Atomic absorption spectroscopy by Manisha Soral

1. BY: MANISHASORAL
M.PHARMACY(PHAMACEUTICS) 1st YEAR
K.L.E. COLLEGE OF PHARMACY, BENGALURU
1

2. contents
• 1. HISTORY
• 2.INTRODUCTION
• 3.PRINCIPLE
• 4.INSTRUMENTATION
• 5.INTERFERENCES
• 6. ANDVANTAGES
• 7.DISADVANTAGES
• 8.APPLICATIONS
2

3. ATOMICABSORPTION
SPECTROPHOTOMETER
3

4. HISTORY
• This technique was introduced in
1955 by Alan Walsh in Australia.
• This technique was introduced
for analyze chemicals.
• The first commercial atomic absorption spectrometer was
introduced in 1959. 4

5. INTRODUCTION
• Atomic absorption spectroscopy measures the discrete
radiation absorbed when ground state atoms are excited to
higher energy levels by absorption of photons of energy.
• The radiant power of the absorbed radiation is related to
the absorption coefficient of the ground state atoms using
the beer lambert equation.
log10I0/I=ɛlc
• Spectroscopic determination of atomic species can only
be performed on a gaseous medium in which individual
atoms or elementary ions such as Fe+, Mg+ etc. are well
separated through one another.
• It is very common technique for detecting and measuring
concentration of metals in the sample. 5

6. It can analyze over 60 to 70 elements.
The elements detected by atomic
absorption spectroscopy are highlighted in
pink.
6

7. PRINCIPLE
• This technique basically based on this principle that free
atoms (gas) generated in atomizer can absorb radiation at
specific frequency.
• Atomic absorption quantifies the absorption of ground
state atoms in gaseous state.
• The atoms absorbs UV and Visible light and make
transitions to higher electronic levels.
• The analyte concentration is determined from the amount
of absorption. 7

8. INSTRUMENTATION
ATOMIC
ABSORPTION
SPECTROSCOPY
RADIATION
SOURCE
SAMPLE
CELL
FLAME
ATOMIZER
MONOCHR
-OMATOR
DETECTOR
RECORDER
8

9. Line diagram of atomic absorption spectrophotometer
9

10. • High-pressure electrical discharge were the first sources used
in atomic absorption spectroscopy.
• These sources consist of a sealed tube filled with a gas
containing two electrodes.
• A voltage is applied between two electrodes and at a given
voltage discharge is initiated.
• Electrons are accelerated by the potential difference between
the electrodes and collide with filler gas to produce excited
molecules, atoms and ions.
• At low gas pressures, the predominant output from these lamps
is atomic line spectra characteristics of filler gas, but as the
pressure is increased the spectral output is broadened and a
continuous spectra are produced. Hydrogen deuterium and
xenon are the most widely used gases.
10
RADIATION SOURCE
CONTINUUM SOURCES

11. 11
• Atomic absorption lines are remarkably narrow (0.002 to
0.005nm) and because electronic transition energies are unique
for each elements. On the other hand narrow line width create
a problem.
• As a result, the use of radiation that has been isolated from
continuum source by monochromator inevitably causes
instrument deviate from beer’s law.
• “Beer’s law states that absorbance is proportional to the
concentrations of attenuating species in the sample.”
• In addition if fraction of radiation absorbed from such beam is
small, the detector receives a signal that is less attenuated(that
is, P0 ‒› P) and sensitivity of the measurement is reduced.

12. HOLLOW CATHODE LAMP
12

13. • This is most widely used as radiation source in atomic
absorption spectroscopy.
• A hollow cathode lamp consists of an anode and a
cylindrical cathode sealed in a glass tube containing an
inert gas. Such as inert gas (usually argon or neon), at a
pressure of 1 to 5 torr.
• The anode is an electrode which is made up of tungsten
wire and which losses electron by ionization takes place.
• The cathode is either fabricated with analyte metal or
serves as a support for a coating of that metal and it
attracts electrons when ionization takes place. 13

14. • When a potential is applied about 300V between the two
electrodes, causes ionization of the inert gas and generation of
a current of 5 to 10 mA.
• As the argon/neon cations and electrons migrates to the two
electrodes, if the potential is sufficiently large the argon cations
strike the cathode with sufficient energy to dislodge some of
the metal atoms and thereby produce an electronic cloud this
process is called sputtering.
• A portion of the sputtered metal atoms are in excited states and
thus emit their characteristics radiation as they return to the
ground state. Eventually, the metal atoms diffuse back to the
glass walls of the tube and are redeposited.
• The lamp window is constructed of either quartz, silica, or
glass.
14

15. ELECTRODELESS DISCHARGE LAMP
• In addition to hollow cathode lamps, electrodeless-discharge
lamp are useful sources of atomic line spectra.
• A typical lamp is constructed from a sealed quartz tube
containing an inert gas, such as argon, and a small quantity of
analyte metal(or its salt). The lamp contains no electrode but
instead is energized by radiofrequency or microwave radiation.
• Ionization of the argon occurs to give ions that are accelerated
by the high frequency component of the field until they gain
sufficient energy to excite the atoms of the metal whose
spectrum is sought.
• Electrodeless-discharge lamps particularly useful for several
elements such as As, Se, and Te, for which hollow cathode
lamp intensities are low.
15

16. ELECTRODE-LESS DISCHARGE
LAMP
16

17. • SOURCE MODULATION:
• In atomic measurement, it is necessary to discriminate
between radiation from hollow cathode lamp or electrode
less- discharge lamp and radiation from the atomizer.
• Much of the radiation coming through the atomizer is
eliminated by the monochromator which is always
located between the atomizer and the detector.
• The effect of analyte emission is overcome by modulating
the output from the hollow cathode lamp so that intensity
fluctuates at constant frequency.
• Modulation can be accomplished by interposing a motor
driven circular chopper between the source and the flame. 17

18. SAMPLE CELL
• This is a beaker like container of the sample which is placed
below the atomizer preferably. A capillary tube drains the
sample to the tip of the burner through nebulizer.
18

19. FLAME ATOMIZER
• A flame atomizer contains a pneumatic nebulizer, which
converts the sample solution into a mist, or aerosols.
• In most atomizers, the high pressure gas is the oxidant,
and the aerosol containing oxidant is subsequently mixed
with fuel.
• The burners used in flame spectroscopy are most often
premixed, laminar flow burners.
19

20. • The aerosol flows into a spray chamber, where it
encounters with the series of baffles that removes all the
finest droplets. As a result, most of the sample collects in
the bottom of the spray chamber, where it is drained to a
waste container.
• Typical solution flow rates are 2 to 5 ml/min.
• The sample spray is mixed with fuel and oxidant gas in
spray chamber.
• The sample spray is mixed with fuel and oxidant gas in
the spray chamber. The aerosol, fuel, and oxidant are then
burned in a slotted burner, which provides a flame that is
usually 5-10 cm in length.
20

21. • Laminar flow burner provides a relatively quite flame and a long path
length. These properties tends to enhance sensitivity for atomic
absorption and reproducibility.
• PROPERTIES OF FLAME:
• When a nebulized sample is carried into a flame, desolvation of the
droplets occur in the primary combustion zone, which is located
just above the tip of the burner.
• The resulting finely divided solid particles are carried to a region in
the center of the flame called the inner cone. Here in the hottest part
of flame, the particles are vaporized and converted into a gaseous
atoms.
• Finally , the atoms, molecules and ions are carried to the outer edge,
or outer cone, where oxidation may occur before the atomization
products disperse into the atmosphere.
21

22. PROPERTIES OF FLAME
• When a nebulized sample is carried into a flame,
desolvation of droplets occurs in the primary combustion
zone, which is located just above the tip of the burner.
• Here, in this hottest part of the flame the particles are
vaporized and converted into gaseous atoms, elementary
ions, and molecular species.
22

23. Sample Mist
Dry
Aerosol
Gaseous
molecule
Free
Atoms
PROCESS OF FLAME
ATOMIZER
Nebulization Desolvation Volatilization Dissociation
23

24. FUELAND OXIDANT
• This is a very important part of the entire process to be
remembered.
• If the heat produced is not sufficient then the sample
doesn’t form neutral atoms.
• If the heat of burner is more, the sample molecules may
ionize instead of forming atoms.
• So, both are undesirable for experimentation. Hence a
proper combination of fuels and oxidant are to be used to
produce recommended temperatures. 24

25. FUELAND OXIDANTS USED
IN COMBUSTION
25

26. GRAPHITE FURNACE:
(ELETROTHERMALATOMIZER)
• One way to increase the sensitivity is to introduce discrete amounts of
sample. This can be achieved by using the conventional
nebulizer/expansion chamber arrangement, or by using a graphite
furnace atomizer.
• The graphite furnace or electro thermal atomizer replaces the flame
burner arrangement in the atomic absorption spectrometer.
• The principle of operation is that a small discrete sample (5-100µl) is
introduced on to the inner surface of a graphite tube through a small
opening.
• The graphite tube is arranged so that light from the hollow cathode lamp
can pass directly through the unit. Various forms of graphite are used,
including pyrolytic graphite(form by heating the graphite tube in
methane atmosphere.)
26

27. • Heating of graphite tube achieved by passage of an electric
current through the unit via water cooled contacts at each end
of the tube.
• Various stages of heating are required to dry the sample,
remove sample matrix and finally to atomize the analyte.
• The drying stage is necessary to remove any residual solvent
from the sample. This can be achieve by maintaining the heat
of the graphite tube above the boiling point of the solvent, e.g.
water at 110ºC for 30s.
• The second step concerns the destruction of sample matrix in
process called ashing; this involves heating the tube between
350 to 1200ºC for 45s.
• The temperature of graphite tube is raised to between 2000 to
3000ºC for 2-3s, allowing atomization of the analyte of
interest.
27

28. • Various problems have been identified in the use of graphite
furnace and various remedies suggested.
• The main problem is that the analyte atoms which are
volatilized from the tube wall come into contact with cooler
gas. This is because an electric current, i.e. the method of
heating, is directly applying to the tube.
28

29. MONOCHROMATOR:
• To perform quantitative analysis, monochromatic light need to
passed through the sample otherwise beer’s law would not hold
true.
• The main purpose of the Monochromator is to isolate a single
atomic resonance line (wavelength) from the lines emitted by
the hollow cathode lamp.
• It is an adjustable filter that selects a specific, narrow region
of the spectrum for transmission to the detector
• A monochromator comprises an entrance slit, a dispersion
device and an exit slit.
 The entrance slit selects a defined beam of (polychromatic)
light from the source.
 The dispersion device causes the different wavelengths of light
in the source beam to be dispersed at different angles.
 The exit slit enables selection of a particular wavelength to
produce the required monochromatic light.
29

30. • Dispersion devices can be prisms but diffraction gratings are
used in AAS as they are cheaper, easier to make and provide
superior performance.
• The diffraction grating is a block of glass with one surface
coated with highly reflective aluminium. The Al surface is
scored with the fine grooves spaced closely together.
• Usually 500-3000 grooves per mm are used. The grooves must
be straight, evenly spaced, parallel and of identical shape.
• Light striking these grooves is reflected and dispersed at
different angle according to its wavelength. By rotating the
grating, a constituent wavelength is focused on to the exit slit
via second mirror. 30

31. MONOCHROMATOR
31

32. DETECTOR
• The light selected by Monochromator is directed onto a
detector whose function is to convert the light signal into
an electrical signal proportional to the to the intensity.
• For atomic absorption spectroscopy, the photomultiplier
tube is most suitable detector.
• In the photomultiplier tube, there is an evacuated
envelope which contains a photocathode, a series of
electrodes called dynodes, and an anode.
• The processing of electrical signal is fulfilled by signal
amplifier. The signals could be displayed for readout.
32

33. PHOTOMULTIPLIER TUBE
• Photomultiplier are typically constructed with an evacuated
glass housing (using an extremely tight and durable glass to
metal seal like other vacuum tubes).
• Containing a photocathode, several dynodes and an anode.
33

34. working
• The detector contains a photo emissive cathode and a series of
dynodes.
• The number of electrons emitted from the cathode is directly
proportional to the intensity of the light beam.
• Electrons emitted from the cathode are accelerated to the first
dynode by 90V potential.
• Approximately 10-12 dynodes are arranged, each dynode is
biased so that it is 90V more positive than the previous dynode
• The number of electrons finally reaching the anode is in the
order of ten million for each incident photon.
• The current measured at the anode collector is still
proportional to the intensity of the light but it has been
amplified over a million times. 34

35. 35

36. RECORDER
• In most of the atomic absorption measurements, chart
recorders are used as read out devices.
• A chart recorder is a potentiometer using a servomotor to
move the recording pen.
• In some atomic absorption digital read out devices are
used.
36

37. RECORDER
CHARTRECORDER DIGITALRECORDER
37

38. SINGLE BEAM INSTRUMENTS
• A typical single beam instrument, comprises of hollow
cathode lamp as radiation source, a chopper or a pulsed
power supply, an atomizer and a simple grating
spectrophotometer with a photomultiplier transducer.
• HCL emits sharp atomic line of the element whose
determination is required. The light is modulated rapidly
by means of rotating chopper located between the light
source and flame.
• Modulation can also be achieved by pulsing the power to
the light source.
• The modulated light is led to the flame where ground
state atoms of the element of interest are present and after
absorption is led to the monochromator which isolates the
wavelength and led to the detector. 38

39. DOUBLE BEAM INSTRUMENTS
• The beam from the hollow cathode source is split by a
mirrored chopper , one half passing through the flame and
other half around it.
• The two beams are then recombined by a half silvered mirror
and passed into a Czerny-turner grating monochromator, a
photomultiplier tube serves as a transducer.
• The ratio between the reference and sample signal is then
amplified and fed to the readout, which may be digital meter or
a computer.
• The reference beam in atomic double beam instruments does
not pass through the flame and thus does not correct for loss of
radiant power due to absorption or scattering by the flame
itself. 39

40. 40

41. INTERFERENCE INATOMIC
ABSORPTION
• ANALYTE INTERFERENCE
• Analyte interference change the magnitude of the analyte
signal itself. Such interferences are usually not spectral in
nature but rather physical or chemical effects.
• PHYSICAL INERFERENCES
• Physical interferences can alter the aspiration, nebulization,
desolvation, and volatilization processes.
• Substances in the sample that change the solution viscosity.
For example, can alter the flow rate and efficiency of the
nebulization process
• Combustible constituents, such as organic solvents, can change
the atomizer temperature and thus affect the atomization
efficiency indirectly.
41

42. • CHEMICAL INTERFERENCE:
• They occur in the conversion of solid or molten particle
after desolvation into free atoms or elementary ions.
Constituents that influences the volatilization of analyte
particle causes this type of interferences and are often
called solute volatilization interferences.
• For example, can alter in some flames the presence of
phosphate in the sample can alter the atomic
concentration of calcium in the flame owing to the
formation of relatively nonvolatile complexes.
• Such effects can sometimes be eliminated or moderated
by the use of higher temperatures.
42

43. • SPECTRAL INTERFERENCES:
• Spectral interferences by elements that absorb at analyte
wavelength are rare in atomic absorption.
• Molecular constituents and radiation scattering can cause
interference, however.
• These are often corrected by background correction
scheme.
• In some case, if the source of interference is known, an
excess of the interferent can be added to both the sample
and the standards. The added substance is sometimes
called as radiation buffer 43

44. BACKGROUND CORRECTION
• Absorption by the flame atomizer or electro thermal
analyzer can cause serious problems in atomic absorption.
• Rarely are there interferences from absorption of the
analyte line by other atoms since the hollow cathode lines
are so narrow. Molecular species can absorbs the
radiation and cause radiation in atomic absorption
measurements.
• However, the total measured absorbance AT in atomic
absorption is the sum of analyte absorbance AA plus the
background absorbance AB.
• AT=AA+AB
• Background correction scheme attempt to measure AB in
addition to AT and to obtain the true analyte absorbance
by subtraction (AA=AT - AB)
44

45. CONTINUUM SOURCE BACKGROUND
CORRECTION
• A popular background correction scheme in commercial
atomic absorption spectrometers is the continuum lamp
technique.
• Here a deuterium lamp and the analyte hollow cathode
are directed through the atomizer at different times.
• The hollow cathode lamp measures the total absorbance
AT while the deuterium lamp provides an estimates of the
background absorbance AB. The computer system or
processing electronics calculates the difference and
reports the background corrected absorbance.
• This method has limitation for elements with lines in the
visible range because the D2 lamp intensity becomes
quite low in this region.
45

46. ZEEMAN EFFECT
BACKGROUND CORRECTION
• Background correction with electro thermal atomizers can be
done by means of the Zeeman effect.
• Here a magnetic field splits normally degenerated spectral
lines into components with different polarization
characteristics.
• Analyte and background absorption can be separated because
of their different magnetic and polarization behaviors.
46

47. 47

48. • The major limitations of flame AAS are as follows:
• The sample introduction system, e.g. nebulizer/expansion
chamber, which is used is inefficient and requires large
volume of aqueous sample.
• The residence time, i.e. the length of time that the atom is
present in the flame, is limited due to the high burning
velocity of the gases, thus leading to rather high detection
limits.
• An inability to analyze solid sample directly (solid
requires dissolution prior to analysis)
48

49. ADVANTAGES OFAAS
• It does not suffer from spectral interference, which occurs
in flame emission spectroscopy.
• It is independent on flame temperature.
• By atomic absorption technique , traces of one element
can easily be determined in presence of high
concentration of other elements.
• It has proved very successful in the analysis of bronze
and copper alloys and in the determination of metals like
platinum, gold etc.
49

50. DISADVANTAGES OFAAS
• This technique has not proved very successful for the
estimation of elements like W, Si, Mo, Ti, and Al etc.
because these elements give rise to oxides in the flame.
• In aqueous solution, the predominant anion affects the
signal to a negotiable degree.
• A separate lamp for each elements to be determined is
required. Attempts are being made to overcome this
difficulty by using continuous source with a very high
resolution Monochromator or alternatively to produce a
line source.
50

51. APPLICATION
• It is used in quantitative and qualitative analysis of
different drug compounds.
• Estimation of zinc in insulin preparation, oils, creams and
in calamine, calcium in number of calcium salts, lead in
calcium carbonate and also as impurity in number of
chemical salts have been reported.
• Atomic absorption spectroscopy is very widely used in
metallurgy, alloys and in inorganic analysis. Almost all
important metals have been analyzed by this method. 51

52. • It is especially useful to analyze ionic metal elements in
blood, saliva, urine samples like sodium, potassium,
magnesium, calcium and other body fluids.
• To detect heavy metals in herbal drugs and synthetic
drugs.
• To determine metal elements in the food industry.
• To estimate Lead in petroleum products.
52

53. REFERENCES
• 1.“FUNDAMENTAL OF ANALYTICAL CHEMISTRY”
Douglas A. skoog; Donald M. West; eighth edition.
• 2. “INSTRUMENTAL METHOD OF CHEMICAL
ANALYSIS” Gurdeep R. chatwal; Sham k. anand; Himalaya
publishing house.
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