li, k.
Page 13 Continuous spectra: It emitted by solids and are characterized by the absence of any sharp lines as a function of wavelength. Band spectra It consists of group of lines. Each group is characteristic of wavelength that becomes closed spaced as they approach the end of the band.The band also called molecular spectra band. Since radiation is emitted out by the excited molecules. Line spectra: These consist of sharply defined and often widely irregularly spaced individual lines of a single wavelength. These spectra are characteristics of elements and are duo to the excitation of gaseous atoms or atomic ions. Hence, line spectra are also called atomic spectra TYPES OF EMISSION SPECTRA
Page 14 Continuous emission spectrum Band emission spectrum Line emission spectrum
Page 15 COMPARISON OF THE ABSORPTION(a) AND EMISSION LINES (b)OF SODIUM
Page 17 PRINCIPLE OF ATOMIC ABSORPTION SPECTROSCOPY The absorption of energy by ground state atoms in the gaseous state forms the basis of atomic absorption spectroscopy. By the help of atomic absorption spectroscopy, one can determine the amount of light absorbed
Page 18 In atomic absorption spectroscopy: Atoms of a metal are volatilized in a flame and their absorbance of a narrow band of radiation produced by a hollow cathode lamp, coated with the particular metal being determined is measured. Absorption will be proportional to the density of atoms in flame.
Page 19 Once absorption is known the concentration of the metallic element can also be known because absorption is proportional to concentration of atoms in the flame. Mathematically, the total amount of light absorbed is given by:
Page 20 PRINCIPLE OF ATOMIC EMISSION SPECTROSCOPY Atomic emission spectroscopy involves the measurement of electromagnetic radiation emitted from atoms. I n this, atoms are thermally excited so that that emit light and the radiation emitted is measured. Both qualitative and quantitative data can be obtained from this type of analysis.
Page 21 The process is as follows: LIQUID STATE FORMATION OF DROPLETS FINE RESIDUE FORMATION OF NEUTRAL ATOMS EXCITATION OF ATOMS EMISSSION OF RADIATION OF SPECIFIC WAVELENGTH WAVELENGTH AND INTESITY OF EMITTED RADIATION The intensity of emitted radiation depends upon the proportion of thermally excited atoms, which in turns depends upon the temperature of flame.
Page 22 Fraction of free atoms thermally excited The wavelength of the radiation emitted is used to identify the element (Qualitative Analysis). The intensity of the radiation emitted depends upon the concentration of element analyzed (Quantitative Analysis)
Page 26 HOLLOW CATHODE LAMP CONSTRUCTION: The lamp consists of thick walled glass envelope which has a transparent window of glass and silica affixed to one end. It consists of one anode and cup shaped cathode which are both connected to tungsten wire. The tube is filled with highly pure inert gas at low pressure of 1 to 2mm. The gases generally used are neon,argon or helium. Mica sheets are placed inside the lamp to limit the radiation to with in the cathode. The choice of window material depend upon the wavelength of the resonance lines of the element concerned.
Page 27 HOW IT WORKS: A potential of about 500v is applied between the electrodes and the current of 2-30mA is used. The filler gas becomes charged at the anode and the ions produced are attracted to the cathode and accelerated by the field. Bombardment of these ions on the inner surface of cathode causes metal atom to sputter out of the cathode cup. Further collisions excite these metal atoms and simple, intense characteristic spectrum of the metal is produced.
Page 29 ELECTRODELESS DISCHARGE LAMP CONSTRUCTION: In this a small amount of the metal or salt of the element for which the source is to be used is sealed inside the quartz bulb. This bulb is placed inside a small self contained RF generator or Driver
Page 30 HOW IT WORKS: When power is applied to the driver, an RF field is created. The coupled energy is vaporized and excite the atom inside the bulb, causing them to emit their characteristic spectrum. The emission from an EDL is higher than that from an HCL, and the line width is generally narrower, but EDLs need a separate power supply and might need a longer time to stabilize.
Page 31 ELECTRODELESS DISCHARGE LAMP
Page 32 ATOMIZERS The principle of atomic absorption requires light absorption by free atoms. However, elements in the sample are in a molecular form. The combination must be broken by some means to free the atoms. This is called ATOMISATION. The most popular method of atomization in AAS is flame excitation source.
Page 33 BASIC STEPS INVOLVED IN FLAME ATOMIZATION Nebulisation: conversion of sample into droplets. Desolvation: removal of solvents. Atomization: thermal or chemical breakdown of solid particles. Condensation of reaction product: removal of residue by exhaust flame gases.
Page 34 BURNERS Two major types of nebulizer burners are used in AAS are PREMIX NEBULISER BURNER and TOTAL CONSUMPTION BURNER. In premix type burner liquid is sprayed into mixing chamber where the droplets are mixed with combustion gas and are send to the burner. In the total consumption burner, nebulizer and burners are combined. This is also called TURBULENT FLOW BURNER
Page 35 MONOCHROMATORS Important device of the optical system of AAS. The function of this device is to separate the spectral line of interest from the other spectral line of different wavelength emitted by the hollow cathode lamp. The desired spectral lines is chosen with the preferred wavelength and bandwidth with the help of grating. GRATING Wavelength dispersion is accomplished with a grating, a reflective surface ruled with many fine parallel lines very close together. Reflection from this ruled surface generates interference phenomenon known as diffraction in which different wavelength of light diverge from grating at different angle.
Page 36 WORKING: Light from the source enters the monochromator at the entrance slit and is directed to the grating where dispersion takes place. The diverging wavelength of light are directed towards exit slit. By adjusting the angle of grating, a selected emission line from the source can be allowed to pass through exit slit and fall onto the detector
Page 37 DETECTOR The most commonly used in AAS is Photomultiplier tube whose output is fed into readout system. A PMT is an electronic tube that is capable converting the photon current into electrical signal and of amplifying this signal..
Page 38 WORKING: It consists of photo cathode and secondary electron multiplier. The photons impact on the cathode surface and sputter electrons from its surface. These electrons are accelerated in an electric field and impact on other electrodes so called dynodes from the surface of each impacting electron sputter secondary electrons. This cascade effect results in significance increase in number of electron. At the end electron impact on an anode and flow off to the mass. The resulting current is measured.
Page 39 READOUT SYSTEM It included meters, chart out device and digital display meter. These days microprocessor controlled system are commercially available where every thing can be done by touch of button.
Page 40 INSTRUMENTATION OF AES PRESENTED BY : HAFIZA RABEEA NISAR
Page 41 ATOMIC EMISSION SPECTROMETER Consists of three main parts: Emission source Optical system Detector
Page 42 EMISSION SOURCE Atomization and excitation of sample is done by emission source: Direct current plasma (DCP) Flame excitation source Inductively coupled plasma (ICP) Spark and arc Laser induced breakdown (LIB) Microwave induced plasma (MIP)
Page 43 1.Direct Current Plasma Excitation Source DCP is created by an electrical discharge between two electrodes. A plasma support gas is necessary (Ar) Sample deposited on one of the electrode. Solid samples, near the discharge so that ionized gas atoms sputter the sample into the gas phase where the analyte atoms are excited.
Page 44 2.Flame Excitation Source The sample solution is directly aspirated into the flame. All desolvation, atomization, and excitation occurs in the flame. Flame provides a high- temperature source for desolvating a sample.
Page 45 3.Inductively-Coupled Plasma Excitation Source Very high temperature excitation source that efficiently desolvates, vaporizes, excites, and ionizes atoms. Sample is nebulized and entrained in the flow of plasma support gas (Ar).
Page 46 The plasma torch consists of concentric quartz tubes, with the inner tube containing the sample aerosol and Ar support gas and the outer tube containing an Ar gas flow to cool the tubes. An RF generator produces an oscillating current in an induction coil that wraps around tubes, this coil creates an oscillating magnetic field, which produces an oscillating magnetic field, which in turn sets up an oscillating current in the ions and electrons of the support gas. These ions and electrons transfer energy to other atoms by collisions to create very high temperature plasma.
Page 47 4.Spark and Arc Spark and arc excitation sources use a current pulse (spark) or a continuous Electrical discharge (arc) between two electrodes to vaporize and excite analyte atoms. Samples are ground with graphite powder and placed into a cup-shaped lower electrode.
Page 48 5.Laser-Induced Breakdown High-energy laser pulse is focused into a gas or liquid, or onto a solid surface, it creates hot plasma. The energy of the laser- created plasma can atomize, excite, and ionize analyte species.
Page 49 6.Microwave Induced Plasma Consists of a quartz tube surrounded by a microwave waveguide. Microwaves produced by microwave generator fill the waveguide or cavity and cause the electrons in the plasma support gas to oscillate. The oscillating electrons collide with other atoms to create and maintain a high- temperature plasma.
Page 50 OPTICAL SYSTEM Once the sample has been introduced into the emission source, atomized, and excited, the emitted photons are diffracted by an optical system consisting of slits, mirrors, and gratings, etc. It may be a Monochromators or Diffraction grating.
Page Monochromators A Monochromators is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or radiation. Two designs are usually employed. 1. Czerny–Turner Monochromators 2. Echelle Monochromators
Page 52 Czerny–Turner Two mirrors are used to reflect and focus the polychromatic and diffracted beams. As the grating rotates, a different wavelength is focused onto the exit slit. Echelle Monochromators It’s made up of two dispersing elements. These elements are arranged in series. They have high resolution and dispersion property.
Page 53 2.Grating Gratings are reflective surfaces containing parallel, equally spaced lines. Their resolving power is proportional to the number of lines, which in turn depends on the line spacing. Diffraction gratings are used in optical system to disperse the emitted radiation.
Page 54 DETECTORS Photomultiplier tube READ OUT SYSTEM
Page 60 The sequential and simultaneous spectrometers are extensively used in the analytical laboratories. The sequential spectrometers are less expensive and more flexible but usually require a higher degree of operator skill and experience. On the other hand, the simultaneous spectrometers are more precise and accurate and are obviously more expensive.
Page 61 SEQUENTIAL SPECTROMETER
Page 62 SIMULTANEOUS SPECTROMETER 1.Polychromators 2.Solid state array based spectrometers
Page 66 INTERFERENCES IN ATOMIC ABSORPTION SPECTROPHOTOMETRY
Page 67 The presence of another atomic absorption line or a molecular absorption band close to the spectral line of the analyte element being monitored. This may be corrected by modulation of the radiation source and the detection system.. 1. Spectral Interferences
Page Chemical interferences These include interferences due to ionisation, formation of low volatility compounds, etc Such interferences can be avoided by increasing the flame temperature.
Page Physical interferences These may be due to variations in the gas flow rates, changes in the solution viscosity which may finally change the atomic concentration in the flame. This can be avoided by matching the matrix and by performing frequent calibrations.
Page 70 INTERFERENCES IN ATOMIC EMISSION SPECTROPHOTOMETRY
Page Spectral Interferences involves the overlap of the spectral lines of two or more elements in the matrix emitting radiation at the same wavelength. These can be minimized by using a high resolution dispersion system.
Page Chemical interferences The chemical interferences include molecular compound formation and ionization effects. These can be minimized by carefully controlling the operating conditions.
Page Physical interferences These interferences are associated with the processes of sample nebulisation and transport. The physical interferences can be reduced by diluting the sample.
Page 74 APPLICATIONS PRESENTED BY KIRAN ILLAHI PRESENTED BY KIRAN ILLAHI
Page 82 ADVANTAGES AND DISADVANTAGES PRESENTED BY: MADIHA KHALID
Page 83 ADVANTAGES OF AAS The atomic absorption technique is specific because the atom of a particular elements can only absorb radiation of their own characteristic wavelength i.e. the light of a particular frequency can easily be absorbed by the specific element to which it is characteristic. Atomic absorption spectroscopy is independent of flame temperature.
Page 84 DISADVANTAGES OF AAS A separate lamp for each element is required. This technique is unsuccessful for estimation of elements like Al, Mo; vanadium because, these elements give rise to red metallic oxidizes in the flame. In aqueous solution the signal is affected by predominant anion.
Page 85 ADVANTAGES OF AES It is very specific; the unique character of the wavelength pattern produced by each element is the reason of its specificity. The method is extremely sensitive, with this technique all metallic elements can be detected even if they are present in very low concentration (0.0001%). Even metalloids (arsenic, silicon, and selenium) have been identified. The method can be used for qualitative analysis as well as to determine concentration as low as 1 ppm.
Page 86 The analysis has the advantage of providing results rapidly because there are situation (e.g. in industrial processes) where time is more important than the high levels of accuracy. If automated, time required is just 30 seconds to a minute. A very small amount of sample (1-10mg) is sufficient for analysis.
Page 87 DISADVANTAGES OF AES The equipment is costly and wide experience is required for its successful handling and interpretation of spectra. The spectrograph is essentially a comparator, for quantitative analysis, standards (usually similar composition to the material under analysis) are necessary. For quantitative results, therefore, a problem is often posed for unknown samples. The sample is destroyed in the process of analysis.
Page 88 This method is not recommended for elements present to a greater extent than 3 percent since for quantities greater than 2-5 percent of the method does not offer accuracy as in gravimetric, titrimetric and some spectrophotometric measurements. The method is limited to the analysis of elements.