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.

17

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%

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Schematic of VGA-77

Transfer tube

Optical path

Pump

Drain

Gas/liquid Separator

Flow controller

Quartz absorption cell

Sample

Acid

NaBH4

Inert gas (N2)

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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