1. Atomic Absorption & Atomic Emission Spectroscopy
Dr.Syed Muzzammil Masaud
Mphill.Pharmaceutical Chemistry

2. BASIC PRINCIPLE
ATOMIC ABSORPTION SPECTROSCOPY (AAS) is an
analytical technique that measures the concentrations of
elements. It makes use of the absorption of light
by these elements in order to measure their
concentration .

3. - Atomic-absorption spectroscopy quantifies the absorption of ground state atoms in the gaseous state .
The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels . The analyte concentration is determined from the amount of
absorption.

4. Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.
- Atomic absorption is a very common technique for detecting metals and metalloids in environmental samples.

5. Elements detectable by atomic absorption are highlighted in pink in this periodic table
                                                                                             

6. The Atomic Absorption Spectrometer
Atomic absorption spectrometers have 4 principal components
1 - A light source ( usually a hollow cathode lamp )
2 – An atom cell ( atomizer )
3 - A monochromator
4 - A detector , and read out device .

7. Schematic Diagram of an Atomic Absorption Spectrometer
Detector and readout device
Light source
(hollow cathode Lamp )
atomizer
monochromator

8. Atomic Absorption Spectrophotometer

9. 1 – Light Source The light source is usually a hollow cathode
lamp of the element that is being measured . It contains a tungsten anode and a hollow cylindrical cathode made of the element to be determined. These are sealed in a glass tube filled with an inert gas (neon or argon ) . Each element has its own unique lamp which must be used for
that analysis .

10. Hollow Cathode Lamp cathode Anode Quartz window Pyrex body Anode

11. How it works
Applying a potential difference between the anode and the cathode leads to the ionization of some gas atoms . These gaseous ions bombard the cathode and eject metal atoms from the cathode in a process called sputtering. Some sputtered atoms are in excited states and emit radiation characteristic of the metal as they fall back to the ground state .

12. Scheme of a hollow cathode lamp

13. The shape of the cathode which is hollow cylindrical concentrates the emitted radiation into a beam which passes through a quartz window
all the way to the vaporized sample.
Since atoms of different elements absorb
characteristic wavelengths of light. Analyzing
a sample to see if it contains a particular element means using light from that element .

14. For example with lead, a lamp containing lead emits light from excited lead atoms that produce the right mix of wavelengths to be absorbed by any lead atoms from the sample .
A beam of the electromagnetic radiation emitted from excited lead atoms is passed through the vaporized sample. Some of the radiation is absorbed by the lead atoms in the sample. The greater the number of atoms there is in the vapor , the more radiation is absorbed .

15. 2 – Atomizer
Elements to be analyzed needs to be in atomic sate Atomization is separation of particles into individual molecules and breaking molecules into atoms .This is done by exposing the analyte to high temperatures in a flame or graphite furnace .

16. The role of the atom cell is to primarily dissolvate a liquid sample and then the solid particles are vaporized into their free gaseous ground state form . In this form atoms will be available to absorb radiation emitted from the light source and thus generate a measurable signal proportional to concentration . There are two types of atomization : Flame and Graphite furnace atomization .

18. Flame Flame AA can only analyze solutions , where
it uses a slot type burner to increase the
path length, and therefore to increase the total
absorbance .
Sample solutions are usually
introduced into a nebuliser by being sucked up a
capillary tube .In the nebuliser the sample is
dispersed into tiny droplets , which can be
readily broken down in the flame.

19. FLAME ATOMIZERS Used in all Atomic Spectroscopic techniques
Converts analyte into free atoms in the form of vapor phase free atoms
Heat is required
Routes for sample introduction

20. Various flame atomization techniques

21. Types of Flames Used in Atomic Spectroscopy

22. Processes that take place in flame

23. Effect of flame temperature on excited state population
# atoms in
Excited state
Boltzmann constant
Temperature
# atoms in
Ground state
Energy
difference
Statistical
factor

24. For Zn: N*/No = %

25. Thus 99.998% of Na atoms are in the ground state
Atomic emission uses Excited atoms
Atomic absorption uses Ground state atoms

26. Effect of flame temperature on excited state population

27. ATOMIZATION DEVICES ATOMIZATION :
A process of forming free atoms by heat
Atomizers are devices that carry out atomization:
Continuous
Non-continuous
Continuous: (Constant temperature with time)
Flame
Plasma
Non-Continuous: (temperature varies with time)
Electrothermal
Spark discharge

28. SAMPLE INTRODUCTION SYSTEMS
In continuous atomizers sample is constantly introduced in form of droplets, dry aerosol, vapor
Nebulizer : A device for converting the solution into fine spray or droplets
Continuous sample introduction is used with continuous nebulizers in which a steady state atomic population is produced. Sample is introduced in fixed or discrete amounts.
Discontinuous samplers are used with continuous atomizers

29. 1- Discrete samples are introduced into atomizers in many
ways:
Electrothermal atomizers
a syringe is used
a transient signal is produced as temperature changes
with time and sample is consumed
2- Indirect insertion (Probe)
sample is introduced into a probe (carbon rod) and mechanically moved into the atomization region
vapor cloud is transient because sample introduced is
limited

30. 3- Flow Injection 4- Hydride Generation
The analyte is introduced into the carrier stream into a nebulizer as mist
4- Hydride Generation
the volatile sample is stripped from the analyte solution and carried out by a gas into the atomizer. This strip is followed by chemically converting the analyte to hydride vapor form.

31. 5- With Arc Spark
Solids are employed
6- Laser Microbe Technique
A beam of laser is directed onto a small solid sample, gets vaporized, atomized by relative heating. Either sample is probed by encoding system or vapor produced is swept into a second absorption or fluorescence

32. Nebulization gas is always compressed, usually acts as the oxidant; it is oxygen (O2) in flame and argon (Ar) in plasma
Nebulization chambers produce smaller droplets and remove or drain larger droplets called aerosol modifiers
Aspiration rate is proportional to compressed gas pressure. The pressure drops through capillary, here 1/4 capillary diameters are recommended. This is inversely proportional to viscocity of the solution
Peristaltic and/or syringe pumps could be used

33. Oxidant and fuel are usually brought into the nebulization chamber through a separate port. They mix and pass the burner head called premixed burner system.
Add organic solvents to reduce the size of the drop

34. The Atomic Absorption Spectrometer Sample Introduction System
Nebuliser
Capillary
Solution

35. The fine mist of droplets is mixed with fuel ( acetylene ) , and oxidant ( nitrous oxide) and burned.
The flame temperature is important because it influences the distribution of atoms. It can be manipulated by oxidant and fuel ratio.

36. Graphite Furnace
The graphite furnace has several advantages over a flame. First it accept solutions, slurries, or solid samples.
Second it is a much more efficient atomizer than a flame and it can directly accept very small absolute quantities of sample. It also provides a reducing environment for easily oxidized elements. Samples are placed directly in the graphite furnace and the furnace is electrically heated in several steps to dry the sample, ash organic matter, and vaporize the analyte atoms.
It accommodates smaller samples but it’s a difficult operation, because the high energy that is provided to atomize the sample particles into ground state atoms might excite the atomized particles into a higher energy level and thus lowering the precision .

37. 3- Monochromators
This is a very important part in an AA spectrometer. It is used to separate out all of the thousands of lines. Without a good monochromator, detection limits are severely compromised. A monochromator is used to select the specific wavelength of light which is absorbed by the sample, and to exclude other wavelengths. The selection of the specific light allows the determination of the selected element in the presence of others.

38. 4 - Detector and Read out Device
The light selected by the monochromator is directed onto a detector that is typically a photomultiplier tube , whose function is to convert the light signal into an electrical signal proportional to the light intensity.
The processing of electrical signal is fulfilled by a signal amplifier . The signal could be displayed for readout , or further fed into a data station for printout by the requested format.

39. Calibration Curve
A calibration curve is used to determine the unknown concentration of an element in a solution. The instrument is calibrated using several solutions of known concentrations. The absorbance of each known solution is measured and then a calibration curve of concentration vs absorbance is plotted.
The sample solution is fed into the instrument, and the absorbance of the element in this solution is measured .The unknown concentration of the element is then calculated from the calibration curve

40. Calibration Curve A 1.0 - b 0.9 - S 0.8 - . o 0.7 - . r 0.6 - .
Concentration ( g/ml )

41. Determining concentration from Calibration Curve
A absorbance measured b S o r b a
n concentration calculated c e
Concentration ( mg/l )

42. Interferences
The concentration of the analyte element is considered to be proportional to the ground state atom population in the flame ,any factor that affects the ground state atom population can be classified as an interference . Factors that may affect the ability of the instrument to read this parameter can also be classified as an interference .

43. The different interferences that are encountered in atomic absorption spectroscopy are :
- Absorption of Source Radiation : Element other than the one of interest may absorb the wavelength being used.
- Ionization Interference : the formation of ions rather than atoms causes lower absorption of radiation .This problem is overcome by adding ionization suppressors.
- Self Absorption : the atoms of the same kind that are absorbing radiation will absorb more at the center of the line than at the wings ,and thus resulting in the change of shape of the line as well as its intensity .

44. - Back ground Absorption of Source Radiation : This is caused by the presence of a particle from incomplete atomization .This problem is overcome by increasing the flame temperature . - Transport Interference : Rate of aspiration, nebulization, or transport of the sample ( e g viscosity, surface tension, vapor pressure , and density ) .

45. 2 Atomic Emission Spectroscopy
Atomic emission spectroscopy is also an analytical technique that is used to measure the concentrations of elements in samples . It uses quantitative measurement of the emission from excited atoms to determine analyte concentration .

46. The analyte atoms are promoted to a higher energy level by the sufficient energy that is provided by the high temperature of the atomization sources .
The excited atoms decay back to lower levels by emitting light . Emissions are passed through monochromators or filters prior to detection by photomultiplier tubes.

47. The instrumentation of atomic emission spectroscopy is the same as that of atomic absorption ,but without the presence of a radiation source . In atomic Emission the sample is atomized and the analyte atoms are excited to higher energy levels all in the atomizer .

48. Schematic Diagram of an Atomic Emission spectrometer

49. Introduction to AES Atomization Emission Sources Advantages of plasma
Flame – still used for metal atoms
Electric Spark and Arc
Direct current Plasmas
Microwave Induced Plasma
Inductively Coupled Plasma – the most important technique
Advantages of plasma
Simultaneous multi-element Analysis – saves sample amount
Some non-metal determination (Cl, Br, I, and S)
Concentration range of several decades (105 – 106)
Disadvantages of plasma
very complex Spectra - hundreds to thousands of lines
High resolution and expensive optical components
Expensive instruments, highly trained personnel required

50. 10A Plasam Source AES Plasma Three main types
an electrically conducting gaseous mixture containing significant concentrations of cations and electrons.
Three main types
Inductively Coupled Plasma (ICP)
Direct Current Plasma (DCP)
Microwave Induced Plasma (MIP)

51. ICP Inductively Coupled Plasma (ICP)
Plasma generated in a device called a Torch
Torch up to 1" diameter
Ar cools outer tube, defines plasma shape
Rapid tangential flow of argon cools outer quartz and centers plasma
Rate of Argon Consumption L/Min
Radio frequency (RF) generator 27 or 41 MHz up to 2 kW
Telsa coil produces initiation spark
Ions and e- interact with magnetic field and begin to flow in a circular motion.
Resistance to movement (collisions of e- and cations with ambient gas) leads to ohmic heating.
Sample introduction is analogous to atomic absorption.

52. Sample introduction Nebulizer Electrothermal vaporizer
Table 8-2 methods of sample introducton

53. Nebulizer convert solution to fine spray or aerosol
Ultrasonic nebulizer
uses ultrasound waves to "boil" solution flowing across disc
Pneumatic nebulizer
uses high pressure gas to entrain solution

54. Electro-thermal vaporizer ETV
electric current rapidly heats crucible containing sample
sample carried to atomizer by gas (Ar, He)
only for introduction, not atomization

55. Plasma structure Brilliant white core Flame-like tail
Ar continuum and lines
Flame-like tail
up to 2 cm
Transparent region
where measurements are made (no continuum)

56. Plasma characteristics
Hotter than flame (10,000 K) - more complete atomization/ excitation
Atomized in "inert" atmosphere
Ionization interference small due to high density of e-
Sample atoms reside in plasma for ~2 msec and
Plasma chemically inert, little oxide formation
Temperature profile quite stable and uniform.

57. DC plasma First reported in 1920s
DC current (10-15 A) flows between C anodes and W cathode
Plasma core at 10,000 K, viewing region at ~5,000 K
Simpler, less Ar than ICP - less expensive
Less sensitive than ICP
Should replace the carbon anodes in several hours

58. Atomic Emission Spectrometer
May be >1,000 visible lines (<1 Å) on continuum
Need
higher resolution (<0.1 Å)
higher throughput
low stray light
wide dynamic range (>1,000,000)
precise and accurate wavelength calibration/intensities
stability
computer controlled
Three instrument types:
sequential (scanning and slew-scanning)
Multichannel - Measure intensities of a large number of elements (50-60) simultaneously
Fourier transform FT-AES

59. Desirable properties of an AE spectrometer

60. Sequential vs. multichannel
Sequential instrument
PMT moved behind aperture plate,
or grating + prism moved to focus new l on exit slit
Pre-configured exit slits to detect up to 20 lines, slew scan
characteristics
Cheaper
Slower
Multichannel instrument
Polychromators (not monochromator) - multiple PMT's
Array-based system
charge-injection device/charge coupled device
Expensive ( > $80,000)
Faster

61. Sequential vs. multichannel

62. Sequential monochromator
Slew-scan spectrometers
even with many lines, much spectrum contains no information
rapidly scanned (slewed) across blank regions (between atomic emission lines)
From 165 nm to 800 nm in 20 msec
slowly scanned across lines
0.01 to nm increment
computer control/pre-selected lines to scan

63. Slew scan spectrometer
Two slew-scan gratings
Two PMTs for VIS and UV
Most use holographic grating

64. Scanning echelle spectrometer
PMT is moved to monitor signal from slotted aperture.
About 300 photo-etched slits
1 second for moving one slit
Can be used as multi channel spectrometer
Mostly with DC plasma source

65. AES instrument types Three instrument types:
sequential (scanning and slew-scanning)
Multichannel - Measure intensities of a large number of elements (50-60) simultaneously
Fourier transform FT-AES

66. Multichannel polychromator AES
Rowland circle
Quantitative det.
20 more elements
Within 5 minutes
In 10 minutes

67. Applications of AES AES relatively insensitive
small excited state population at moderate temperature
AAS still used more than AES
less expensive/less complex instrumentation
lower operating costs
greater precision
In practice ~60 elements detectable
10 ppb range most metals
Li, K, Rb, Cs strongest lines in IR
Large # of lines, increase chance of overlap

68. Detection power of ICP-AES

69. ICP/OES INTERFERENCES
Spectral interferences:
caused by background emission from continuous or recombination phenomena,
stray light from the line emission of high concentration elements,
overlap of a spectral line from another element,
or unresolved overlap of molecular band spectra.
Corrections
Background emission and stray light compensated for by subtracting background emission determined by measurements adjacent to the analyte wavelength peak.
Correction factors can be applied if interference is well characterized
Inter-element corrections will vary for the same emission line among instruments because of differences in resolution, as determined by the grating, the entrance and exit slit widths, and by the order of dispersion.

70. Physical interferences of ICP
cause
effects associated with the sample nebulization and transport processes.
Changes in viscosity and surface tension can cause significant inaccuracies,
especially in samples containing high dissolved solids
or high acid concentrations.
Salt buildup at the tip of the nebulizer, affecting aerosol flow rate and nebulization.
Reduction
by diluting the sample
or by using a peristaltic pump,
by using an internal standard
or by using a high solids nebulizer.

71. Interferences of ICP Chemical interferences:
include molecular compound formation, ionization effects, and solute vaporization effects.
Normally, these effects are not significant with the ICP technique.
Chemical interferences are highly dependent on matrix type and the specific analyte element.

72. Memory interferences:
When analytes in a previous sample contribute to the signals measured in a new sample.
Memory effects can result
from sample deposition on the uptake tubing to the nebulizer
from the build up of sample material in the plasma torch and spray chamber.
The site where these effects occur is dependent on the element and can be minimized
by flushing the system with a rinse blank between samples.
High salt concentrations can cause analyte signal suppressions and confuse interference tests.

73. Typical Calibration ICP curves

74. Calibration curves of ICP-AES

75. 10B. Arc and Spark AES Arc and Spark Excitation Sources:
Limited to semi-quantitative/qualitative analysis (arc flicker)
Usually performed on solids
Largely displaced by plasma-AES
Electric current flowing between two C electrodes

76. Carbon electrodes
Sample pressed into electrode or mixed with Cu powder and pressed - Briquetting (pelleting)
Cyanogen bands (CN) nm occur with C electrodes in air -He, Ar atmosphere
Arc/spark unstable
each line measured >20 s
needs multichannel detection

77. Arc and Spark spectrograph

78. spectrograph Beginning 1930s photographic film Cheap
Long integration times
Difficult to develop/analyze
Non-linearity of line "darkness“
Gamma function
Plate calibration

79. Multichannel photoelectric spectrometer
multichannel PMT instruments
for rapid determinations (<20 lines) but not versatile
For routine analysis of solids
metals, alloys, ores, rocks, soils
portable instruments
Multichannel charge transfer devices
Recently on the market
Orignally developed for plasma sources

80. Comparison Between Atomic Absorption and Emission Spectroscopy
- Measure trace metal concentrations in complex matrices .
- Atomic absorption depends upon the number of ground state
atoms .
Emission - Measure trace metal concentrations in complex matrices . - Atomic emission depends upon the number of excited atoms .

81. - It measures the radiation absorbed by the ground state atoms.
- Presence of a light source ( HCL ) .
- The temperature in the atomizer is adjusted to atomize the analyte atoms in the ground state only.
- It measures the radiation emitted by the excited atoms .
- Absence of the light source .
- The temperature in the atomizer is big enough to atomize the analyte atoms and excite them to a higher energy level.