1. Atomic Absorption Spectroscopy
Lecture 13 1

2. Flame Absorbance profiles
The temperature of a flame depends on the position from its tip.
So, it is necessary to concentrate our work on one spot in a flame and preferably adjust the height of the flame to get best signal.
In fact, not all elements require a specific height above burner tip but rather each element has its own requirements which largely reflect some of its properties.
For example:
For Silver (Ag) analysis:
Using higher distances from the tip so that higher temperatures are achieved to analyze for silver.
This is possible since silver will not be easily oxidized.

4. For Chromium analysis:
Occurs at lower heights (fuel rich flames) since at higher heights oxygen from atmosphere will force chromium to convert to the oxide which will not be atomized at flame temperatures.
For magnesium analysis:
Increasing the height above tip will increase the signal due to increased atomization at higher temperatures.
However, at higher distances the oxide starts to form leading to a decrease in signal.

5. Flame Atomizers (Continuous Atomizers)
There are several types of flame atomizers available.
1) The turbulent flow burner:
The simplest type that is very similar to conventional Bunsen burner.
Disadvantages:
This type of burner suffers from fluctuations in temperature since there is no good mechanism for homogeneous mixing of fuel and oxidant.
The drop size of nebulized sample is also inhomogeneous which adds to fluctuations in signal.
The path length of radiation through the flame is small which suggests a lower sensitivity of the technique.
Turbulent flow burners are also susceptible to flashback.

6. 2) Laminar flow burner (also called premix burner) :
The drawbacks of the turbulant burnaer are overcome using this most widely used burner.
Advantages:
Quite flames obtained.
Long path length are obtained.
Flashback is avoided.
Very homogeneous mixing between fuel, oxidant, and droplets take place.
Larger droplets are excluded and directed to a waste container.

7. A schematic representation of the burner

12. Fuel and Oxidant Regulators
The adjustment of the fuel to oxidant ratio and flow rate is undoubtedly very crucial.
1) Stoichiometric ratios are usually required.
2) Optimization is necessary in order to get highest signal.
In the determination of metals that form stable oxides:
A flame with excess fuel is preferred in order to decrease oxide formation.

13. Performance Characteristics of Flame Atomizers
Reproducibility of flame methods are usually superior to other atomization techniques.
However, the residence time of an atom in a flame is in the order of 10-4 s which is very short.
This is reflected in a lower sensitivity of flame methods as compared to other methods.
Also, conventional flames with reasonable burning velocities can produce relatively low temperatures which make them susceptible to interference from molecular species.

14. 2. Electrothermal Atomization
Advantages:
These have better sensitivities than flame methods.
Explained by:
A longer atom residence time is achieved (can be more than 1 s).
Also, atomization of the whole sample in a very short time.
Procedure:
As the name implies, a few mL of the sample are injected into the atomization chamber (a cylinder of graphite coated with a film of pyrolytic carbon).

16. Electrothermal Atomization processes:
Evaporation:
The solvent associated with the sample is evaporated in a low temperature (~120 oC) slow process (seconds).
b. Ashing:
Sample is ashed to burn organics associated with the sample at moderate temperatures (~600 oC, seconds).
c. Atomization:
The current is rapidly increased after ashing so that a temperature in the range from oC is obtained in less than1 second.

17. Electrothermal Atomizers (Discrete Atomizers)
The cylindrical graphite tube:
The heart of the atomizer.
Contain efficient heating elements and electronics.
Cylindrical graphite tube opened from both ends and has a central hole for sample introduction.
What type of Graphite:
It was found that porous graphite results in poor reproducibility since some of the analyzed materials will diffuse through porous graphite and will thus lead to a history effect.

19. Pyrolytic graphite :النوع المفضل
The cylindrical graphite is made from a special type of nonporous high quality graphite.
Description:
The length of the cylinder is 2-5 cm.
The diameter is less than 1 cm.
When the tube is fixed in place electrical contacts are achieved which are water cooled.
Two inert gas streams (argon) flow at the external surface and through the internal space of the tube to prevent oxidation and clean the tube after each measurement.
Usually, samples are analyzed in triplicates where three consecutive reproducible signals are required for each sample.

21. Atomic Absorption Spectroscopy
Lecture 14

22. Performance Characteristics of Electrothermal Atomizers
Advantages:
Electrothermal atomization is the technique of choice in case of small sample size.
Higher sensitivities than flames are ordinarily obtained.
Disadvantages:
The analysis time is in the few minutes range.
The relative precision is in the range of 5-10% as compared to 1% in flame methods.
In addition, the linear dynamic range is usually small (~ two orders of magnitude) which requires extra sample manipulation.
It may be also mentioned that better experienced personnel can achieve the merits of the technique.

23. Atomic Absorption Instrumentation
Atomic absorption instruments consist of:
1) A source of radiation.
2) A monochromator.
3) A flame or electrothermal atomizer in which sample is introduced.
4) and a transducer.

24. 1) Radiation Sources Radiation in the UV-Vis region is required.
We can not use broad band sources.
Because:
The best monochromators can not provide a bandwidth that is narrower than the atomic absorption line.
If the bandwidth of the incident radiation is wider than the line width, measurement will fail as absorption will be only a tiny fraction of a large signal which is difficult to measure and will result in very low sensitivities (Figure a).
Therefore, line sources with bandwidths narrower than that of the absorption lines must be used.

25. X 

26. This can be achieved by using a lamp producing the emission line of the element of interest where analyte atoms can absorb that line.
Conditions are established to get a narrower emission line (from lamp) than the absorption line.
This can in fact be achieved by getting an emission line of interest at the following conditions:

27. Conditions for getting an emission line of interest:
Low temperatures:
To decrease Doppler broadening (which is easily achievable since the temperature of the source is always much less than the temperature in flames).
2. Lower pressures:
This will decrease pressure broadening and will thus produce a very narrow emission line.

28. Difficulty:
Separate lamp for each element is neede for a which is troublesome and inconvenient.
Overcome:
However, recent developments lead to introduction of multielement lamps.
Requirement:
In this case, the lines from all elements should not interfere and must be easily resolved by the monochromator so that, at a specific time, a single line of one element is leaving the exit slit .

29. Types of Line sources: 1)Hollow Cathode Lamp (HCL)
This is the most common source in atomic absorption spectroscopy.
It is formed from a tungsten anode and a cylindrical cathode the interior surface of which is coated by the metal of interest.
The two electrodes are usually sealed in a glass tube with a quartz window and filled with argon at low pressure (1-5 torr).
Ionization of the argon is forced by application of about 300 V DC where positively charged Ar+ heads rapidly towards the negatively charged cathode causing sputtering.
A portion of sputtered atoms is excited and thus emit photons as atoms relax to ground state.
The cylindrical shape of the cathode serves to concentrate the beam in a limited region and enhances redeposition of sputtered atoms at the hollow surface.

32. Properties:
High potentials usually result in high currents which, in turn, produce more intense radiation.
However, Doppler broadening increases as a result.
In addition, the higher currents will produce high proportion of unexcited atoms that will absorb some of the emission beam which is referred to as self absorption (a lower intensity at the center of the line is observed in this case).

33. 1) Electrodeless Discharge Lamps (EDL)
Is a sealed quartz tube containing a few torr of an inert gas and a small quantity of the metal of interest.
Excitation of the metal is achieved by a radiofrequency or a microwave powered coil through ionization of argon, due to high energetic radiofrequency.
Ionized argon will hit the metal causing excitation of the atoms of the metal of interest.
The output power of the EDL lamp is higher than the HCL lamp.
However, compared to HCL lamps, EDL lamps are rarely used.

36. Emission in Flames
There can be significant amounts of emission produced in flames: due to
Presence of flame constituents (molecular combustible products).
Sometimes impurities in the burner head.
This emitted radiation must be removed for successful sensitive determinations by AAS.
What happened If it not removed??
1) In case of absorption measurments:
A negative error will always be observed.
We can visualize this effect by considering the schematic below:

38. The detector will see the overall signal:
Which is the power of the transmitted beam (P) in addition to the power of the emitted radiation from flame (Pe).
Therefore if we are measuring absorbance, this will result in a negative error as the detector will measure what it appears as:
A high transmittance signal (actually it is P + Pe).

39. Explanation:
Absorbance is defined as:
A = log (Po/P)
However, in absence of a sample the detector will measure S1 , where:
S1 = Po + Pe
In presence of a sample, the detector will measure S2, where:
S2 = P + Pe
Therefore A = log (Po + Pe)/(P + Pe)
At high absorbances, Pe may become much larger than P and the absorbance will be a constant since both Po and Pe are constants:
A = log (Po + Pe)/(Pe)

40. 2) In case of emission measurements:
Here will always be a positive error since emission from flame is an additive value to the actual sample emission.
It is therefore obvious that we should get rid of this interference from emission in flames.

41. Correction method for background of flame emission Source Modulation
For excluding the emission signal from flames can easily.
1) Can be done by an addition of a chopper to the instrumental design.
What is the chopper:
The chopper is a motor driven device that has open and solid (mirrors in some cases) alternating regions as in the schematic:

43. The function of the chopper:
Is to chop the light leaving the source so that when the incident beam hits the chopper at the solid surface, the beam will be blocked and detector will only read the emitted signal from the flame.
As the chopper rotates and the beam emerges to the detector, the detector signal will be the sum of the transmitted signal plus that emitted from the flame.
The signal processor will be able to subtract the first signal from the second one, thus excluding the signal from emission in flames.

44. Beam chopper for
subtracting flame
background emission
Lamp and flame
emission reach detector
Only flame
emission reaches
detector
(c) Resulting signal

45. This can be represented by the following equations:
Signal 1 (Blocked Beam)  = Pe
Signal 2 (Transmitted Beam) = P + Pe
Overall Difference Signal = (P + Pe) - Pe = P (Corrected Signal)
This correction method for background emission in flames is called source modulation.

46. The schematic of the AAS instrument with source modulation correction can be represented by the following schematic:

47. It should be recognized that addition of extra components to an instrument will decrease the signal to noise ratio and addition of a moving component is usually regarded as a disadvantage due to higher need for maintenance.
2) Another procedure:
Which can overcome the emission from flames is to use a modulated power supply that will give fluctuating intensities at some frequency (say for example pulsed radiation at a specific frequency).

48. The emission from flames is a continuous signal.
But that from the source is modulated.
Now if we use a high pass RC filter, only the fluctuating signal will be measured as signal while the DC signal will be considered zero as it can not pass through the electronic filter.
The high pass RC filter:
Is a device which uses a resistor and a capacitor the impedance of which is inversely proportional to the frequency of the modulated signal.
Therefore, only high frequencies will have low impedance and can pass through the capacitor while signals of low frequencies will suffer very high resistance and will not be able to go through the capacitor.

49. AAS Instruments
Instruments in AAS can be regarded as 1) Single or 2) double beam instruments.
1) Single Beam Atomic Absorption spectrophotometers.
A single beam instruments is the same as the one described above (source modulation section) or generally:

50. The term “spectrophotometer” implies that:
1- The instrument uses a dispersive monochromator (containing a prism or a grating).
2) Also, the detector is a photomultiplier tube in most cases.

53. 2) Double Beam Atomic Absorption Spectrophotometers
In this type of instruments:
The incident beam is split into two beams of equal intensity by a chopper with the solid surface being a mirror.
One of the beams will traverse the sample in the atomizer while the other is considered as a reference.
Detector signals will be consecutive readings of both the reference and sample beams.
The ratio of the reference to the sample beams is recorded to give the final signal.

54. A schematic representation of a double beam instrument is shown below:

55. It should be emphasized here that in the absence of sample:
Pr is not equal to P since the reference beam traverses through air while the other beam traverses through the flame.
In flames, particulates and molecular species scatter and absorb a portion of incident radiation, which results in a lower intensity of the beam.
To act as a real double beam, The AA spectrophotometer reference beam should pass through a reference flame.

56. But even if we do that:
There are no guarantees that both beams will be of equal intensities,,,
Because it is almost impossible to obtain exactly equivalent flames.
It is therefore important to understand that:
The excellent features of a double beam configuration are not achievable in AAS instrumentation.

57. Atomic Absorption Spectroscopy
Lecture 16

58. Interferences in Atomic Absorption Spectroscopy
There are two major classes of interferences in AAS:
1)The first class: Spectral interferences.
Related to spectral properties of components other than atomized analyte.
2) The other class: Chemical interferences.
Related to the chemical processes occurring in flames and electrothermal atomizers and their effects on signal.
*These are usually more important than spectral interferences.

59. First: Spectral Interferences
1. Spectral line Interference
Usually, interferences due to overlapping lines is rare since atomic lines are very narrow.
In cases of line interference, it can be simply overcome by:
Choosing to perform the analysis using another line that has no interference with other lines.
Therefore, line interference is seldom a problem in atomic spectroscopy.

60. 2. Scattering
Caused by particulates from combustion products and sample materials which lead to scatter radiation that will result in positive analytical error.
The error from scattering can be corrected for:
By making a blank measurement.
Scattering phenomenon is most important when concentrated solutions containing elements that form refractory oxides (like Ti, Zr, and W) are present in sample matrix.

61. Types of particles that make scattering:
Metal oxide particles with diameters larger than the incident wavelength (a real problem).
2) Samples containing organic materials or organic solvents which can form carbonaceous (especially in cases of incomplete combustion) particles.

62.  3. Broad Band Absorption
Broad band spectrum limit the sensitivity of the technique.
Resulted from:
Molecular species from combustion products or sample matrix that formed in flames or electrothermal atomizers.
Spectral interferences by matrix products are not widely encountered in flame methods.
Largely overcome by:
1) Adjusting various experimental conditions like fuel/oxidant ratio or temperature.

63. 2) Using radiation buffer :
Much higher concentration of interferent is used than that initially present in sample material, in both sample and standards.
The contribution from sample matrix will thus be insignificant.
Spectral interferences due to matrix are severe in electrothermal methods and must thus be corrected for.

64. Background Correction Methods
a.      The Two Line Correction Method:
In this method, a reference line from the source (from an impurity in cathode or any emission line) is selected where this line should have the following properties:
1.      Very close to analyte line
2.      Not absorbed by analyte
If such a line exists, since the reference line is not absorbed by the analyte, its intensity should remain constant throughout analysis.

66. However, if its intensity decreases:
This will be an indication of absorbance or scattering by matrix species.
The decrease in signal of the reference line is used to correct for the analyte line intensity (by subtraction of the absorbance of the reference from that of the analyte).
This method is very simple but unfortunately it is not always possible to locate a suitable reference line.

67. b. The Continuum Source Method
Is the most common method although, for reasons to be discussed shortly, it has major drawbacks and fails a lot.
In this technique:
Radiation from a deuterium lamp and a HCL lamp alternately pass through the graphite tube analyzer.
It is essential to keep the slit width of the monochromator sufficiently wide in order to pass a wide bandwidth of the deuterium lamp radiation.

68. In this case, the absorbance by analyte atoms is negligible and absorbance can be attributed to molecular species in matrix.
The absorbance of the beam from the deuterium lamp is then subtracted from the analyte beam (HCL) and thus a background correction is obtained.

71. Problems Associated with Background Correction Using D2 Lamp
1. The very hot medium inside the graphite tube is inhomogeneous and thus signal is dependent on the exact path a beam would follow inside the tube.
Therefore, exact alignment of the D2 and HCL lamps should be made.
2. The radiant power of the D2 lamp in the visible is insignificant which precludes the use of the technique for analysis of analytes in the visible region.
3.  Addition of an extra lamp and chopper will decrease the signal to noise ratio.

72. c. Background Correction Based on Zeeman Effect
Theory: Zeeman has observed that:
When gaseous atoms (but not molecules) are placed in a strong magnetic field (~ 1 tesla), splitting of electronic energy levels takes place.
The simplest splitting:
One energy level results in three energy levels:
One at a higher energy, (- s satellite)
Second at a lower energy (+ s satellite lines)
- Third remains at the same energy as the level in absence of the magnetic field (central p line).
Furthermore, the p line has twice the absorbance of a s line and absorbs polarized light parallel ║to direction of the magnetic field while the two s lines absorb light perpendicular ┴ to magnetic field.

73. ┴ ║ ┴

74. Zeeman Effect – Magnetic

75. Light from a HCL lamp will pass through a rotating polarizer that passes polarized light parallel to external magnetic field at one cycle and passes light perpendicular to field in the other cycle.
The idea of background correction using this method :
Is to allow light to traverse the sample in the graphite furnace atomizer and record the signal for both polarizer cycles using the wavelength at the p line.

77. a.  First cycle:
Light parallel to field; the p line of the analyte absorbs in addition to absorbance by matrix (molecular matrix absorb both polarized light parallel or perpendicular to field)
Signal a = Ap + Amatrix
b. Second cycle:
Light perpendicular to field; the p line of analyte will not absorb light perpendicular to field and s lines will also not affect absorbance at the p line wavelength.
Only matrix will absorb.
Signal b = AMatrix

78. The overall signal is the difference of the two signals = Ap
Therefore, excellent background correction is achieved using the Zeeman effect.
This background correction method results in good correction and is usually one of the best methods available.

79. Zeeman effect correction
B - Rotating polarizer
E – absorption at // polarization
Very sensitive correction technique

80. Atomic Absorption Spectroscopy
Lecture 17

81. Second: Chemical Interferences
These are interferences resulting from chemical processes occurring in flames and electrothermal atomizers and affect the absorption signal.
Chemical processes occurring in flames:
The burnt gases as behaving like a solvent.
This is necessary since our knowledge of gaseous state reaction equilibria is rather limited.

82. 1. Formation of Compounds of Low Volatility
Chemical interferences include three major processes:
1. Formation of Compounds of Low Volatility
Anionic species: (-ve error)
Forming compounds of low volatility are the most important.
Decrease the sensitivity.
For example: the absorption signal of calcium will be decreased as higher concentrations of sulfate or phosphate are introduced.
2) Cations species: (-ve error)
Forming combined products with the analyte will also decrease the signal obtained for the analyte.
For example: aluminum forms a heat stable compound with magnesium.

83. Elimination of Low Volatility Compounds
Addition of a releasing agent: cations that can replace the analyte (preferentially react with the anion).
In this case the analyte is released from the compound of low volatility and replaced by the releasing agent.
Examples of RA:
Lanthanum or strontium are good releasing agents in the determination of calcium in presence of phosphate or sulfate.
Also, lanthanum or strontium are good releasing agents in the determination of magnesium in presence of aluminum since both can replace magnesium.

84. Addition of a protective agent: organic ligands that form stable volatile species with analytes are called protective agents.
Example:
EDTA and 8-hydroxyquinoline which will form complexes with calcium even in presence of sulfate and phosphate or aluminum.
3. Use  of higher temperature: is the simplest procedure to try if it is possible

85. 2. Dissociation Equilibria
Dissociation reactions occur in flames: where the outcome of the process is desired to produce the atoms of analyte.
For example: metal oxides and hydroxides will dissociate in flames to produce the atoms as in the equations
MO = M + O
M(OH)2 = M + 2 OH

86. Remember that: we are not working in solution to dissociate the compounds into ionic species.
In fact: not much is known about equilibrium reactions in flames.
Alkaline earth oxides and hydroxides are relatively stable and will definitely show characteristic broad band spectra (more intense than line spectra), except at very high temperatures.
Alkali metals oxides and hydroxides: Show opposite behavior for which are instable even at lower flame temperatures and thus produce line spectra.

87. An equilibrium can be established for the dissociation of compounds containing atoms other than oxygen, like NaCl where:
NaCl = Na + Cl
Now, if the signal from a solution of NaCl was studied in presence of variable amounts of Cl (from HCl, as an example):
The signal will be observed to decrease as the concentration of Cl is increased; a behavior predicted by the Le Chatelier principle in solutions.

88. The same phenomenon is observed when a metal oxide is analyzed using a fuel rich flame (low O2 ) or a lean flame (high O2).
Signal will be increased in fuel rich flames since the dissociation of metal oxides is easier due to less oxygen while the opposite takes place in lean flames (oxygen rich).

89. A good example on dissociation equilibria can be presented for the analysis of vanadium in presence of aluminum and titanium:
Fuel rich flames:
Result in higher absorbance signal for vanadium since the little oxygen present in flames will be mainly captured by Al and Ti, thus more V atoms are available.
However, in lean flames:
Excess oxygen is present and thus vanadium will form the oxide and addition of extra Ti and Al will not affect the signal. 

90. 3. Ionization Equilibria
Ionization in fuel/air flames is very limited due to relatively low temperatures.
However, in fuel/nitrous oxide or fuel/oxygen mixtures, ionization is significant.
Therefore, at higher temperatures an important portion of atoms can be converted to ions:
M = M+ + e
K = [M+][e]/[M]

91. Ionization in flames:
Lead to the decrease in absorption signal for alkali metals at very high temperatures.
After signal increase by increasing Temp. it starts to decrease as temperature is further increased; a consequence of ionization.
Therefore:
Usually lower flame temperatures are used for determination of alkali metals.
Ionization suppressor:
A material that is added to samples in order to produce large number of electrons, which results in higher sensitivities.

93. Practical Details in AAS
Sample Preparation:
- The need for introduction of samples in the solution form.
This necessitates the dissolution of the sample (requires very good experience).
Care taken in order not to lose any portion of the analyte.
Make sure that the reagents, acids, etc. used in the dissolution and pretreatment of the sample are free from analyte impurities.

94. For direct introduction of sample to an electrothermal atomizer without pretreatment (like serum samples), definitely, electrothermal atomizers will have an obvious advantage over flame methods which require nebulization.

95. Organic Solvents
Increased nebulization rate due to lower surface tension of organic solvents which produces smaller droplets as well as faster evaporation of solvents in flames will result in better sensitivities.
Immiscible organic solvents containing organic ligands are used to extract metal ions of interest and thus concentrate them in a small volume (thus increasing sensitivity) and excluding possible interferences due to matrix components.

96.   Calibration Curves
The absorbance of a solution is directly proportional to its concentration.
Deviation from linearity:
Due to the large number of variables in AAS.
Using standard procedure to construct a relation between the absorbance and concentration for a series of solutions of different concentrations.
The thus constructed graph is called a calibration curve.

97. The unknown analyte absorbance is found and the concentration is calculated or located on the curve.
Neither interpolation nor extrapolation is permitted to the calibration curve.
A sample can be diluted or the calibration curve may be extended but always the analyte absorbance should be within the standard absorbance range recorded.
Usually, the concentration axis has the ppm or ppb units.

98. Standard Addition method
Used to overcome interferences.
In addition, the use of this method provides better correlations between standards and sample results due to constant nebulization rates.
The method involves addition of the same sample volume to a set of tubes or containers.

99. Variable volumes of a standard are added to the tube set followed by completion to a specific volume.
Now, all tubes contain the same amount of sample but different concentrations of analyte.
A plot is then made for the volume of standard and absorbance.
This plot will have an intercept (b) with the y axis and a slope equals m.

101. The concentration of the analyte can be determined by the relation:
Cx = bCs/mVx
Where, Cx and Vx are concentration and volume of analyte and Cs is the concentration of standard.
One can only use two points to get the analyte concentration using the relation:
Cx = AxCsVs/(At –Ax)Vx

102. Detection Limits
Usually, atomic absorption based on electrothermal atomization has better sensitivities and detection limits than methods based on flames.
In general:
Flame methods:
have detection limits in the range from 1-20 ppm.
Electrothermal methods:
have detection limits in the range from 1-20 ppb.

103. This range can significantly change for specific elements where not all elements have the same detection limits.
For example, detection limits fro mercury and magnesium using electrothermal atomization are 100 and 0.02 ppb.
While the detection limits for the same elements using flame methods are 500 and 0.1 ppm, respectively.

104. Accuracy
Flame methods are superior to electrothermal methods in terms of accuracy.
The relative error in flame method can be less than 1% while that for electrothermal method occurs in the range from 5-10%.
Also, electrothermal methods are more susceptible to molecular interferences from the matrix components.
Therefore, unless a good background correction method is used, large errors can be encountered in electrothermal methods depending on the nature of sample analyzed.

105. Flame Photometry It is a flame emission technique.
We introduce it here because we will not be back to flame methods in later chapters.
The basics of the technique are extremely simple:
Sample is nebulized into a flame.
Atomization occurs due to high flame temperatures.
- Also excitation of easily excitable atoms can occur.

106. Emission of excited atoms is proportional to concentration of analyte.
Flame emission is good for such atoms that do not require high temperatures for atomization and excitation, like Na, K, Li, Ca, and Mg.
The instrument is very simple and excludes the need for a source lamp.
The filter is exchangeable in order to determine the analyte of interest and, in most cases, a photomultiplier tube is used as the detector.