1. Department of Chemistry Analytical and Environmental Chemistry (CH-204) Atomic Spectroscopy
Michael J. Hynes

2. Atomic Spectroscopy Lecturer Michael J. Hynes Texts
(1) Quantitative Chemical Analysis , by Daniel C. Harris
Pub. Freeman. Chapter 21 (4th Edition)
(2) Atomic Absorption and Emission Spectroscopy by E. Metcalfe & F.E. Prichard, Pub. Wiley (ACOL series).
(3) Most Analytical Chemistry Textbooks.
(4) Handout

3. Principle of Atomic Absorption pectroscopy
Atomic Absorption (AA) is based on the principle that a ground state atom is capable of absorbing light of the same characteristic wavelength as it would emit if excited to a higher energy level.
In flame AA, a cloud of ground state atoms is formed by aspirating a solution of the sample into a flame of a temperature sufficient to convert the element to its atomic state.
The degree of absorption of characteristic radiation produced by a suitable source will be proportional to the population of ground state atoms in the flame, and hence to the concentration of the element in the analyte.

4. Heat
Compound
Atoms
Spectra of atoms consist of SHARP LINES.
Each element has a characteristic spectrum.
Due to sharpness of lines, there is little overlap between the spectral lines of different elements.
Therefore, there is little interference.
Atomic Spectroscopy
High
Sample
Vapour
Temperature
Measure absorbance or emission of the atomic vapour.
Atomic spectroscopy deals with atoms.
Fe2+ and Fe3+ will not be distinguished.

6. Sensitivity Atomic spectroscopy is very sensitive for most elements.
Concentrations at the ppm level may be routinely determined using flame atomisation.
Using electrothermal atomisation, concentrations at the ppb may be determined.
1 ppm = 10-6g/g or 1g/g
The density of dilute aqueous solutions is approximately 1.00 so that:
1 g/g of aqueous solution = 1g/ml = 1 ppm ppm Fe = 1 x 10-6 g Fe/ml = 1.79 x 10-5 mol dm-3
1 ppm = 1 second in 11.6 days
1 ppb = 1 second in 31.7 years.

7. Atomic Absorption Spectroscopy
Absorbance = -log(It/I0)
Io = incident radiation (on sample)
It = transmitted radiation.
Atomic Emission Spectroscopy
Absorbance = -log(It/I0)
I0 =intensity of radiation reaching detector
in the absence of sample.
It = intensity of radiation reaching detector
when sample is being aspirated.

8. Both methods are used to determine the concentration of an element in solution.
Both methods use a standard curve.
Difference between UV and IR spectroscopy is that sample must be atomised.
Sample may be atomised by:
(1) A flame
(2) Electrically heated furnace
(3) A Plasma

9. Molecular Absorption Bandx
Abs
W1/2 x 
Band Width = W1/2 = width of band at half the height
W1/2 =
0.003 nm
Abs
25 nm+  
Molecular Absorption Band
Atomic Vapour
Absorption Band

10. Atomic Absorption and Emission Lines
Resonance Lines
E3(Excited state)
E2(Excited state)
E1(Excited state) E
Eo(Ground state)
Absorption
Emission
Most Intense Line
3 Absorption Lines
6 Emission lines
E = E1 - E0 = h = hc/

11. Comparison of the atomic emission spectrum of
silicon compared with the molecular absorption
spectrum of ethanal.

15. Laminar Flow Burner

16. EXAMPLES OF FLAME TEMPERATURES
Acetylene flow rate: 1000 ml/minute
Air flow rate: ml/minute
Sample uptake: ml/minute
 80% of sample goes to waste.
Therefore, dilution factor =  15,000
Highly inefficient!

17. Effect of temperature in atomic spectroscopy
Boltzman Distribution
E1 (g = 2)
(Excited state)
E1
E0 (g = 1)
(Ground state)
g is the multiplicity
Na E1 = 3.37 x J/atom
At 2600 K
At 2610 K N1/No = 1.74 x 10-4

18. Effect of Temperature on Sodium Atoms
The effect of a 10 K temperature rise on the ground state population is negligible (0.02 %).
In the excited state the fractional change is
So!

19. Effect of Temperature
Small changes in flame temperature (10 K) have little effect in atomic absorption but have significant effects in atomic emission spectroscopy.
Must have good flame control in atomic emission spectroscopy.

22. Radiation Source Beer’s Law only applies to monochromatic radiation.
In practice, monochromatic implies that the linewidth of the radiation being measured is less than the bandwidth of the absorbing species.
Atomic absorption lines very sharp with an inherent linewidth of nm.
Due to Doppler effect and pressure broadening, linewidths of atoms in a flame are typically nm.
Therefore, we require a source having a linewidth of less than 0.01 nm.
Typical monochromator has a bandwidth of 1 nm i.e. x100 greater than the linewidth of the atom in a flame.

23. Hollow Cathode Lamp
Used because of the requirement for a source of narrow lines of the correct frequency.
Hollow Cathode lamp
filled with argon or neon at a pressure of Pa (1 - 5 torr)

24. Monochromator bandwidth
(~100x greater than atomic lines

25. Hollow Cathode Lamp

26. Reactions in the Hollow Cathode Lamp
Apply sufficiently high voltage between the cathode and the anode:
(1) Ionization of the filler gas:
Ne + e- = Ne+ + 2e-
(2) Sputtering of the cathode element (M):
M(s) + Ne+ = M(g) + Ne
(3) Excitation of the cathode element (M)
M(g) + Ne+ = M*(g) + Ne
(4) Emission of radiation
M*(g)  (M(g) + h

27. Hollow Cathode Lamp
The cathode of the hollow cathode lamp (HCL) contains the element being analysed.
Therefore the atomic radiation emitted by the HCL has the same frequency as that absorbed by the analyte atoms in the flame or furnace.
The linewidth from the HCL is relatively narrow (compared to linewidths of atoms in the flame or furnace) because of low pressure in lamp and lower temperature in lamp (less Doppler broadening).
Thus the linewidth from the HCL is nearly “monochromatic” (vs sample).
Different lamp required for each element although some are mulit-element.

29. Hollow Cathode Lamp - The Filler Gas

30. Electrothermal Atomisation - Graphite Furnace
Sample holder consists of a graphite tube.
Tube is heated electrically
Beam of light passes through the tube.
Offers greater sensitivity than flames.
Uses smaller volume of sample
typically l ( ml).
All of sample is atomised in the graphite tube.
Atomised sample is confined to the optical path for several seconds (residence time in flame is very short).
Uses a number of stages as shown on next slide.

32. Stages in a Graphite Furnace
Typical conditions for Fe:
Drying stage: 125o for 20 sec
Ashing stage 1200o for 60 sec
Atomisation 2700o for 10 sec
Requires high level of operator skill.
Method development difficult.

33. Schematic Diagram of a Graphite Furnace
HCL

34. Advantages and Disadvantages of Flame AAS
equipment relatively cheap
easy to use (training easy compared to furnace)
good precision
high sample throughput
relatively facile method development
cheap to run
Disadvantages
lack of sensitivity (compared to furnace)
problems with refractory elements
require large sample size
sample must be in solution

35. Advantages and Disadvantages of Electrothermal Atomisation
very sensitive for many elements
small sample size
Disadvantages
poor precision
long cycle times means a low sample throughput
expensive to purchase and run (argon, tubes)
requires background correction
method development lengthy and complicated
requires a high degree of operator skill (compared to flame AAS)

37. Monochromator
The operation and sensitivity of the atomic absorption spectrometer spectrometer depends on the spectral band width of the resonance line emitted by the primary radiation source (the hollow cathode lamp).
The function of the monochromator is to isolate the resonance line from non-absorbing lines close to it in the source spectrum and from background continua and molecular emissions originating in the flame.
Hollow cathode lamps emit a number of lines, in the case of multi-element lamps, the number can be quite large and it is necessary to isolate the line of interest.

38. Detector/Measuring System
The intensity of the line source is measured with a photomultiplier, which produces an electrical signal proportional to the intensity of the incident light.
This signal is amplified and processed electronically to produce an output which on older instruments was read on a digital or analogue meter in either absorbance or concentration mode.
In modern instruments, the output is usually displayed on the computer screen of the PC controlling the instrument.
In order to eliminate the unwanted emissions from the flame, the light source is modulated by a chopper which is located between the hollow cathode lamp and the flame. The amplifier which modifies the signal from the photomultiplier is tuned in to the same frequency .
(Alternatively, the hollow cathode lamp may be modulated by applying an AC voltage at say 50 Hz.).

39. A B

40. Interferences
Interference is any effect that changes the signal when analyte concentrations remain unchanged.
While atomic absorption spectroscopy is relatively free from interferences, there are a number of interferences which must be dealt with.

41. Spectral Interference
This refers to overlap of analyte signals with signals originating from other elements in the sample or with signals due to the flame or furnace.
Example:
Al nm
V nm
Solution:
Separate elements or use a different line (which may be less sensitive).

42. Chemical Interference Formation of Stable or Refractory Compounds
Elements that form very stable compounds are said to be refractory because they are not completely atomised at the temperature of the flame or furnace.
Solution
Use a higher flame temperature (nitrous oxide/acetylene)
Use a release agent
Use protective chelation

43. Examples
Determination of calcium in the presence of sulfate or phosphate (e.g. in natural waters)
3Ca2+ + 2PO43- = Ca3(PO4)2
(stable compound)
Release agent
Add 1000 ppm of LaCl3
2LaCl3 + Ca3(PO4)2 = 3CaCl2 + 2La(PO4)
CaCl2 readily dissociates
Protective chelation
Ca3(PO4)2+3EDTA = 3Ca(EDTA) + 2PO43-
Ca(EDTA) dissociates readily.

44. Ionisation Interference
M(g)  M+(g) + e-
A problem in the analysis of alkali metal ions at low flame temperatures and other elements at higher temperatures.
Because alkali metals have the lowest ionisation potentials, they are most extensively ionised in flames.
At 2450 K and a pressure of 0.1 Pa, sodium is 5% ionised.
Potassium is 33% ionised under the same conditions.
Ionised atoms have energy levels which are different to the parent atoms
therefore the analytical signal is reduced.

45. Solution Add an ionisation suppressor
Add an easily ionised element such as Cs.
Add 1000 ppm of CsCl when analysing Na or K.
Cs is more readily ionised than either Na or K.
This produces a high concentration of electrons in the flame.

46. Matrix effects
The amount of sample reaching the flame is dependent on the physical properties of the solution:
viscosity
surface tension
density
solvent vapour pressure.
To avoid differences in the amount of sample and standard reaching the flame, it is necessary that the physical properties of both be matched as closely as possible.
Example:
Analysis of blood

47. Non-Atomic Absorption
Non-atomic absorption is caused by molecular absorption or light scattering by solid particles in the flame.
The absorption measurement obtained with a hollow cathode lamp is the sum of the atomic absorption and the non-atomic absorption.
The interference is corrected for by making a simultaneous measurement of the non-atomic absorption using a continuum source (usually deuterium)
this is called background correction

48. Operational Parameters
Sensitivity
the concentration of an element which will reduce the transmission by 1%
This corresponds to an absorption of

49. Calculations For an absorbance of 1.0 we require
1.0/ = 230 times the sensitivity.
For Cu, sensitivity = 0.05 ppm
For an absorbance of 1 we require a concentration of 11.5 ppm.
Using scale expansion of 10 we can usually obtain an absorbance of 1 using a 1 ppm solution of copper.

50. Detection Limit
The concentration of an element that gives a signal equal to three times the peak to peak noise level of the baseline.
Measure the baseline while aspirating a blank solution.

51. Terms to understand in atomic spectroscopy (1)
Atomic absorption spectroscopy
Atomic emission spectroscopy
Atomisation
Background correction
Boltzman distribution
Chemical interference
Detection limit
Graphite furnace
Hollow-cathode lamp

52. Terms to understand in atomic spectroscopy (2)
Inductively coupled plasma
Ionisation interference
Ionisation suppressor
Matrix
Matrix modifier
Nebulisation
Premix burner
Releasing agent
Spectral interference

53. Applications of AAS Agricultural analysis Clinical and biochemistry
soils
plants
Clinical and biochemistry
whole blood, plasma and serum Ca, Mg, Li, Na, K, Cu, Zn, Fe etc.
Metallurgy
ores, metals and alloys

54. Applications of AAS Lubricating oils Greases
Ba, Ca, Mg and Zn additives
Greases
Li, Na, Ca

55. Applications of AAS Water and effluents Food Animal feedstuffs
many elements e.g. Ca, Mg, Fe, Si, Al, Ba
Food
wide range of elements
Animal feedstuffs
Mn, Fe, Co, Cu, Zn, Cr, Se
Medicines
range of elements