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Flame emission & atomic absorption spectroscopy

Details about flame emission and atomic absorption spectroscopy and comparison between them.

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Flame emission & atomic absorption spectroscopy

  1. 1. Flame emission and Atomic absorption Spectroscopy Presented by: Guided by: Himal Barakoti Arup Chakroborty M. Pharm, 1st Sem Assistant Professor Department of Pharmacy Department of Pharmacy Assam Down Town University Assam Down Town University
  2. 2. Contents:  Introduction  Theory and principle of Flame emission Spectroscopy  Instrumentation  Interferences  Application  Atomic Absorption Spectroscopy  Theory and Principle 2
  3. 3. Contents:  Instrumentation  Interferences  Application  Comparison between Flame emission and Atomic absorption Spectroscopy  References 3
  4. 4. Introduction  Atomic spectroscopy is thought to be the oldest instrumental method for the determination of elements.  These techniques are introduced in the mid of 19th Century during which Gustav Kirchhoff and Robert Bunsen showed that the radiation emitted from the flames depends on the characteristic element present in the flame.  The developments in the instrumentation area led to the widespread application of atomic spectroscopy.  The absorption and emission of radiant energy by atoms provide powerful analytical tools for both quantitative and qualitative analysis of substance. 4
  5. 5. Flame emission Spectroscopy  Flame emission spectroscopy (FES) is a method of chemical analysis that uses intensity of light emitted from flame, arc or spark at particular wavelength to determine quantity of element in sample.  The basis of flame photometric working is that, the species of alkali metals (Group 1) and alkaline earth metals (Group II) metals are dissociated due to the thermal energy provided by the flame source.  Due to this thermal excitation, some of the atoms are excited to a higher energy level where they are not stable.  The absorbance of light due to the electrons excitation can be measured by using the direct absorption techniques. 5
  6. 6.  The subsequent loss of energy will result in the movement of excited atoms to the low energy ground state with emission of some radiations, which can be visualized in the visible region of the spectrum.  The intensity of radiation emitted by these excited atoms returning to the ground state provides the basic for analytical determination in FES.  The wavelength of emitted light is specific for specific elements.  In flame emission spectroscopy, o Wavelength of spectral lines give identity of elements. o Intensity of emitted light is directly proportional to the number of atoms present. 6
  7. 7. Instrumentation  Flame photometers are the simplest type of atomic spectrometers.  Solution is introduced into a fine spray.  Solvents evaporates leaving dehydrated salt.  Certain fraction of atoms absorbs energy and are raised to excited state.  These excited atoms on returning to ground state emits photons of certain wavelength.  Flame emission passes through monochromator which filters all emitted light expect the wavelength of our interest.  Photoelectric Detector measures the intensity of filtered light. 7
  8. 8. 1. Atomizer/Nebulizer  Process of conversion of sample to a fine mist of finely divided droplets using a jet of compressed gas. o Pneumatic nebulizers o Electro thermal vaporizer o Ultrasound nebulizer 2. Burner  They must have ability to evaporate the liquid droplets from sample solution and capacity to excite atoms formed and cause them to emit radiant energy. o Mecker burner o Total consumption burner o Laminar flow burner o Shileded burner o Lundergraph burner 8
  9. 9. 3. Monochromator (Grating or prism monochromator) They disperse radiation coming from the flame and falling on it. Dispersed radiation goes to detector from exit slit. Filters chosen has wavelength range transparent to emission from element of interest. 4. Detectors (photoemissive cells or photomultiplier tubes) They measure intensity of radiation falling on it 5. Amplifier & readout devices It suitably amplify and display detector’s output. Amplifier sensativity can be changed to be able to analyze sample of varying concentration. 9
  10. 10. Types of flame used The most common instrument uses air as oxidant. The temperature of the flames produced is relatively low so the technique is only suitable for the elements that are easily excited such as alkali and alkali earth elements. 10 Flame Temp/˚C Gas/Air 1700-1900 Gas/O2 2700-2800 H2/Air 2000-2100 H2/O2 2550-2700 C2H2/Air 2100-2400 C2H2/O2 3050-3150 C2H2/N2O 2600-2800
  11. 11.  A higher temperature will tend to increase the number of atoms in the excited state and hence the signal.  Some detection limits: 11 Elements Spectral lines/nm Flame Detection limits/ppm Al 396 C2H2/N2O 0.1 Ba 553 C2H2/N2O 0.01 K 766 C2H2/O2 0.001 Li 671 C2H2/N2O 0.0001
  12. 12. Interferences  Number of factors beside the analyte affect the intensity of emitted radiation. The analytical signals often include contributions from constituents other than analyte termed as interferences and are found to interfere the outcome of procedure.  3 types: 1. Spectral Interferences: interference that affect spectral intensity or resolution. a. 1st type of interference arises when two elements exhibit spectra, which partially overlap, and both emit radiation at particular wavelength. The detector cannot distinguish between the source of radiation and records the total sign, thus resulting in incorrect answer. This type of interferences are more common in high flame temperature as numerous spectral lines are produced in high temperature. E.g., Fe line at 324.73 nm overlaps with Cu line at 324.75 nm. This Can be overcome by taking alternative wavelength with no overlap. 12
  13. 13. b. 2nd type of spectral interference deals with spectral lines of two or more elements which are close but their spectra do not overlap. The filter may allow spectral lines separated by 5.0-10.0 nm to pass through, thus resulting in error in analysis. This can be reduced by increasing the resolution of spectral isolation system. c. 3rd type of spectral interference occurs due to presence of continuous background which arises due to high concentration of salts in the sample. Some organic solvents also produce continuous background. This can be corrected by using suitable scanning technique. 2. Ionization Interference: In certain cases, high temperature flame may cause ionization of the metal atoms. E.g., In case of sodium Na Na+ + e- The Na+ ion possesses an emission spectrum of its own frequencies, which are different from those of Na atom. This reduces the radiant power of atomic emission. This type of interference can be eliminated by addition of large quantity of potassium salt to standard and sample solution. Potassium itself undergoes ionization due to low ionization energy and suppresses the ionization of sodium. 13
  14. 14. 3. Chemical Interferences: This arises out of reaction between different interferents and the analyte. a. Cation-cation interference: The presence of certain anions, such as oxalate, phosphate, sulphate and aluminate in a solution may affect the intensity of radiation emitted by an element, resulting in serious analytical error. For e.g., calcium in presence of phosphate ion forms a stable substance as Ca3(PO4)2 which does not decompose easily, resulting in production of lesser atom. Thus calcium signal is depressed. This can be removed either by extraction of anion or by using calibration curve prepared from standard solutions containing same concentrations of the anion as found in the sample. b. Cation-cation interference: In many cases, mutual interferences of cations results in reducing element signal intensity. These interference are neither spectral nor ionic in nature and the actual mechanism has not been well understood. E.g., Al interferes with Ca and Mg. Na interferes with Mg. c. Oxide formation: arises due to formation of stable metal oxide if oxygen is present in flame, resulting in reduced signal intensity. E.g., Alkaline earth metal. This can be eliminated by either using high flame temperature to dissociate the oxides or by using oxygen-deficient environment to produce excited atom. 14
  15. 15. Limitations  As natural gas and air flame is employed for excitation, the temperature is not high enough to excite transition metals, therefore the method is selective towards detection of alkali and alkaline earth metals.  Low temperature makes this method susceptible to certain disadvantages, most of them related to flame stability and aspiration conditions. Fuel and oxidation flow rates and purity, aspiration rates, solution viscosity affects these.  FES is a means of determining the total metal concentration of a sample; it tells us nothing about the molecular form of the metal in original sample.  Only liquid samples can be used. 15
  16. 16. Applications  Flame Photometers are widely used in quality control where a simple and quick determination of alkali or alkali earth metal is required. They have the advantage of being significantly lower priced than most other atomic spectrometers.  Biological/medical applications- notable applications are the determinations of Na, K, Ca and Mg in body fluids and other biological samples.  Food industry – Determination of calcium and iron in beer 16
  17. 17. Atomic Absorption Spectroscopy  Guystav Kirchoff and Robert Bunsen first used atomic absorption spectroscopy—along with atomic emission—in 1859 and 1860 as a means for identify atoms in flames and hot gases.  Modern atomic absorption spectroscopy has its beginnings in 1955 as a result of the independent work of Alan. C. Walsh and C. T. J. Alkemade. 17
  18. 18.  Atomic absorption spectroscopy (AAS) is an absorption spectroscopic method where radiation from a source is absorbed by non-excited atoms in vapour state.  AAS deals with the absorption of specific wavelength of radiation by neutral atoms in the ground state.  Reliable for detecting over 70 elements with metals and metalloids. 18
  19. 19. Theory and Principle  The technique uses basically the principle that free atoms (gas) generated in an atomizer can absorb radiations at specific frequency.  AAS quantifies the absorption of ground state atoms in the gaseous form.  Atoms absorb UV/visible light and make transition to higher electronic energy levels. The analyte concentration is determined from the amount of absorption.  Concentration measurement are usually determined from a working curve after calibrating instrument with standard known concentration. Liquid sample Formation of droplets Fine residue Formation of neutral ion Neutral atom absorbs specific wavelength of radiation from hollow cathode lamp Measurement of intensity of radiation absorbed by the neutral atom using the detector 19
  20. 20.  The extent to which radiation of a particular frequency is absorbed by an atomic vapour is related to the length of the path transverse and to the concentration of absorbing atoms in the vapour.  This is analogues to the Beer-Lamberts law relating to samples in solution. Thus, for a collimated monochromatic beam of radiation of incident Io passing through an atomic vapour of thickness I Iv = Io eл-kvI Where, Iv is the intensity of the transmitted radiation Kv is absorption co efficient 20
  21. 21. Instrumentations21
  22. 22. 1. Sharp line radiation source (hollow cathode lamp)  Hollow cathode lamps are most commonly used radiation containing tungsten anode and hollow cylindrical cathode made of element to be determined.  These are sealed with gas tube filled with an inert gas.  When a current flows between the anode and cathode in this lamps, metal atoms are sputtered from the cathode cup, and collisions occur with the filler gas. A number of metal atoms become excited and give off their characteristics radiation. 22
  23. 23. 2. A solution Nebulizer and burner  Nebulizer suck up liquid samples at controlled rate and create a fine aerosol spray for introduction into flame.  Aerosol, fuel and oxidant are thoroughly mixed for introduction into flame.  Atomization is carried out by separation of particles into individual molecules and breaking molecules into atoms. This is done by exposing analyte to high temperature in flame or graphite furnace.  In graphite furnace, samples are deposited in a small graphite coated tube which can then be heated to vaporize and atomize the analyte.  Graphite tubes are heated using a high current power supply. 23
  24. 24. 3. Monochromator  They are used to select specific wavelength of light which s absorbed by the sample, and to exclude other wavelengths.  Selections of specific light allows the determination of selected element in the presence of others. 4. Detectors  The light selected by monochromator is directed onto a detector that is typically a photomultiplier tube which convert the light signal into electrical signal proportional to the light intensity.  The signal is amplified, displayed for readout of fed to data station for print out. 24
  25. 25. Interferences  Spectral Interferences: Spectral interferences are caused by presence of another atomic absorption line or a molecular absorbance band close to the spectral line of element of interest.  Chemical interference: If a sample contains a species which forms a thermally stable compound with the analyte that is not completely decomposed by the energy available in the flame then chemical interference exists. Refractory elements such as Ti, W, Zr, Mo and Al may combine with oxygen to form thermally stable oxides. Analysis of such elements can be carried out at higher flame temperatures using nitrous oxide – acetylene flame instead of air-acetylene to provide higher dissociation energy. 25
  26. 26.  Matrix interference: When a sample is more viscous or has different surface tension than the standard it can result in differences in sample uptake rate due to changes in nebulization efficiency. Such interferences are minimized by matching as closely as possible the matrix composition of standard and sample.  Ionization interference: Excess energy of the flame can lead to excitation of ground state atoms to ionic state by loss of electrons thereby resulting in depletion of ground state atoms. 26
  27. 27. limitations  Expensive  Low precision  Low sample throughput  Requires high level of operator skill  Sample must be in solution or at least volatile  Individual source lamps required for each element 27
  28. 28. Applications  Determination of metal at trace level in solution.  Determination of purity: o Presence of heavy metal in body fluids o Pollution of water by metals o Food stuffs o Soft drinks and beer o Analysis of Geochemical exploration for mineral o Soils, crude oils, petroleum products, plastics. 28
  29. 29. Comparison between FES and AAS Flame Emission spectroscopy Atomic Absorption spectroscopy 1. Amount of light emitted by excited atom is measured. 1. Amount of light absorbed by ground state atom is measured. 2. Absorption intensity and signal response greatly influenced by temperature variation. 2. Absorption intensity and signal response does not depend upon temperature. 3. Beer’s law is not obeyed. 3. Beers law is obeyed. 4. Intensity of emitted radiation is directly proportional to the number of atoms in excited state. 4. Intensity of absorbed radiation is directly proportional to the number of atoms in ground state. 5. Relation between emission intensity vs. concentration is not much linear. 5. Absorption intensity vs concentration of analyte is much linear. 6.Atomization and excitation flame used. 6. Atomization flame used. 7. Intensity vs concentration data is obtained. 7. Absorbance vs concentration data is obtained 8. Limited to alkali and alkali earth metals. 8. Useful for more than 70 metals. 29
  30. 30. References  G. Gauglitz and T. Vo-Dinh “Handbook of spectroscopy” pg 421-493  J. Michael Hollas “Modern Spectroscopy” 4th Ed, pg 64-65     Map%3A_Analytical_Chemistry_2.0_(Harvey)/10_Spectroscopic_Methods/10.4%3 A_Atomic_Absorption_Spectroscopy  30
  31. 31. Thank you 31