INSTRUMENTAL ANALYSIS CHEM 4811

1. INSTRUMENTAL ANALYSIS CHEM 4811 CHAPTER 6 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2. CHAPTER 6 ATOMIC ABSORPTION SPECTROMETRY

3. ATOMIC ABSORPTION - Absorption by gas phase free atoms in the ground state - Elemental analysis - Powerful tool for quantitative analysis - Can determine ppm and ppb levels of most metal elements - Minimized interference of analyte by other elements - Spectrometry instead of spectroscopy since it is exclusively used for quantitative analysis

4. ABSORPTION OF RADIANT ENERGY - Gas phase atoms absorb radiant energy and move to the excited state - No vibrational or rotational energy is involved in the electronic excitation - The result is few narrow absorption lines as opposed to broad bands by molecules - Energy is absorbed when the correct magnitude of ΔE is applied to the gas phase atom

5. ABSORPTION OF RADIANT ENERGY - This causes excitation in which ΔE = hν - Frequency of absorption can then be calculated if ΔE is known - Absorption lines due to transitions from the ground state are called resonance lines - Electrons in the excited state can absorb radiant energy and move to higher energy levels - The range of radiant energy absorbed falls within the UV-VIS region

6. ABSORPTION OF RADIANT ENERGY - Only valence electrons are excited by UV-VIS radiation (energy is not high enough to excite core electrons) - Electrons can spin in two ways so results in two similar energy levels - There are therefore two possible absorption lines Example - Sodium absorption lines are 589.5 nm and 589.0 nm

7. MAXWELL-BOLTZMANN EQUATION - For a molecule with two energy levels Eo and E1 - Ground state energy level = Eo - Excited state energy level = E1 E1 - Eo = ΔE - An atom (or molecule) may exist in more than one state at a given energy level - Number of states is referred to as degeneracies

8. MAXWELL-BOLTZMANN EQUATION - Degeneracy at Eo = go - Degeneracy at E1 = g1 E1, g1 Emission Absorption ΔE Eo, go

9. MAXWELL-BOLTZMANN EQUATION - Describes relative populations of different states at thermal equilibrium - N1/No is the relative population at equilibrium - k = Boltzmann constant = 1.381 x 10-23 J/K

10. MAXWELL-BOLTZMANN EQUATION - N1 = number of atoms in the higher energy level - No = number of atoms in the lower lower energy level - g = degeneracy of level - g1 = number of states having equal energy at level 1 - g1 = number of states having equal energy at level 2 - T = temperature (K)

11. MAXWELL-BOLTZMANN EQUATION - The total amount of radiation absorbed depends on the number of atoms available for excitation in the lower energy state - Ground state to excited state transitions are the greatest - Excited state to excited state transitions are rare - Lists of absorption frequencies of for AAS determination are available

12. MAXWELL-BOLTZMANN EQUATION - AAS is useful for ~ 70 metals and metalloids - The energy required for first excited state transition of nonmetals is greater than photons of greater 190 nm wavelength - Nonmetals cannot be excited by UV radiation but by vacuum UV

13. SPECTRAL LINEWIDTH - Atomic spectra have narrow lines (~ 10-4 nm) Factors That Cause Line Broadening 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 leading to line broadening - Increase in temperature results in greater effect

14. SPECTRAL LINEWIDTH Factors That Cause Line Broadening Stark Broadening - Strong local electrical field encountered by atoms Natural Linewidth - Uncertainty in the energy of electrons due to excited state lifetime giving rise to broadening - Zeeman splitting in the presence of magnetic field

15. EXTENT OF ABSORPTION - Absorbance (A) = fraction of absorbed radiant energy From Beer’s Law - A = abc = (constant x f)(b)(N/cm2) - f = the oscillator strength (constant for a particular transition) - N = number of ground state atoms in the light path

16. EXTENT OF ABSORPTION - The amount of radiation absorbed is slightly dependent on temperature - Low ionization energy (alkali metals) and high temperatures result in the formation of ions rather than atoms - Ions do not absorb at atomic absorption wavelengths so result in loss of signal

17. INSTRUMENTATION Components - Radiation source - Atomizer - Sample introduction device - Wavelength selector - Detector - Readout device and data processor

18. INSTRUMENTATION Atomization - The process of breaking analyte into gaseous atoms Po P Radiation source Atomizer (Flame) Readout Device monochromator (λselector) detector Sample Introduction Device

19. INSTRUMENTATION - Light from radiation source passes through an atomizer - Atomizer serves as the sample cell - Sample introduction device is used to introduce the sample into the atomizer - Atomizer converts sample to gas phase ground state atoms that absorb the incident radiation - Light from atomizer passes through monochromator to detector - Detector measures how much light is absorbed by sample

20. RADIATION SOURCE - Line source is required to reduce interference from other elements - Continuous radiation sources cannot be used due to very narrow absorption lines (would be difficult to detect lines) Two radiation sources are used - The hollow cathode lamp (HCL) and - The electrodeless discharge lamp (EDL)

21. RADIATION SOURCE Hollow Cathode Lamp (HCL) - Produces very narrow emission lines specific for the element used to construct the cathode - Cathode is a hollow cylinder of pure metal (must conduct current) - Cathode is made from the element of interest - Multielement HCLs are available but are not efficient - Cathode and an inert anode are sealed in a glass cylinder filled with Ar or Ne (filler gas) at low pressure

22. RADIATION SOURCE Hollow Cathode Lamp (HCL) - Quartz window is sealed to the end of the lamp (glass may be used for only VIS radiation) - High voltage is applied across the cathode and anode to emit narrow intense lines from the cathode element - Atoms from filler gas are ionized at the anode and are accelerated towards the cathode - Accelerated ions strike the surface of the cathode and surface atoms are displaced (sputtering)

23. RADIATION SOURCE Hollow Cathode Lamp (HCL) - Displaced atoms become excited upon collision with electrons - Excited atoms emit characteristic atomic emission lines - The emission lines are absorbed by the unexcited atoms in the sample - HCL provides high sensitivity, high selectivity, but have limited lifetime - Different HCL must be used for each different analyte element

24. RADIATION SOURCE Electrodeless Discharge Lamp (EDL) - An alternative of HCL used for volatile elements (e.g. germanium, cadmium, arsenic, selenium) - Small amount of metal or salt of the metal of interest is sealed in a quartz bulb - Bulb is filled with an inert gas (Ar) at low pressure - Bulb is centered in coils which are RF generators

25. RADIATION SOURCE Electrodeless Discharge Lamp (EDL) - A radio frequency (RF) field is generated when power is applied to the coils - Generated energy vaporizes and excites the metal atoms in the bulb - Characteristic emission spectrum in the metal is produced - Light intensity is about 100 times greater than that of HCL - Better detection limits but less stable than HCL

26. ATOMIZER - Is the sample cell of the AAS - Produces ground state free gas phase analyte atoms present in sample Two common atomizers - Flame atomizer and - Electrothermal (furnace) atomizer

27. ATOMIZER Flame Atomizer - An oxidant gas is mixed with a fuel gas in a premix burner - The mixture is lit to create a flame Two types of flames are used - Air-acetylene flame - Air is the oxidant and acetylene is the fuel - Nitrous oxide-acetylene flame - Nitrous oxide is the oxidant and acetylene is the fuel

28. ATOMIZER Flame Atomizer - The premix burner generates laminar gas flow which results in a very steady flame - Sample is introduced into the burner in the form of solution - Sample is sucked by a nebulizer (capillary tube) - Nebulizer sprays the solution into the mixing chamber in the form of fine mist (aerosol)

29. ATOMIZER Flame Atomizer Three Nebulizer Designs - Concentric (the most common) - Cross-flow - Modified Babington

30. ATOMIZER Flame Atomizer - The fuel and oxidant gases carry sample aerosol to the base of the flame - The aerosol sample is desolvated, vaporized, and atomized to form free gas phase atoms - The process is efficient for droplets 4 µm or less in diameter - The smaller the droplets the more efficient the process

31. ATOMIZER Flame Atomizer - Nebulizer is usually made of stainless steel - Sample solution goes into nebulizer at ~ 5µL/min - Long pathlength is necessary to compensate for loss of sample in the mixing chamber

32. ATOMIZER Flame Atomizer Advantages - Fast - High element selectivity - Instrument is easy to operate Disadvantages - Most atoms remain in the unexcited state - Restricted detection limits to the ppm range - Possibility of flashback occuring which may result in explosion in mixing chamber (when gas flow rate > the burning velocity)

33. ATOMIZER Electrothermal Atomizer (ETA) - Also called furnace atomizer - The most common type is the graphite furnace atomizer (GFAAS) - Uses a tube of graphite coated with pyrolytic graphite - System is heated with electrical resistance - Graphite tube is ~ 6 mm diameter and 25-30 mm long - Inert gas is used to prevent graphite from being oxidized

34. ATOMIZER Electrothermal Atomizer (ETA) - Quartz windows are at each end of furnace to permit light from radiation source to pass through to furnace - Lontudinal graphite tubes result in ‘carry-over’ or ‘memory effect’ problems due to temperature gradient - Transverse graphite tubes are more efficient due to even heating (electrical contacts are transverse to the light path)

35. ATOMIZER Other Atomizers Cold vapor-AAS technique (CVAAS) - For determination of mercury Hydride generation technique (HGAAS) - For elements that form volatile hydrides (As, Se, Sb) Glow Discharge and Laser Ablation - For direct analysis of solid samples

36. MONICHROMATOR - High resolution is not required due to narrow lines produced by radiation source - Absorption lines of interest are separated from other lines - Diffraction grating is used as the dispersion element (may be equipped with two gratings) - Useful wavelength range is ~ 190 – 850 nm - Quartz lenses (or concave mirrors) are used to focus radiation at different parts of the optical system

37. MONICHROMATOR - The system of slits and gratings enables analyst to select the desired wavelength of radiation - Entrance slit prevents stray radiation from entering - Radiation passes through entrance slit to the gratings - Gratings disperse radiation and directs it towards the exit slit - Desired absorption line is permitted through the exit slit to the detector - Slits may be fixed or variable (variable allows for max flexibility)

38. DETECTOR - The most common detector is the PMT - PDA and CCD are also used Modulation - Switching on and off the radiation source very rapidly - Required for accurate results

39. DETECTOR - Many metals emit strongly at the same wavelength at which they absorb - If absorption (I) is being measured, the signal recorded would be (I+E) - This problem is overcome by modulation of the radiation source - Done by using a rotating chopper placed directly in front of the source - Chopper is a metal disc with opposite quadrants cut off

40. DETECTOR - An alternative way is to pulse the power to the lamp at a given frequency - Signal (I) then becomes alternating current (AC) - E is however direct current (DC) - Detector is programmed to measure AC but not DC - Results in measurement of I but not I+E

41. SAMPLE PREPARATION-FLAME ATOMIZATION - Sample must be in the form of solution - Sample must be prepared by acid digestion, fusion, ashing to acidic solution or combustible nonaqueous solution Examples of solid samples Metal alloys, ceramics, glasses, polymers, food, soil, rock, animal tissue, fertilizers, paint chips, coal, pharmaceuticals Examples of liquid samples Wine, blood, oil, beverage, wastewater, organic solvents, petroleum, milk, seawater, serum

42. SAMPLE PREPARATION-FLAME ATOMIZATION The Atomization Process - Nebulization (M+ + X-; solution → aerosol) - Desolvation (MX; solid) - Liquefaction (MX; liquid) - Atomization (Mo + Xo; gas) - Excitation (M*, gas) - Ionization (M+ + e-; gas)

43. SAMPLE PREPARATION-FLAME ATOMIZATION - Analyst must use the appropriate fuel/oxidant ratio for maximum sensitivity (lists are available) - Flames are classified as oxidizing (fuel-lean, excess oxidant) or reducing (fuel-rich, excess fuel) - Air-acetylene flame can be used in either oxidizing mode or reducing mode - Nitrous oxide-acetylene flame is usally in the reducing mode - Elements that form stable oxides are run in the reducing mode (Al, B, Mb, Vr)

44. SAMPLE PREPARATION-GRAPHITE FURNACE - Analyte should be in the form of solution - About 5 – 50 µL is injected into the graphite tube - The tube is subjected to a multistep temperature program A typical program consists of the following steps - Dry - Pyrolyze (ash, char) - Cool - Atomize - Clean out - Cool down

45. SAMPLE PREPARATION-GRAPHITE FURNACE - Dry step is used to remove the solvent (temperature ramp) - Pyrolyze (ashing or char) step is used to remove as much matrix as possible (temperature ramp) - A cool down step is used for longitudinal heated furnaces - Atomization step produces gas phase free analyte atoms (no purge gas flow and temperature is raised rapidly) - Clean out step is when the temperature is raised above the atomization step to burn out remaining residue - The furnace is then allowed to cool

46. 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

47. 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

48. INTERFERENCE - Physical or chemical process that can cause the signal from an analyte to be higher or lower than of an equivalent standard - Result of change in signal when analyte concentration is unchanged - Can cause negative or positive errors in quantitative analysis Two Classes - Spectral interference and - Nonspectral interference

49. INTERFERENCE Spectral Interference - Overlap of analyte signal by other signals from other species or flame or furnace - Causes the amount of light absorbed to be too high due to absorption of other species other than the analyte - Commonly caused by stable oxides Sources - Atomic spectral interference - Background absorption

50. INTERFERENCE Nonspectral Interference - Affect the formation of analyte free atoms Examples - Chemical interference - Ionization interference - Matrix interference (solvent effects)