Atomic Absorption

1. Atomic Absorption
Spectrophotometer (AAS)

2. Overview of AA
spectrometer.
Light Source Detector
Sample
Compartment
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5. Introduction
• AAS is a technique used for determining the concentration of a
particular metal element within a sample.
• AAS is used to analyse the concentration of over 62 different
metals in a solution.
• AAS is a method of analysis based on absorption of radiation by
atoms when a solution of metallic salt is aspirated (drawing)
into a flame.
• Only a drop of sample needed
• The metals need not be removed from other components.
• Sensitive in the ppm range
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6. Principles of AAS
• AAS method is similar to that of spectrophotometer.
• The only exception is the replacement of the sample cell by a
flame.
• In AAS, a monochromatic light for a particular element is
produced by a hollow cathode lamp utilizing that element as the
cathode.
• The monochromatic light produced by the lamp is beamed
through a long flame into which is aspirated the solution to be
analysed.
• The heat energy dissociates the molecules and converts the
components to atoms.
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7. • At flame temperature, some atoms in the solution are activated,
but most of the atoms are remain in the ground state.
• The ground state atoms of the same element as in the hollow
cathode cup absorb their own resonance (reflected) lines.
• The amount of light absorbed varies directly with their
concentration in the flame.
• The transmitted light that is not absorbed reaches the
monochromator.
• The monochromator passes only the wavelengths close to the
resonance lines of the particular element to be analysed.
• Then the transmitted light strikes a detector and the decrease in
transmitted light is measured.
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8. Components of AAS
• The components of an AAS are
1. Hollow cathode lamp
2. Beam chopper
3. The flame or furnace
4. Nebulizer
5. Monochromator
6. Detector
7. Amplifier.
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10. 1. Hollow Cathode Lamp
• The most widely used light source is the hollow cathode lamp.
• These lamps are designed to emit the atomic spectrum of a
particular element, and specific lamps are selected for use
depending on the element to be determined.
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11.  The cathode of the lamp is a hollow-out cylinder of
the metal whose spectrum is to be produced.
 Each analyzed element requires a different lamp.
 The anode and cathode are sealed in a glass
cylinder normally filled with either neon or argon
at low temperature.
 At the end of the glass cylinder is window
transparent to the emitted radiation.
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12. • The cathode lamps are stored
in a compartment inside the
AA spectrometer.
• The specific lamp needed for a
given metal analysis is rotated
into position for a specific
experiment.
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14. Sputtering
• When an electrical potential is
applied between the anode and
cathode, some of the fill gas atoms
are ionized.
• The positively charged fill gas ions
accelerate through the electrical
filed (gather in a line) to collide
with the negatively charged
cathode and dislodge individual
metal atoms in a process called
sputtering.
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15. Excitation and Emission
• Then the sputtered metal atoms are
elevated to an excited state.
• When the atoms return to the ground
state, the characteristic line spectrum
(light) of that atom is emitted.
• Then the emitted light is directed at the
flame where unexcited atoms of the
same element absorb the radiation and
are themselves raised to the excited
state.
• Then the absorbance is measured.
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17. 2. Beam chopper
• In AAS, the hollow cathode lamp and flame are light emitting
source.
• The phototube responds to radiation from the hollow cathode
lamp as well as flame.
• This will create interference in absorption measurement.
• This problem is corrected by a beam chopper.
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18. • Beam chopper is a motor
driven device that has open
and solid (or mirrors in some
cases) alternating regions.
• One half of their rotation, i.e.,
open region, permits the
beam obtained from lamp to
pass through.
• During the other half of their
rotation, i.e., mirror region,
the beam is reflected and not
allowed to pass through.
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19. Function of the Chopper
 When the incident beam hits the solid surface, the beam is blocked and the
detector is only read the emitted signal from the flame.
 As the chopper rotates to the open surface and the beam emerges to the
detector.
 Now the detector signal is the sum of the transmitted light signal plus that
emitted from the flame.
 The signal processor is able to subtract the first signal from the second-one,
thus excluding the signal from the emission in flames.
 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 flame is called source
modulation.
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20. 3. The flame / Burner system
 The third unique component in the system is the burner,
through which the sample is introduced. The burner consists of
three parts, namely,
 nebulizer,
 premix chamber and
 burner head.
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21. Nebulizer
 Suck up liquid sample at a
controlled rate
 Create a fine aerosol for
introduction into the flame
 Mix the aerosol and fuel
and oxidant thoroughly
for introduction into the
flame.
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25. • Sample solution is aspirated
through a nebulizer and
sprayed as a fine aerosol into
the mixing chamber.
• The design of the nebulizer is
to limit the size of the
atomized sample introduced
to the flame to a very small
size (~ 5 – 10nm).
• Droplets larger than this are
stopped by spoilers and end
up flowing to waste.
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26. Premix Chamber
• The nebulizer chamber / premix
chamber thoroughly mixes
acetylene (the fuel) and oxidant (air
or nitrous oxide) and by doing so,
creates a negative pressure at the
end of the small diameter, plastic
nebulizer tube.
• This negative pressure acts to suck
(uptake) liquid samples up the tube
and into the nebulizer chamber, a
process called aspiration.
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27. • A small glass impact bead and / or a fixed impeller inside the
chamber create a heterogeneous mixer of gas (fuel + oxidant)
and suspended aerosol (finely dispersed sample).
• This mixture flows immediately into the burner head where it
burns a smooth, laminar flame evenly distributed along a
narrow slot in the well-machined metal burner.
• Liquid sample not flowing into the flame collects on the bottom
of the nebulizer chamber and flows by gravity through a waste
tube to a glass waste container.
• For some elements that form refractory oxides (molecules hard
to break down in the flame) nitrous oxide (N2O) needs to be
used instead of air (78% N2 + 21% O2) for the oxidant.
• In that case, a slightly different burner head with a shorter
burner slot length is used.
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28. Laminar burner
• The mixture of gases and sample is
directed into the burner head and
flame.
• The burner is a specially one for
AAS.
• It has a long, flat topped head
positioned directly below and
parallel to the beam of light from
the lamp.
• The gases flow through a 10cm
long slot at the top of the burner
head so that a long, thin curtain of
flame is produced.
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29.  The two holes, left and right,
are where the light beam enters
and leaves after passing
through the flame.
 The dark place at the top is a
stain from the heat of the flame.
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30.  The light from the hollow cathode lamp
passes through the full 10cm length of
the flame, greatly enhancing the
absorption of light by the ground state
atoms in the flame.
 The narrowness of the slot also
concentrates the atoms and results in
greater efficiency of light absorption.
• Burner head is made up of solid
titanium which is corrosion resistant
and free of most of the elements
commonly determined by absorption.
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31. Job of the flame
• Destroy any analyte ions and
breakdown complexes.
• Create atoms (the elemental
form) of the element of
interest. FeO, CuO, ZnO etc.
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33. 3. Monochromotors
• Monochromotors are the devices that can selectively provide
radiation of desired wavelength out of the range of wavelengths
emitted by the source (lamp) or emitted by the analyte sample.
• In AAS, the monochromotor select a given emission line and
isolate it from other lines.
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34. • Light from the source (i.e., flame)
enters the monochromotor at the
entrance slit and is directed to the
grating where dispersion takes
place.
• The diverging wavelength of the
light are directed toward the exit
slit.
• By adjusting the angle of the
grating, a selected emission line
from the source can be allowed to
pass through the exit slit and fall
onto the detector.
• All other lines are blocked from
exiting.
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36. Detectors (Photomultiplier Tubes)
• In photomultiplier tubes
numerous dynodes are aligned in
a circular or in a linear manner.
• Here most of the electrodes act as
both an anode and a cathode
with each dynode (electrode pair)
having a potential difference of
+90 V; thus, the potential
increases by 90 V as an electron
goes from one electrode to the
next.
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37. • When nine dynodes are used, a common feature in
PMTs, the net result is a yield of 106 to 107
electrons from a single emitted photon.
• This causes considerable amplification of a weak
signal compared to a photon tube (in old
instruments) that does not amplify the signal.
• PMTs were excellent detectors for FAAS and FAES
measurements due to the low intensity of radiation
in these systems and have dominated these
instruments for decades.
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38. Data Processing
• Data collection has greatly advanced with the aid of computer
technology that has replaced the strip charts.
• The resulting data can be presented in a variety of ways, but
typically a print out is made.
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39. Determine the concentration of a
solution from a calibration curve.
• AA can be used to identify the presence of an element
(qualitative analysis), or the concentration of a metal (quantitative
analysis)
• Quantitative analysis can be achieved by measuring the
absorbance of a series of solutions of known concentration.
• A calibration curve and the equation for the line can be used to
determine an unknown concentration based on its absorbance.
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41. Analytical procedure
• The sample to be analysed is prepared in the form of solution.
• Plant and animal tissues are first ashed by wet or dry ashing
techniques.
• Then a solution of ash is prepared in hydrochloric acid.
• This solution is finally diluted with water to appropriate
concentration.
• The hollow cathode lamp for the required element is put on for
30min for warming up.
• The correct monochromator wavelength is selected and the slit
width is adjusted to isolate the line to be measured.
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42. • The read is adjusted to zero transmission when the light is cut
off from the detector.
• When blank solution is aspirated, the read out is adjusted to 100
% transmission.
• The standards and samples are aspirated and the readings are
taken.
• A plot of concentration against absorbance gives almost a
straight line.
• The amount of metals present in the given sample is calculated
from the calibration graph.
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43. The Calcium Flame
The calcium
flame is red.
This is
intensely red
because the
calcium
content is
high.
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44. The Copper Flame
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45. The Potassium Flame
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46. The Manganese Flame
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47. The Cobalt Flame
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48. Applications of AAS
• Water analysis (Ca, Mg, Fe, Si, Al, Ba content).
• Food analysis.
• Analysis of animal feedstuffs (Mn, Fe, Cu, Cr, Se, Zn).
• Analysis of additives in lubricating oils and greases (Ba, Ca, Na,
Li, Zn and Mg).
• Analysis of soil.
• Clinical analysis (blood samples, plasma serum – Ca, Mg, Li, Na,
K and Fe).
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