1. Part 2

2. Relationship Between Atomic Absorption and Flame Emission
Flame Emission: it measures the radiation emitted by the excited atoms that is related to concentration.
 Atomic Absorption: it measures the radiation absorbed by the unexcited atoms that are determined.
Atomic absorption depends only upon the number of unexcited atoms, the absorption intensity is not directly affected by the temperature of the flame.
The flame emission intensity in contrast, being dependent upon the number of excited atoms, is greatly influenced by temperature variations.
Prof. Dr. Hisham Ezzat Abdellatef

3. Atomizers in emission techniques
Type Method of Atomization Radiation Source
Arc sample heated in an sample
electric arc ( oC)
Spark sample excited in a sample
high voltage spark
Flame sample solution sample
aspirated into a flame
(1700 – 3200 oC)
Argon sample heated in an sample
plasma argon plasma ( oC)
Prof. Dr. Hisham Ezzat Abdellatef

4. Atomizers in absorption techniques
Type Method of Atomization Radiation Source
Atomic sample solution aspirated HCL
(flame) into a flame
atomic sample solution evaporated HCL
(nonflame) & ignited ( oC)
(Electrothermal)
Hydride Vapor hydride generated HCL
generation
Cold vapor Cold vapor generated (Hg) HCL
Prof. Dr. Hisham Ezzat Abdellatef

5. Atomizers in fluorescence techniques
Type Method of Atomization Radiation Source
atomic sample aspirated sample
(flame) into a flame
atomic sample evaporated sample
(nonflame) & ignited
x-ray not required sample
fluorescence
Prof. Dr. Hisham Ezzat Abdellatef

6. Prof. Dr. Hisham Ezzat Abdellatef
Flame Atomization: In a flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel, and carried into a flame where atomization occurs. The following processes then occur in the flame.
Desolvation (produce a solid molecular aerosol)
Dissociation (leads to an atomic gas)
Ionization (to give cations and electrons)
Excitation (giving atomic, ionic, and molecular emission)
Prof. Dr. Hisham Ezzat Abdellatef

7. Processes that take place in flame or plasma
Sample Atomization
For techniques samples need to be atomized
Techniques are useful for element identification
Molecular information destroyed by atomization
Flame Atomization
Sample nebulized
Mixed with fuel
Carried to flame for atomization
T 1

8. The Atomization Process
nebulization
desolvation
vaporization
[M+,X-]aq
[M+,X-]aq
[MX]solid
[MX]gas
solution
mist
atomization
atomization
excitation or absorption
emission
[M0]gas
[M*]gas
[M0]gas
[X0]gas
(via heat or light)
ground state
excited state
[M+]gas
[X+]gas
Prof. Dr. Hisham Ezzat Abdellatef

9. Prof. Dr. Hisham Ezzat Abdellatef
Types of Flames:
Several common fuels and oxidants can be employed in flame spectroscopy depending on temperature needed. Temperatures of 1700oC to 2400oC are obtained with the various fuels when air serves as the oxidant. At these temperature, only easily decomposed samples are atomized. For more refractory samples, oxygen or nitrous oxide must be employed as the oxidant. With the common fuels these oxidants produce temperatures of 2500oC to 3100oC.
Prof. Dr. Hisham Ezzat Abdellatef

10. Prof. Dr. Hisham Ezzat Abdellatef

11. Prof. Dr. Hisham Ezzat Abdellatef
Burning Velocity:
The burning velocities are of considerable importance because flames are stable in certain ranges of gas flow rates only. If the gas flow rate does not exceed the burning velocity, the flame propagates itself back in to the burner, giving flashback. As the flow rate increases, the flame rises until it reaches a point above the burner where the flow velocity and the burning velocity are equal. This region is where the flame is stable. At higher flow rates, the flame rises and eventually reaches a point where it blows off of the burner.
Prof. Dr. Hisham Ezzat Abdellatef

12. Prof. Dr. Hisham Ezzat Abdellatef
Flame Structure:
Important regions of a flame include:
primary combustion zone
interzonal region
secondary combustion zone
Primary combustion zone: Thermal equilibrium is ordinarily not reached in this region, and it is, therefore, seldom used for flame spectroscopy.
Prof. Dr. Hisham Ezzat Abdellatef

13. Prof. Dr. Hisham Ezzat Abdellatef

14. Prof. Dr. Hisham Ezzat Abdellatef
2. Interzonal region: This area is relatively narrow in stoichiometric hydrocarbon flames, is often rich in free atoms and is the most widely used part of the flame for spectroscopy.
3. Secondary combustion zone: In the secondary reaction zone, the products of the inner core are converted to stable molecular oxides that are then dispersed into the surroundings.
Prof. Dr. Hisham Ezzat Abdellatef

15. Prof. Dr. Hisham Ezzat Abdellatef
Temperature Profiles:
A temperature profile of a typical flame for atomic spectroscopy is shown in Fig The maximum temperature is located in the flame about 1 cm above the primary combustion zone. It is important– particularly for emission methods – to focus the same part of the flame on the entrance slit for all calibrations and analytical measurements.
Prof. Dr. Hisham Ezzat Abdellatef

16. Prof. Dr. Hisham Ezzat Abdellatef

17. Prof. Dr. Hisham Ezzat Abdellatef
secondary combustion zone
interzonal region
primary combustion (reaction) zone
pre heating zone
appearance depends on fuel/oxidant
length of zones depends on gas flow rate
DANGER
 gas flow too high- lifts flame off burner
gas flow too low- flash back
pre heating zone gases heated rapidly
primary combustion region contains free radicals but not in thermodynamic equilibrium
interzonal region used for spectrometry has free radicals
secondary combustion zone combustion products formed i.e. CO and H2O
Prof. Dr. Hisham Ezzat Abdellatef

18. Prof. Dr. Hisham Ezzat Abdellatef
Types of fuel/oxidant
air/acetylene
2300oC most widely used.
nitrous oxide/acetylene
2750oC hot and reducing red feather zone - due to CN very reactive free radical scavenger for 02 → lowers partial pressure of 02 in zone reducing atmosphere
C2H N2 → 2CO2 + H2O + 10N2
stoichiometric reaction
C2H2 + 5N2O → 2 CO2 + H2O + 5N2
Prof. Dr. Hisham Ezzat Abdellatef

19. Prof. Dr. Hisham Ezzat Abdellatef
Why do you need a different burner for different oxidants?
because to prevent flash back linear gas flow rate
needs to 3 x speed of which flame can travel,
burning velocity).
Prof. Dr. Hisham Ezzat Abdellatef

20. Prof. Dr. Hisham Ezzat Abdellatef
Role of Chemistry in the Flame
 sample atomised by thermal and chemical
dissociation
H2 + Q → H + H
O2 + Q → O + O
H + O2 → OH + O
O+ H2 → OH + H
equilibrium achieved by 3rd body collision (B)
i.e. N2, O2
H + H + B → H2 + B + Q
H + OH + B → H2O + B + Q
Free reductions may react with sample to produce
atoms
i.e. H + HO + NaCl → H2O + Na + Cl
Na + Q → Na* 10
Prof. Dr. Hisham Ezzat Abdellatef

21. Prof. Dr. Hisham Ezzat Abdellatef
Flame Atomisation Process
Sample must be in the form
of a fine mist so as not to put out flame.
Breaks down sample into very fine drops to form
liquid aerosol or mist.
This assist atomisation as sample only in flame ≈ 0.025s
Sample drawn up capillary tube at high velocity
Sample
oxidant
Prof. Dr. Hisham Ezzat Abdellatef

22. Prof. Dr. Hisham Ezzat Abdellatef
Suction caused by high flows of oxidant gas and Venturi effect.
The high gas flow rate at the end of the capillary creates a pressure drop in the capillary – the pressure in capillary is below atmospheric pressure and sample solution is pulled up.
The high speed gas breaks the solution into a fine mist by turbulence as it emerges from capillary.
How do we get a better aerosol?
use impact bead (glass or alloy) to encourage aerosol formation and remove large droplets.
Prof. Dr. Hisham Ezzat Abdellatef

23. Prof. Dr. Hisham Ezzat Abdellatef
Flame absorbance Profiles:
Fig. 9-4 shows typical absorption profiles for three elements. Magnesium exhibits a maximum in absorbance at the middle of the flame. The behavior of silver, which is not readily oxidized, is quite different, a continuous increase in the number of atoms, and thus the absorbance, is observed from the base to the periphery of the flame. Chromium, which forms very stable oxides, shows a continuous decrease in absorbance beginning close to the burner tip.
Prof. Dr. Hisham Ezzat Abdellatef

24. Flame absorbance profile for three elements
Prof. Dr. Hisham Ezzat Abdellatef

25. Prof. Dr. Hisham Ezzat Abdellatef
Flame Atomizers:
Figure 9-5 is a diagram of a typical commercial laminar flow burner that employs a concentric tube nebulizer. The aerosol is mixed with fuel. The aerosol, oxidant, and fuel are then burned in a slotted burner that provides a flame that is usually 5 or 10 cm in length.
Prof. Dr. Hisham Ezzat Abdellatef

26. Prof. Dr. Hisham Ezzat Abdellatef
Laminar-Flow Burner
Prof. Dr. Hisham Ezzat Abdellatef

27. Prof. Dr. Hisham Ezzat Abdellatef
Advantages:
1. Uniform dropsize
2. Homogeneous flame
3. Quiet flame and a long path length
Disadvantages:
1. Flash back if Vburning > Vflow
2. ~90% of sample is lost
3. Large mixing volume
Prof. Dr. Hisham Ezzat Abdellatef

28. Sample introduction techniques
Prof. Dr. Hisham Ezzat Abdellatef

29. Methods of Sample Introduction in Atomic Spectroscopy
Prof. Dr. Hisham Ezzat Abdellatef

30. Prof. Dr. Hisham Ezzat Abdellatef
Nebulization
Nebulization is conversion of a sample to a fine mist of finely divided droplets using a jet of compressed gas.
The flow carries the sample into the atomization region.
Pneumatic Nebulizers (most common)
Four types of pneumatic nebulizers:
Concentric tube - the liquid sample is sucked through a capillary tube by a high pressure jet of gas flowing around the tip of the capillary (Bennoulli effect).
This is also referred to aspiration. The high velocity  breaks the sample into a mist and carries it to the atomization region.
Prof. Dr. Hisham Ezzat Abdellatef

31. Types of pneumatic nebulizers
Concentric tube; Cross flow
Concentric tube; Cross flow
Fritted disk
Babington
Prof. Dr. Hisham Ezzat Abdellatef

32. Prof. Dr. Hisham Ezzat Abdellatef
Cross-flow
The jet stream flows at right angles to the capillary tip. The sample is sometimes pumped through the capillary.
Fritted disk
The sample is pumped onto a fritted disk through which the gas jet is flowing. Gives a finer aerosol than the others.
Babington
Jet is pumped through a small orifice in a sphere on which a thin film of sample flows. This type is less prone to clogging and used for high salt content samples.
Ultrasonic Nebulizer
The sample is pumped onto the surface of a vibrating piezoelectric crystal.
The resulting mist is denser and more homogeneous than pneumatic nebulizers.
Electro-thermal Vaporizers (Etv)
An electro thermal vaporizer contains an evaporator in a closed chamber through which an inert gas carries the vaporized sample into the atomizer.
Prof. Dr. Hisham Ezzat Abdellatef

33. Prof. Dr. Hisham Ezzat Abdellatef
Liquid samples introduced to atomizer through a nebulizer
Pneumatic nebulizer
Ultrasonic-Shear Nebulizer
Prof. Dr. Hisham Ezzat Abdellatef

34. Prof. Dr. Hisham Ezzat Abdellatef
Atomization
Atomizers
Flame
Electrothermal
Special
Glow Discharge
Hydride Generation
Cold-Vapor
Prof. Dr. Hisham Ezzat Abdellatef

35. Prof. Dr. Hisham Ezzat Abdellatef
Flame Chemistry
Flames are used in atomic emission spectrometry for excitation (emission spectrometry) but in atomic absorption flames are used as Atom Cells to produce gaseous atoms.
Why must the atoms not be excited for atomic absorption spectrometry?
If the atom is already in the excited state it cannot absorb the light.
Prof. Dr. Hisham Ezzat Abdellatef

36. Different Atomization Sources for Atomic Spectroscopy
Typical Source Temperature
Source Type
° C.
Combustion Flame
° C
Electrothermal Vaporization (ETV) on graphite platform
° C
Inductively coupled plasma (ICP)
° C
Direct-current plasma (DCP)
° C
Microwave induced plasma (MI))
non-thermal
Glow Discharge plasma (GDP)
~ 40,000° C(?)
Spark Sources (dc or ac Arc)
Prof. Dr. Hisham Ezzat Abdellatef

37. Prof. Dr. Hisham Ezzat Abdellatef
Flame Atomizers
Superior method for reproducible liquid sample
introduction for atomic absorption and fluorescence
spectroscopy.
Other methods better in terms of sampling efficiency
and sensitivity.
Prof. Dr. Hisham Ezzat Abdellatef

38. Prof. Dr. Hisham Ezzat Abdellatef
Laminar-Flow Burner
Prof. Dr. Hisham Ezzat Abdellatef

39. Prof. Dr. Hisham Ezzat Abdellatef
Atomizers
Flame Atomic Absorption Spectrometry
Atomization occurs in a flame created by mixing a fuel with an oxidant
Analyte and background ions are atomized simultaneously
Only a small percentage of the aqueous sample is atomized – much of the sample goes to waste
Prof. Dr. Hisham Ezzat Abdellatef

40. Prof. Dr. Hisham Ezzat Abdellatef
Laminar flame atomizer
Prof. Dr. Hisham Ezzat Abdellatef

41. Prof. Dr. Hisham Ezzat Abdellatef

42. Prof. Dr. Hisham Ezzat Abdellatef

43. Performance Characteristics
Of Flame Atomizers
In terms of reproducible behavior, flame atomization appears to be superior to all other methods for liquid sample introduction. In terms of sampling efficiency and thus sensitivity, however, other atomization methods are markedly better. A large portion of the sample flows down the drain and the residence time of individual atoms in the optical path in the flame is brief (~10-4s).
Prof. Dr. Hisham Ezzat Abdellatef

44. Electrothermal Atomization
Atomization of entire sample in short period
Average sample time in optical path is seconds
Evaporation of sample
Microliter volume
Low temperature
Sample ashed at higher temperature
Increase current
Sample temperature goes to °C
Sample measured above heated surface
High sensitivity for small samples
Prof. Dr. Hisham Ezzat Abdellatef

45. Electrothermal Atomization
It provides enhanced sensitivity because the entire sample is atomized in a short period, and the average residence time of the atoms in the optical path is a second or more. A few microliters of sample are first evaporated at a low temperature and then ashed at a somewhat higher temperature in an electrically heated graphite tube or in a graphite cup. Then the current is rapidly increased to several hundred amperes, which caused the temperature to soar to perhaps 2000oC to 3000oC; atomization of the sample occurs in a period of a few milliseconds to seconds.
Prof. Dr. Hisham Ezzat Abdellatef

46. Prof. Dr. Hisham Ezzat Abdellatef

47. Prof. Dr. Hisham Ezzat Abdellatef
Atomizers
Electrothermal or Graphite Furnace Atomizer
Atomization occurs in an electrically heated graphite tube
The graphite tube is flushed with an inert gas (Ar) to prevent the formation of (non-absorbing) metal oxides
graphite tube
Prof. Dr. Hisham Ezzat Abdellatef

48. Prof. Dr. Hisham Ezzat Abdellatef
Performance Characteristics:
Electrothermal atomizers offer the advantage of unusually high sensitivity for small volumes of sample. Typically, sample volumes between 0.5 and 10 L are used; absolute detection limits lie in the range of to g of analyte. Furnace methods are slow-typically requiring several minutes per element. A final disadvantage is that the analytical range is low, being usually less than two orders of magnitude.
Prof. Dr. Hisham Ezzat Abdellatef

49. Electrothermal atomizer
Sample concentration
Prof. Dr. Hisham Ezzat Abdellatef

50. Prof. Dr. Hisham Ezzat Abdellatef
Atomization
Prof. Dr. Hisham Ezzat Abdellatef
From Skoog et al. (2004); Table 28-1, p.840

51. Atomization and Excitation
Atomic Emission Spectroscopy
The heat from a flame or an electrical discharge promotes an electron to a higher energy level
As the electron falls back to ground state, it emits a wavelength characteristic of the excited atom or ion
Prof. Dr. Hisham Ezzat Abdellatef
From Skoog et al. (2004); Figure 28-1, p.840

52. Prof. Dr. Hisham Ezzat Abdellatef
Atomic Line Spectra
Each spectral line is characteristic of an individual energy transition
E = hn =
Prof. Dr. Hisham Ezzat Abdellatef

53. Prof. Dr. Hisham Ezzat Abdellatef
Flame atomic absorption spectrometry
Beer Lamberts Law
A = log (Po/P)
A =  b c
where  is the molar absorptivity coefficient in units of mol-1 dm3 cm-1
b is the pathlength in cm
and c is the concentration in mol dm-3
In limits (below 0.8 Absorbance)
A vs. concentration Po P
sample b
Prof. Dr. Hisham Ezzat Abdellatef