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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 20

ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 20. DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university. CHAPTER 20 ATOMIC SPECTROSCOPY. ATOMIC SPECTROSCOPY. - Used for elemental analysis

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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 20

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  1. ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 20 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

  2. CHAPTER 20 ATOMIC SPECTROSCOPY

  3. ATOMIC SPECTROSCOPY - Used for elemental analysis - Deals with the absorption and emission of radiation by atoms - Deals with free atoms - Line spectra are observed - Can be used for both qualitative and quantitative analysis

  4. ATOMIC SPECTROSCOPY - Atomic spectra have narrow lines (~ 10-4 nm) Two Major effects That Cause Line Broadening (yield linewidths of ~ 10-3 to 10-2 nm) Doppler Broadening - Species may move towards or away from detector - Result in doppler shift and broadening of spectral lines Pressure Broadening - Species of interest may collide with other species and exchange energy - Increase in temperature results in greater effect

  5. ATOMIC SPECTROSCOPY - Liquid sample is sucked - Sample passes through a plastic tube into a flame - Flame breaks molecules into atoms (atomization) - Monochromator selects wavelength that reaches the detector - The concentration of elements is measured by emission or absorption radiation - Concentrations are measured at the ppm level

  6. ATOMIC SPECTROSCOPY Atomization - The process of breaking analyte into gaseous atoms Po P Light source monochromator (λselector) detector readout Flame Sample

  7. ATOMIC SPECTROSCOPY Source - Line source is required to reduce interference from other elements Hollow Cathode Lamp (HC) - Produces emission lines specific for the element used to construct the cathode - Cathode is made from the element of interest - Cathode must conduct current

  8. ATOMIC SPECTROSCOPY Electrodeless Discharge Lamp - A salt of the metal of interest is sealed in a quartz tube along with an inert gas - A radio frequency (RF) field excites the inert gas - Excited gas ionizes metal - Light intensity is about 100 times greater than that of HC - Less stable than HC

  9. ATOMIC EMISSION SPECTROSCOPY - Does not require light source - Excited atoms in the flame emit light that reaches the detector (luminescence) Techniques Based on Excitation Source - Flame Photometry - Furnace (Electrical Excitation) - Inductively Coupled Plasma

  10. ATOMIC EMISSION SPECTROSCOPY Qualitative Analysis - Techniques rely on specific emission lines Element Hg Cu Ag Zn K Emission Line (Ǻ) 2537 3248 3281 3345 3447

  11. ATOMIC EMISSION SPECTROSCOPY Quantitative Analysis - Techniques rely on intensity of emission lines I = kPoc k is a proportionality constant Po is the incident radiant power c is the concentration of emitting species

  12. ATOMIC EMISSION SPECTROSCOPY Flame Photometry - For liquids and gases - Most flame spectrometers use premix burner (sample, fuel, and oxidant are mixed before reaching the flamw) - Flame decomposes sample into metal atoms (M) - Oxides (MO) and hydroxides (MOH) may also form

  13. ATOMIC EMISSION SPECTROSCOPY Flame Photometry - Flame may be rich (rich in fuel) or lean - Rich flame reduces MO and MOH formation (excess carbon reduces MO and MOH to M) - Lean flame has excess oxidant and is hotter - Good for Groups 1A and 2A elements (easier to ionize)

  14. ATOMIC EMISSION SPECTROSCOPY Furnace (Electrical Excitation) - For liquids and solids - More sensitive than flame - Lower detection limits than flame (~ 100 times) - Requires less sample than flame - Graphite furnace is highly sensitive - Operates at a maximum temperature of 2550 oC

  15. ATOMIC EMISSION SPECTROSCOPY Inductively Coupled Plasma (ICP) - Makes use of plasma (partially ionized gas) - Similar to flame photometry but reaches much higher temperatures (greater than 10000 K) - More sensitive - A radio frequency (RF) is used to excite an inert gas (Ar) - Excited gas ionizes the sample

  16. ATOMIC ABSORPTION SPECTROSCOPY (AAS) - Atoms absorb light from the source - Unabsorbed light reaches the detector - Quantitative analysis is based on the absorption of light by free atoms - Makes use of Beer’s Law

  17. ATOMIC ABSORPTION SPECTROSCOPY (AAS) Drawback Flame Photometry - Most atoms remain in the unexcited state Furnace (Electrical Excitation) - Most atoms remain in the unexcited state Inductively Coupled Plasma (ICP) - Problem of atoms remaining in the unexcited state is minimal

  18. ATOMIC ABSORPTION SPECTROSCOPY (AAS) Compared to Emission Advantages - Less dependent on temperature - Fewer interferences - Better sensitivity Disadvantage - Quantitative analysis only - Only used for metals since most nonmetals form oxides

  19. EEFECT OF TEMPERATURE - More atoms are excited as temperature increases - However, most are still in the atomic state number Minimum energy for ionization T1 T2 T3 T1< T2< T3 Energy

  20. EEFECT OF TEMPERATURE - For a molecule with two energy levels Eo and E* - Ground state energy level = Eo - Excited state energy level = E* E* - Eo = ΔE - At atom (or molecule) may exist in more than one state at a given energy level - Number of states is referred to as degeneracies

  21. EEFECT OF TEMPERATURE Degeneracy at Eo = go Degeneracy at E* = g* E*, g* Emission Absorption ΔE Eo, go

  22. EEFECT OF TEMPERATURE Boltzmann Distribution - Describes relative populations of different states at thermal equilibrium - N*/No is the relative population at equilibrium - T is he temperature (K) - k is the Boltzmann’s constant (1.381 x 10-23 J/K)

  23. EEFECT OF TEMPERATURE The Excited State Population - Increase in temperature has very little effect on the ground state population (though an increase in population occurs) - Has no noticeable effect on the signal in atomic absorption - Increase in temperature increases the excited state population (however small) - Rise in emission intensity is observed

  24. EEFECT OF TEMPERATURE Atomic Absorption - Not sensitive to temperature variation Atomic Emission - Sensitive to temperature variation ICP is mostly used for emission

  25. BACKGROUND CORRECTION - Backgorund emission or absorption should be accounted for Two Common Approaches D2 Correction - Light from source and D2 lamp pass through sample alternately - D2 output is not very good at wavelengths greater than 350 nm Zeeman Correction - Atomic vapor is exposed to a strong magnetic field - Splitting of the atoms electronic energy level occurs - Background absorption can then be directly measured

  26. INTERFERENCE - Result of change in signal when analyte concentration is unchanged Spectral Interference - Overlap of analyte signal by other signals from other species or flame or furnace - Commonly caused by stable oxides Chemical Interference - Chemical reactions of other species with analyte - Caused by substances that decrease the extent atomization of analyte - Minimized by high flame temperatures

  27. INTERFERENCE Ionization Interference - Ionization decreases the concentration of neutral atoms - Prevalent in analysis of metals with low ionization energies (alkali metals) - Ionization suppressor may be added to decrease the ionization of analyte (CsCl is used for K analysis) - The method of standard addition eliminates interference - Known amounts of analyte are added to unknown - Standard addition curve is plotted

  28. INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY (ICP-MS) - Very sensitive and good for trace analysis - Plasma produces analyte ions - Ions are directed to a mass spectrometer - Ions are separated on the basis of their mass-to-charge ratio - A very sensitive detector measures ions - Very low detection limits

  29. INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY (ICP-MS) Drawback Isobaric Interference - Cannot distinguish ions of similar mass-to-charge ratio - HCl and H2SO4 create isobaric interferences so are avoided - 138Ba2+ interferes with 69Ga+

  30. SUMMARY Flame Absorption - Low cost - Different lamp required for each element - Poor sensitivity Furnace Absorption - High cost - Different lamp required for each element - High background signals - High sensitivity

  31. SUMMARY Inductively Coupled Plasma Emission - High cost - No lamp required - Low background signals - Low interference - Moderate sensitivity Inductively Coupled Plasma-Mass Spectrometry - Very high cost - No lamp required - Least background signals - Least interference - Very high sensitivity

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