Atomic absorption spectrophotometry (AAS) is a technique used to determine the concentration of metals in solutions. It works by vaporizing the sample into atoms and measuring how much light of a specific wavelength is absorbed. The key components are a hollow cathode lamp, burner to create a flame, monochromator to select the wavelength, and detector. Standards of known concentration are used to generate a calibration curve to determine unknown sample concentrations from their absorbance readings. AAS can be used to analyze over 60 metal elements present in parts per million concentrations in various materials like water, foods, soils, and biological samples.

1. Atomic Absorption
Spectrophotometer (AAS)

2. Overview of AA
spectrometer.
Light Source Detector
Sample
Compartment
8/29/2023 2

3. 8/29/2023 3

4. 8/29/2023 4

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
8/29/2023 5

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.
8/29/2023 6

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.
8/29/2023 7

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.
8/29/2023 8

9. 8/29/2023 9

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.
8/29/2023 10

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.
8/29/2023 11

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.
8/29/2023 12

13. 8/29/2023 13

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.
8/29/2023 14

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.
8/29/2023 15

16. 8/29/2023 16

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.
8/29/2023 17

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.
8/29/2023 18

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.
8/29/2023 19

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.
8/29/2023 20

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.
8/29/2023 21

22. 8/29/2023 22

23. 8/29/2023 23

24. 8/29/2023 24

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.
8/29/2023 25

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.
8/29/2023 26

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.
8/29/2023 27

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.
8/29/2023 28

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.
8/29/2023 29

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.
8/29/2023 30

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.
8/29/2023 31

32. 8/29/2023 32

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.
8/29/2023 33

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.
8/29/2023 34

35. 8/29/2023 35

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.
8/29/2023 36

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.
8/29/2023 37

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.
8/29/2023 38

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.
8/29/2023 39

40. 8/29/2023 40

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.
8/29/2023 41

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.
8/29/2023 42

43. The Calcium Flame
The calcium
flame is red.
This is
intensely red
because the
calcium
content is
high.
8/29/2023 43

44. The Copper Flame
8/29/2023 44

45. The Potassium Flame
8/29/2023 45

46. The Manganese Flame
8/29/2023 46

47. The Cobalt Flame
8/29/2023 47

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).
8/29/2023 48

49. 8/29/2023 49

50. 8/29/2023 50

51. 8/29/2023 51

52. 8/29/2023 52