Flame Atomic Absorption
Basic Principles
2
Importance of Flame AA as an Analytical
Technique
Analyze concentrations of metals in solution.
67 elements by flame AA.
PPB to percentage levels.
Precision typically better than 1 percent RSD.
Few interferences.
Sample preparation is simple.
Instrument is easy to tune and operate.
3
ICP-MS
ICP-OES
GF-AAS
Flame AA
200
150
100
80
40
ppq ppm ppb 0.1 % 100% ppt
4
Terminology
1) Sensitivity
2) Detection limit
3) Quantitation limit
4) Accuracy
5) Precision
6) Standard Deviation
5
Periodic Table
6
Flame versus Furnace AAS
Criteria Flame Furnace
Elements 67 48
Sensitivity ppm - % ppt – ppb
Precision Good Fair
Interferences Few Many
Speed Rapid Slow
Simplicity Easy More Complex
Flame Hazards Yes No
Automation Yes Yes (unattended)
Operating Cost Low Medium
7
Absorption versus Emission
Fraunhofer
Absorption Lines
Elemental Emission
Lines
Qualitative detection of elements.
8
Principles of Atomic Spectroscopy
• 분자들은 원자와 이온으로 해리.
• 원자와 이온들에 흡수, 방출된 빛은 아래의 식으로 에너지로
계산 될 수 있음.
E = h = hc/
h = Planck’s constant (6.63x10-34 Js)
c = speed of light (3x108 m/s)
= frequency of the absorbed light (Hz)
= wavelength (m)
• 위의 식을 이용하여 에너지를 반대로 파장으로 표현.
9
Atoms, Molecules and Bonding
원자: 양자 와 중성자들이 가운데 위치하고 전자구름이
둘러싸고 있는 형태.
분자:두 개 혹은 그 이상의 원자들이 서로 연결된 형태.
C C
C C
C
O
O
Principles of Atomic Spectroscopy
10
Bohr Model of Ground State Atom
Be (5n, 4p, 4e) Bohr Model of the Atom
핵 – 가운데 위치한 구형
• 양자(protons) – 양의 전하
• 중성자(neutrons) – 중성 전하
핵 주변으로 다른 에너지 형태가 궤도(orbitals)를 돌고 있음
• 전자(electrons) – 음의 전하
모든 중성 원자들은 같은 수의 양자와 전자를 갖고 있음.
Neutrons
Protons Electrons
Orbitals
11
Electron Energy Shift
11
Excited State Atom or Ion
Ground State Atom or Ion
h Energy emitted
Valence (Outer) Electrons
Energy
absorbed
12
Emission of Light Energy
들뜬 원자는 불안정함.
– 들뜬 상태의 원자는 재빨리 빛 형태의 에너지를 방출.
– 전자는 높은 에너지 궤도에서 낮은 궤도로 이동.
높은 에너지에서 낮은 에너지의 전자전이는 스펙트럼에서
빛의 라인을 생성.
– 방출스펙트럼이 구성.
13
Absorption Energy Diagram
Absorption (Excitation)
14
Emission Energy Diagram
(Many Lines/Element)
Emission
15
Energy Level Diagram for Pb
Electron Energy Transitions
16
Atomic Absorption Technique (1)
같은 source를 사용.
– Copper if the analyte is copper.
불꽃은 원자의 생성과 light path의 운반에 쓰임.
원자의 들은 빛에 노출 되어 resonance line을 흡수.
빛의 투과는 흡광도로 계산됨.
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Atomic Absorption Technique (2)
18
Atomic Absorption Process
Resonance lines must originate from ground
state.
19
Basis for Spectrochemical Methods
각 원소들이 방출 혹은 흡수하는 빛은 고유 파장을 가짐.
20
Beer –Lambert Law (1)
Where:
= Incident Light Intensity.
= Transmitted Light Intensity.
= Absorption Coefficient.
= Concentration.
= Path Length.
21
Beer- Lambert Law (2)
Absorbance (A) is defined as:
Therefore: A = abc
Because both a and b are constants for a
particular measurement.
22
Percentage Transmittance versus ABS
Transmittance Absorbance
100% 0
10% 1
1% 2
.01% 3
23
Beer – Lambert Law (3)
The Law predicts that a plot of absorbance
versus concentration will give a straight line.
AA Hardware
25
Spectrometer Components (1)
Five main components
– 광원.
Hollow Cathode Lamp, UltrAA Lamp,
Continuum
– 원자화 장치. Flame (or Furnace or Vapour
Generator)
– 단색화장치
– 검출기
– Amplifier readout system
26
Spectrometer Components (2)
27
Emission Line Overlaps Absorption Line
28
Hollow Cathode Lamp Design
29
HCL Operation (1)
30
HCL Operations (2)
31
Deuterium Lamp Intensity versus Wavelength
32
Lamp Output (1)
램프방출 빛의 구성:
– Atomic resonance lines.
– Impurity lines.
– Fill gas emission lines.
– Non-resonance lines.
램프전류로 세기를 조절.
33
Lamp Output (2)
34
Monochromator Function
Hollow cathode lamp는 많은 파장을 방출.
단색화 장치는 램프에서 단일 resonance line을 분리.
이상적인 단색화장치는 오직 하나의 파장을 분리.
– Sometimes easy – Cu.
– Sometimes more difficult- Fe.
35
Spectral Isolation of Analytical Wavelength
36
Monochromator
Angle of the
grating
determines the
wavelength
focused on the
exit slit.
37
Grating Schematic
38
Effect of Spectral Band Width
Resonance Line Resonance Line Resonance Line
39
Resolution Considerations
40
Photomultiplier Tube Operation
*100 Million Amplification of Signal
Flame Atomic Absorption
Flame Atomization
42
Atomization
Process by which atoms are made available for absorption
measurement.
Need to convert molecules/compounds to FREE GROUND
STATE ATOMS.
– Expose to light of characteristic wavelength for that element.
– Highly complex process leading to atomization complete in a few
milliseconds.
43
Flame Atomization
Convert the analyte solution into free atoms in the light path of the hollow
cathode lamp.
Primary aim. – Generate an aerosol.
– Introduce aerosol into flame. While NOT blocking the nebulizer.
While NOT blocking the burner.
Accomplished using: – Nebulizer
– Spray chamber
– Burner head
44
Atomization Process (1)
45
Atomization Process (2)
Flame heat evaporates solvent.
– Near base of flame
– Converts aerosol into VERY SMALL solid droplets.
Particles fuse or melt.
Vaporization. – Form molecules.
Molecules dissociate. – Form ground state atoms.
46
Nebulizer (1)
Pneumatic device that draws solution through capillary.
Shatters solution into droplets. – Non-uniform droplet size.
Droplets and oxidant passes through venturi.
Directed onto glass bead. – Shatters droplets.
– More uniform droplet size. Better detection limits.
– More smaller droplets. Better sensitivity.
47
Nebulizer (2)
Mark VI Nebulizer
48
Nebulizer (3)
Designed for maximum flexibility. – Set for maximum sensitivity.
Hi-vac setting
– Set for maximum resistance to blockage. Hi-solids setting
All components constructed from inert materials.
– Fluorinated Polypropylene
– Pt/Ir nebulizer capillary Corrosion resistant
49
Impact Bead (Mark VI)
50
Impact Bead (Mark VII)
51
Function of Impact Bead
Externally adjustable.
Breaks large aerosol droplets into smaller ones.
– Increased signal.
– Decreased noise.
52
Spray Chamber
All components constructed from inert materials. – Fluorinated polypropylene.
Removed large droplets.
Mixes remaining small droplets with flame gases. – Crucial for uniform mixing.
– Some evaporation occurs during this stage.
Passes mixture into the burner.
53
Schematic of the Spray Chamber
54
Spray Chamber Assembly
55
Mark VII Burner Heads
56
Position in Light Path
Flame MUST be positioned to place maximum atom population
in light path.
– Vertical alignment.
– Horizontal alignment.
– Rotational alignment.
Maximum atom population = maximum signal.
57
Absorbance Contours in the Flame
58
Types of Flames Used
59
Elements by Air/Acetylene Flame
Almost universally used for easily atomized elements. – Cu, Pb, K, Na, etc.
Temperature of about 2300 degrees Celsius.
Interferences negligible.
Chemical environment usually NOT critical. – Oxidizing.
– Stoichiometric.
– Reducing.
Not hot enough to break down refractory oxides.
60
Atomization Mechanism (1)
Easily atomized element with air-acetylene.
61
Atomization Mechanism (2)
Refractory element with air-acetylene.
62
Elements by Nitrous Oxide/Acetylene Flame
Good for refractory oxides. – AI, Si, W, Etc.
Temperature 3000 degrees Celsius.
Chemical environment important. – Oxidizing – lean flame – minimum acetylene.
Will NOT produce atoms from strongly bound oxides.
– Stoichiometric – no excess fuel or oxidant.
– Reducing – rich flame – excess acetylene. Excess C and H break down strongly bound oxides.
63
Burning Velocities
FLAME TEMPERATURE BURNING VELOCITY
Degrees Celsius CM/SEC
Air – Propane 1950 80
Air – Acetylene 2300 160
N20 – Acetylene 3000 180
02 – Acetylene 3050 2480
64
Flame Optimization
Burner rotation.
Nebulizer capillary setting. – Uptake.
Impact bead position.
Fuel/oxidant ratio.
Burner height.
Burner horizontal position.
65
Sensitivity versus Impact
Bead Position
66
Optimum Viewing Height (1)
Cu
67
Optimum Viewing Height (2)
Ca
68
Optimum Viewing Height (3)
Compromise
Flame Interferences
70
1) Spectral interference
2) Chemical interference
3) Ionization interference
4) Matrix interference
5) Non-specific interference
Interferences
71
General Background Correction
Total absorbance measured (HCL). – Atomic + non-specific.
Background measured (Deuterium Lamp). – Non-specific only.
Measurements are time separated. – A few milliseconds.
Atomic absorption calculated. – Total absorbance – background absorbance = atomic absorbance.
72
Deuterium Technique
Most common.
Continuum source to measure background.
– Deuterium Lamp.
Operating range from 190 to 420 nm.
Background is most significant at shorter wavelength.
– Deuterium works well most of the time.
73
Deuterium Lamp Intensity versus Wavelength
74
Deuterium Background Correction
75
Deuterium Background Correction (2)
76
Deuterium Background Correction (3)
Hollow cathode lamp energy attenuated by both atomic and
background species.
– Total absorption.
Hollow cathode lamp signal = AA + BGD
Deuterium energy attenuated by background species.
– Background only.
– Atomic component too small to detect. Deuterium lamp signal = BGD only.
Electronically processed signal = AA only.
77
Vapor Generation Atomizer
- As, Se, Sn, Sb, Te, Bi, Hg
78
VGA
산화제 : Hg, As공용
HCl + H2O = 1:1
환원제
Hg의 경우 : NaBH4 0.3%, NaOH 0.5%
As의 경우 : NaBH4 0.6%, NaOH 0.5%
79
Schematic of VGA-77
Transfer tube
Optical path
Pump
Drain
Gas/liquid Separator
Flow controller
Quartz absorption cell
Sample
Acid
NaBH4
Inert gas (N2)
80
Hydride Formation
NaBH4 + 2 HCl + 3H2O == H3BO3 + NaCl + 8H
M+++ + 8H MH3 + 4H2
or
M++ + 8H MH2 + 4H2
MH3 + Heat M + 3H +
Hg++ + SnCl2 Hg +
81
Thank you
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