1. Lecture 6 ATOMIC SPECTROSCOPY
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2. INTRODUCTION TO ATOMIC SPECTROMETRY
Sample is atomized (atoms/ions) absorption or emission measured
INTRODUCTION TO ATOMIC SPECTROMETRY

3. INTRODUCTION TO ATOMIC SPECTROMETRY
ENERGY LEVEL DIAGRAMS
Every elements has unique set of atomic orbitals
p,d,f... levels split by spin-orbit coupling
Spin (s) and orbital (l) motion create magnetic fields that perturbeach other (couple)
if fields parallel - slightly higher energy
if fields antiparallel - slightly lower energy
INTRODUCTION TO ATOMIC SPECTROMETRY

4. INTRODUCTION TO ATOMIC SPECTROMETRY

5. INTRODUCTION TO ATOMIC SPECTROMETRY

6. INTRODUCTION TO ATOMIC SPECTROMETRY
ELECTRONIC TERM SYMBOLS
INTRODUCTION TO ATOMIC SPECTROMETRY

7. INTRODUCTION TO ATOMIC SPECTROMETRY

8. INTRODUCTION TO ATOMIC SPECTROMETRY
• Similar pattern between atoms but different spacing • Spectrum of ion different to atom • Separations measured in electronvolts (eV) As # of electrons increases, # of levels increases Emission spectra become more complex Li 30 lines, Cs 645 lines, Cr 2277 lines
INTRODUCTION TO ATOMIC SPECTROMETRY

9. INTRODUCTION TO ATOMIC SPECTROMETRY
As # of electrons increases, # of levels increases Emission spectra become more complex Li 30 lines, Cs 645 lines, Cr 2277 lines
INTRODUCTION TO ATOMIC SPECTROMETRY

10. INTRODUCTION TO ATOMIC SPECTROMETRY
Desire narrow lines for accurate identification Broadened by (i) uncertainty principle (ii) pressure broadening (iii) Doppler effect (iv) (electric and magnetic fields)
INTRODUCTION TO ATOMIC SPECTROMETRY

11. Uncertainty Principle:
Quantum mechanical idea states must measure for some minimum time to tell two frequencies apart
ATOMIC LINE WIDTHS

12. INTRODUCTION TO ATOMIC SPECTROMETRY
Shows up in lifetime of excited state • if lifetime infinitely long, E infinitely narrow • if lifetime short, E is broadened
INTRODUCTION TO ATOMIC SPECTROMETRY

13. INTRODUCTION TO ATOMIC SPECTROMETRY
Example Lifetime of Hg*=2x10-8 s. What is uncertainty broadening for 254 nm line?
INTRODUCTION TO ATOMIC SPECTROMETRY

14. INTRODUCTION TO ATOMIC SPECTROMETRY
Differentiating with respect to frequency: sometimes called natural linewidth.
INTRODUCTION TO ATOMIC SPECTROMETRY

15. INTRODUCTION TO ATOMIC SPECTROMETRY

16. INTRODUCTION TO ATOMIC SPECTROMETRY
(ii) Pressure broadening: Collisions with atoms/molecules transfers small quantities of vibrational energy (heat) - ill-defined ground state energy Effect worse at high pressures • For low pressure hollow cathode lamps (1-10 torr) Å • For high pressure Xe lamps (>10,000 torr) Å (turns lines into continua!)
INTRODUCTION TO ATOMIC SPECTROMETRY

17. INTRODUCTION TO ATOMIC SPECTROMETRY
(iii) Doppler broadening: Change in frequency produced by motion relative to detector
INTRODUCTION TO ATOMIC SPECTROMETRY

18. INTRODUCTION TO ATOMIC SPECTROMETRY
In gas, broadens line symmetrically Doppler broadening increases with T • At room T ~ Å Total linewidth typically Å
INTRODUCTION TO ATOMIC SPECTROMETRY

19. INTRODUCTION TO ATOMIC SPECTROMETRY
Other Effects of T on Atomic Spectrometry: T changes # of atoms in ground and excited states Boltzmann equation
INTRODUCTION TO ATOMIC SPECTROMETRY

20. INTRODUCTION TO ATOMIC SPECTROMETRY
Important in emission measurements relying on thermal excitation Na atoms at 2500 K, only 0.02 % atoms in first excited state! Less important in absorption measurements % atoms in ground state!
INTRODUCTION TO ATOMIC SPECTROMETRY

21. INTRODUCTION TO ATOMIC SPECTROMETRY
Methods for Atomizing and Introducing Sample Sample must be converted to atoms first
INTRODUCTION TO ATOMIC SPECTROMETRY

22. INTRODUCTION TO ATOMIC SPECTROMETRY
Must transfer sample to atomizer - easy for gases /solutions but difficult for solids
INTRODUCTION TO ATOMIC SPECTROMETRY

23. Lecture 7 ATOMIC EMMISION SPECTROSCOPY
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24. INTRODUCTION TO ATOMIC SPECTROMETRY
ATOMIC EMISSION SPECTROSCOPY (AES) Identification of elements but not compounds
INTRODUCTION TO ATOMIC SPECTROMETRY

25. Excitation and Atomization: Traditionally based on • flame but • arc and spark • plasma excitation offers (i) increased atomization/excitation (ii) wider range of elements (iii) emission from multiple species simultaneously (iv) wide dynamic range
ATOMIC EMISSION SPECTROSCOPY

26. Flame Excitation Sources: Primary Combustion Zone Interzonal Region Secondary Combustion Zone
ATOMIC EMISSION SPECTROSCOPY

27. ATOMIC EMISSION SPECTROSCOPY

28. Laminar Flow Burner: • Cheap • Simple • Flame stability • Low temperature
ATOMIC EMISSION SPECTROSCOPY

29. ATOMIC EMISSION SPECTROSCOPY

30. Arc and Spark Excitation Sources: • Limited to semiquantitative/qualitative analysis (arc flicker) • Usually performed on solids • Largely displaced by plasma-AES Electric current flowing between two C electrodes
ATOMIC EMISSION SPECTROSCOPY

31. Electric current flowing between two C electrodes
Sample pressed into electrode or mixed with Cu powder and pressed
briquetting
Cyanogen bands (CN) nm occur with C electrodes in air -
He, Ar atmosphere
ATOMIC EMISSION SPECTROSCOPY

32. Arc/spark unstable - each line measured >20 s (needs multichannel detection) photographic film: • Cheap • Long integration times • Difficult to develop/analyze • Non-linearity of line "darkness"
ATOMIC EMISSION SPECTROSCOPY

33. ATOMIC EMISSION SPECTROSCOPY

34. ATOMIC EMISSION SPECTROSCOPY

35. multichannel PMT instruments: • for rapid determinations (<20 lines) but not versatile • routine analysis of solids - metals, alloys, ores, rocks, soils • portable instruments
ATOMIC EMISSION SPECTROSCOPY

36. Plasma Excitation Sources: gas containing high proportion of cations and electrons (1) Inductively Coupled Plasma (ICP)
ATOMIC EMISSION SPECTROSCOPY

37. • Torch up to 1" diameter
• Ar cools outer tube, defines plasma shape
• Radio-frequency (RF) up to 2 kW
• Ar flow up to 20 L/min
ATOMIC EMISSION SPECTROSCOPY

38. ATOMIC EMISSION SPECTROSCOPY
Plasma Structure:
• Brilliant white core - Ar continuum and lines
• Flame-like tail up to 2 cm
• Transparent region - measurements made
• Hotter than flame (10,000 K) - more complete atomization/excitation
• Atomized in "inert" atmosphere
• Little ionization - too many electrons in plasma
ATOMIC EMISSION SPECTROSCOPY

39. (2) Direct Current (DC) Plasma
• DC current (10-15 A) flows between C anodes and W cathode
• Plasma core at 10,000 K, viewing region at ~5,000 K
• Simpler, less Ar than ICP - less expensive
ATOMIC EMISSION SPECTROSCOPY

40. Atomic Emission Spectrometers May be >1,000 visible lines (<1 Å) on continuum Need • high resolution (<0.1 Å) • high throughput • low stray light • wide dynamic range (>106) • precise and accurate wavelength calibration/intensities • stability • computer controlled
ATOMIC EMISSION SPECTROSCOPY

41. Three instrument types:
sequential (scanning and slew-scanning)
Multichannel
(Fourier transform FT-AES)
ATOMIC EMISSION SPECTROSCOPY

42. Sequential monochromators:
Slew-scan spectrometers - even with many lines, much spectrum contains no information
• rapidly scanned (slewed) across blank regions
• slowly scanned across lines
• computer control/preselected lines to scan
ATOMIC EMISSION SPECTROSCOPY

43. Multichannel AES:
ATOMIC EMISSION SPECTROSCOPY

44. Sequential instrument - PMT moved behind aperture plate, or grating+prism moved to focus new  on exit slit
Cheaper
Slower
Pre-configured exit slits to detect up to 20 lines, slew scan
Multichannel instrument - multiple PMT's
Expensive
Faster
ATOMIC EMISSION SPECTROSCOPY

45. Solution Sample Introduction:
(1) Electrothermal vaporizer* (ETV)
electric current rapidly heats crucible containing sample
sample carried to atomizer by gas (Ar, He)
only for introduction, not atomization
ATOMIC EMISSION SPECTROSCOPY

46. ATOMIC EMISSION SPECTROSCOPY

47. (2) Nebulizer - convert solution to fine spray or aerosol
Ultrasonic nebulizer uses ultrasound waves to "boil" solution flowing across disc
Pneumatic nebulizer uses high pressure gas to entrain solution
ATOMIC EMISSION SPECTROSCOPY

48. Cross-flow Nebulizer
ATOMIC EMISSION SPECTROSCOPY

49. Solid Sample Introduction: (1) Electrothermal vaporizer
Solid Sample Introduction: (1) Electrothermal vaporizer* (2) Direct Insertion(*) uses powder placed inside flame, plasma, arc or spark atomizer (atomizer acts as vaporizer) Coating on electrode in atomizer (3) Ablation uses coating of electrodes in discharge cell and sample entrained in Ar or He gas Laser ablation uses laser to vaporize sample
ATOMIC EMISSION SPECTROSCOPY

50. APPLICATION OF AES AES relatively insensitive (small excited state population at moderate temperature) AAS still used more than AES (i) less expensive/complex instrumentation (ii) lower operating costs (iii) greater precision
ATOMIC EMISSION SPECTROSCOPY

51. In practice ~60 elements detectable
10 ppb range most metals
Li, K, Rb, Cs strongest lines in IR
Large # of lines, increase chance of overlap
ATOMIC EMISSION SPECTROSCOPY

52. ATOMIC EMISSION SPECTROSCOPY

53. Lecture 8 ATOMIC ABSORPTION SPECTROSCOPY
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54. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

55. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

56. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

57. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

58. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

59. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

60. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

61. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

62. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

63. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

64. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

65. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

66. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

67. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

68. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

69. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

70. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

71. ATOMIC ABSORPTION SPECTROSCOPY (AAS)

72. ATOMIC ABSORPTION SPECTROSCOPY (AAS)
AAS intrinsically more sensitive than AES
similar atomization techniques to AES
addition of radiation source
high temperature for atomization necessary
flame and electrothermal atomization
very high temperature for excitation not necessary / generally no plasma/arc/spark AAS
ATOMIC ABSORPTION SPECTROSCOPY (AAS)

73. FLAME AAS:
simplest atomization of gas/solution/solid
laminar flow burner - stable "sheet" of flame
flame atomization best for reproducibility (precision) (<1%)
relatively insensitive - incomplete volatilization, short time in beam
ATOMIC ABSORPTION SPECTROSCOPY (AAS)

74. ATOMIC ABSORPTION SPECTROSCOPY

75. ATOMIC ABSORPTION SPECTROSCOPY
Primary combustion zone - initial decomposition, molecular fragments, cool
Interzonal region - hottest, most atomic fragments, used for emission/fluorescence
Secondary combustion zone - cooler, conversion of atoms to stable molecules, oxides
element rapidly oxidizes - largest [atom] near burner
element poorly oxidizes - largest [atom] away from burner
ATOMIC ABSORPTION SPECTROSCOPY

76. most sensitive part of flame for AAS varies with analyte
Consequences?
sensitivity varies with element
must maximize burner position
makes multielement detection difficult
ATOMIC ABSORPTION SPECTROSCOPY

77. Electrothermal Atomizers:
entire sample atomized short time ( °C)
sample spends up to 1 s in analysis volume
superior sensitivity ( g analyte)
less reproducible (5-10 %)
ATOMIC EMISSION SPECTROSCOPY

78. Graphite furnace ETA
ATOMIC ABSORPTION SPECTROSCOPY

79. external Ar gas prevents tube destruction internal Ar gas circulates gaseous analyte
ATOMIC ABSORPTION SPECTROSCOPY

80. Three step sample preparation for graphite furnace:
Dry - evaporation of solvents (10->100 s)
Ash - removal of volatile hydroxides, sulfates, carbonates ( s)
Fire/Atomize - atomization of remaining analyte (1 s)
ATOMIC ABSORPTION SPECTROSCOPY

81. ATOMIC ABSORPTION SPECTROSCOPY

82. Atomic Absorption Instrumentation:
AAS should be very selective - each element has different set of energy levels and lines very narrow
BUT for linear calibration curve (Beers' Law) need bandwidth of absorbing species to be broader than that of light source difficult with ordinary monochromator
ATOMIC ABSORPTION SPECTROSCOPY

83. Solved by using very narrow line radiation sources
minimize Doppler broadening
pressure broadening
lower P and T than atomizerand using resonant absorption
Na emission 3p2s at nm used to probe Na in analyte
ATOMIC ABSORPTION SPECTROSCOPY

84. Hollow Cathode Lamp:
ATOMIC ABSORPTION SPECTROSCOPY

85. ATOMIC ABSORPTION SPECTROSCOPY
300 V applied between anode (+) and metal cathode (-)
Ar ions bombard cathode and sputter cathode atoms
Fraction of sputtered atoms excited, then fluoresce
Cathode made of metal of interest (Na, Ca, K, Fe...)
different lamp for each element
restricts multielement detection
Hollow cathode to
maximize probability of redeposition on cathode restricts light direction
ATOMIC ABSORPTION SPECTROSCOPY

86. ELECTRODELESS DISCHARGE LAMP
ATOMIC ABSORPTION SPECTROSCOPY

87. AAS Spectrophotometers:
ATOMIC ABSORPTION SPECTROSCOPY

88. ATOMIC ABSORPTION SPECTROSCOPY
Signal at one wavelength often contains luminescence from interferents in flame Chemical interference: (i) reverses atomization equilibria (ii) reacts with analyte to form low volatility compound releasing agent - cations that react preferentially with interferent - Sr acts as releasing agent for Ca with phosphate protecting agent - form stable but volatile compounds with analyte (metal-EDTA formation constants)
ATOMIC ABSORPTION SPECTROSCOPY

89. IONIZATION
ATOMIC ABSORPTION SPECTROSCOPY

90. ATOMIC ABSORPTION SPECTROSCOPY

91. hotter atomization means:
more ionization
emission from interferents
ATOMIC ABSORPTION SPECTROSCOPY

92. ATOMIC ABSORPTION SPECTROSCOPY
Spectral interference - emission or absorption from interferent overlaps analyte
Beam usually chopped or modulated at known frequency
Signal then contains constant (background) and dynamic (time varying) signals
ATOMIC ABSORPTION SPECTROSCOPY

93. ATOMIC ABSORPTION SPECTROSCOPY

94. DETECTION LIMITS for AAS/AES?
AA/AE comparable (ppb in flame)
AAS less suitable for
weak absorbers (forbidden transitions)
metalloids and non-metals (absorb in UV)
metals with low IP (alkali metals)
ATOMIC ABSORPTION SPECTROSCOPY

95. ATOMIC ABSORPTION SPECTROSCOPY

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109. ATOMIC ABSORPTION SPECTROSCOPY

110. ATOMIC ABSORPTION SPECTROSCOPY

111. ATOMIC EMISSION SPECTROSCOPY

112. INTRODUCTION TO ATOMIC SPECTROMETRY

113. INTRODUCTION TO ATOMIC SPECTROMETRY

114. INTRODUCTION TO ATOMIC SPECTROMETRY

115. INTRODUCTION TO ATOMIC SPECTROMETRY

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119. INTRODUCTION TO ATOMIC SPECTROMETRY

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121. INTRODUCTION TO ATOMIC SPECTROMETRY

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123. INTRODUCTION TO ATOMIC SPECTROMETRY

124. INTRODUCTION TO ATOMIC SPECTROMETRY

125. INTRODUCTION TO ATOMIC SPECTROMETRY

126. INTRODUCTION TO ATOMIC SPECTROMETRY

127. INTRODUCTION TO ATOMIC SPECTROMETRY

128. INTRODUCTION TO ATOMIC SPECTROMETRY

129. INTRODUCTION TO ATOMIC SPECTROMETRY