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CHM260 - AAS (i)
1. Atomic Spectroscopy based on
Flame Atomization;
Theory, Instrumentation and
Application
Nurul Auni binti Zainal Abidin
Faculty of Applied Science
UiTM Negeri Sembilan
2. ATOMIC SPECTROSCOPY
• Three techniques (methods) included in atomic
spectroscopy:
1) atomic absorption spectroscopy
2) atomic emisssion spectroscopy
3) atomic fluorescene spectroscopy
3. Atomic spectroscopy
• In order to perform atomic spectroscopy, atoms of
the analyte must first be formed, usually in the form
of an atomic vapor.
• Atomization – the process by which a sample is
converted to an atomic vapor.
• Atomizer – a device used to converted a sample to
an atomic vapor.
5. Atomization process
• Solution of the analyte is evaporated rapidly at an
elevated temperature to yield a finely divided solid.
• Further heating will break down into gaseous atoms.
6. Atomic Absorption Spectroscopy
• Uses absorption of light to measure the
concentration of gas-phase atoms.
• Since samples are usually liquids or solids, the
analyte atoms must be vapourised in a flame
(or graphite furnace).
6
9. Atomization process
• Solution of the analyte is evaporated rapidly at an
elevated temperature to yield a finely divided solid.
• Further heating will break down into gaseous atoms.
11. Flame Atomization
• Nebulization – Conversion of the
liquid sample to a fine spray.
• Desolvation - Solid atoms are
mixed with the gaseous fuel.
• Volatilization – Solid atoms are
converted to a vapor in the flame.
• There are three types of particles
that exist in the flame:
1) Atoms
2) Ions
3) Molecules
12. 1. Types of Flames
Fuel / Oxidant Temperature
H–C C-H acetylene / air 2100 – 2400 C (most common)
acetylene / N2O 2600 – 2800 C
acetylene / O2 3050 – 3150 C
• Selection of flame type depends on the volatilization of interest.
2. Flame Structure
• Interzonal region is the hottest part of the flame
and best for atomic absorption.
• Oxidation of the atoms occurs in the secondary
combustion zone where the atoms will form
molecular oxides and are dispersed into the
surroundings.
13. FUELS/OXIDANTS
• Low T flames : easily reduced elements
(Cu, Pb, Zn, Cd)
• High T flames: difficult to reduce elements
(e.g. alkaline earths).
• Fuels: natural gas, propane, butane, H2,
and acetylene;
• Oxidants- Air and O2 (low temperature
flames). N2O (high temperature flames).
• Flame characteristics:
• Sample enters flame, is vaporized, reduced
and eventually oxidized.
• Exact region of flame in which each of
these occurs depends upon:
• flow rate,
• drop size, and
• oxidizability of sample.
• Optimum position for flame for many
metals.
15. 3. Burners
• Two types of burners in flame spectroscopy
i. Turbulent flow (total consumption burner)
ii. Laminar flow (premix burner)
16. Turbulent Flow Burner
• Nebulizer & burner are combined into a single unit.
• Sample is drawn up the capillary & nebulized
• Sample flow rate: 1 to 3 mL/min
17. Turbulent Flow Burner
• Advantage:
– introduce relatively large & representative sample
into the flame.
• Disadvantages:
– A relatively short path length through flame
– Problems with clogging of the tip
– Burners noisy from electronic and auditory stand
point
18. Laminar Flow Burner
• Sample is nebulized by the flow of
oxidant past a capillary tip.
• Resulting aerosol then mixed with
fuel & flows past a series of baffles
that remove all but the finest
droplets.
• Majority of the sample collects in
the bottom of mixing chamber,
drained to a waste container.
• Aerosol, oxidant & fuel are burned
in a slotted burner that provided a
flame, 5 – 10 cm in length.
19. Laminar Flow Burner
• Advantages:
i. Provide quiet flame
ii. Longer path length, enhance sensitivity &
reproducibility
• Disadvantages:
i. Lower rate of sample introduction
ii. Possibility of selective evaporation of mixed
solvents in the mixing chamber, create analytical
uncertainties.
iii. Mixing chamber contains a potentially explosive
mixture that can flash back if the flow rates are
too low.
20. Atomic Absorption Spectroscopy
(based on flame)
• Currently the most widely used of all the atomic
methods.
• Simplicity, effectiveness, relatively low cost.
• Flame atomization best for reproducibility
(precision)(<1%)
21. Atomic Absorption Spectroscopy
• AAS is commonly used for metal analysis
• A solution of a metal compound is sprayed
into a flame and vaporises
• The metal atoms absorb light of a specific
frequency, and the amount of light absorbed
is a direct measure of the number of atoms of
the metal in the solution
21
22. Atomic Absorption Spectroscopy
(based on flame)
• The spectra result form the atomized sample absorbing
photons of radiation of the appropriate energy
(wavelength).
• Energy of radiation absorbed by a vaporized atom is
identical with that needed to bring about excitation to a
higher electronic state.
• Transitions: excitation of an electron from ground state
to a higher energy level E
1
E
24. Spectrophotometers
• Contain same basic components as an instrument
designed for absorption analysis of solution.
• Source sample container (flame or hot surface)
selector detector readout
system
27. Double-Beam Instrument
• Radiation from hollow cathode lamp is split into 2 beams
– One passes through the flame
– The other around the flame
• A half-silvered mirror returns both beams to a single path
then pass through the monochromator then detector.
• Note: monochromator placed between sample and
detector
– Eliminates most of the radiation originating from the
flame.
28. A. Line Sources
LINE SOURCE – is required because width of the
absorption line is very narrow
Examples of line source used in AAS:
Hollow cathode Electrodeless discharge
lamp (HCL) lamp (EDL)
done
29. 1. Hollow Cathode Lamp (HCL)
• Light source in AA instrument
• Light from this lamp exactly light required for the
analysis, even no monochromator is used.
• Atoms of metal tested are present within the lamp.
30. How does the Hollow Cathode Lamp works?
• When lamp on, atoms are supplied with energy
• Causes atoms elevate to excited states
• Upon returning to ground state, exactly the same
wavelength that are useful in the analysis are emitted
• Since the analyzed metal with exactly the same energy
levels that undergoes excitation
• Hollow cathode lamp MUST contain the element to be
determined
31. Mechanism of excitation process in Hollow
Cathode Lamp
• Lamp is a sealed glass envelope filled with argon (or
Neon) gas
• When lamp is ON, Ne atoms are ionized with electrons
drawn to anode (+ charge electrode)
• The Ne ions, Ne+ bombard the surface of the cathode (-
charge electrode)
• Metal atoms, M, in the cathode are elevated to the
excited state and are rejected from the surface as a
result of this bombardment
32. Mechanism of excitation process in Hollow
Cathode Lamp
• When atoms return to ground state, characteristic line
spectrum of that atom is emitted
• This light is directed at the flame, where unexcited
atoms of the same element absorb the radiation and
are themselves raised to the excited state
• Absorbance is measured, and related to the
concentration
34. 1. Electrodeless Discharge Lamp (EDL)
• Constructed of a metal or salt of interest sealed in a quartz tube
filled with a noble gas (Ne or Ar) at low pressure (1 – 5 torr).
• The noble gas is ionized and accelerated by a strong radio-
frequency (RF) or microwave field and in turn excite the metal or
salt of interest.
• Provide radiant intensities usually one to two orders of magnitude
greater than HCL.
35. Source Modulation
• Function:
Employed in AAS to distinguish between
the component of radiation arising from the
source and the component of arising from
the flame background
• Device: light chopper
36. Source Modulation
Light chopper
• A chopper or a modulated power supply is used to
modulate the source radiation that passes through the
atomizer (flame).
• The chopper rotates resulting in an alternating atomic
absorption and atomic emission signal.
• The signal from emission of radiation will be continuous
and can be substracted from the modulated AA signal.
37. Light chopper
• Eliminate the effects of radiation from the flame
• Light is “chopped” with a rotating half-mirror so that
detector sees alternating light intensities
• One moment, only light emitted by flame is read
since the light from the source is cut off
• Next moment, light from both the flame emission &
transmission of the source‟s light is measured since
the source‟s light is allowed to pass
• The elements of the detector is such that the
emission signal is substracted from the total signal &
this difference is what we measured.
38. Questions
1. In flame atomic absorption spectroscopy, briefly
describe the „atomization‟ process which the analyte
undergoes.
2. Why is source modulation employed in atomic
absorption spectroscopy? Name the device used for this
purpose.
3. Name the common line source used in AAS.
40. Spectral interference
• Arise when particulate matter from atomization
scatters the incident radiation from the source.
• Or when absorption or emission of an
interfering species either overlaps or lies so
close to the analyte absorption that resolution
by the monochromator becomes impossible.
41. Spectral interference (simple words)
• Is one in which the spectral line of the
elements being determined overlaps with a
spectral line (or band) from another element
present in the sample.
• The effect of the element will also be measured
& thus the results will be incorrect.
42. Spectral interference
The most common method of solving this problem
i) Tune the monochromator to a different
spectral line for the element of interest so that there is
no overlap.
ii) secondary lines (can be found in the literature)
43. Chemical interferences
• 2 common types of chemical interferences (that
reduce the concentration of free gaseous atoms)
(i) ionization
(ii) formation of molecular species
44. Chemical interference – Ionization
• Sample (liquid/solid) must be vaporized & atomized in a
high temperature source such as flame.
• This high temperature environment lead to ionization of
the analyte atoms.
• Analyte ionization can be supressed by adding a source
of electrons, which shifts the equilibrium of the analyte
from the ionic to the atomic form.
• Cesium and potassium are common ionization
supressors that are added to analyte solutions.
Analyte Analyte+ + e-
45. Chemical interference – Ionization
• Cesium and potassium are common ionization
suppressors that are added to analyte solutions
• These atoms (Cs & K) are easily ionized &
produce a high concentration of free electrons in
the flame.
46. Chemical interference – Refractory
Formation
• Some elements can form refractory compounds that are
not atomized in flames.
• E.g. in the presence of phosphates, which interferes
with calcium measurements due to formation of
refractory calcium phosphate.
• Formation of refractory compounds can be prevented or
reduced by adding a releasing agent.
47. Chemical interference – Refractory
Formation
• For calcium measurement:
– Adding lanthanum to the sample (and
standard) solutions binds the phosphates as
LaPO4.
– LaPO4 has a very high formation constant, Kf
and effectively ties up the phosphate
interferent.
48. QUANTITATIVE ANALYSIS BY ATOMIC
ABSORPTION SPECTROSCOPY
– Calibration Curve
– Standard Addition Method
Absorption behavior follows Beer‟s Law &
concentration of unknown are determined the same
way.
All atomic species have an absorptivity, a
Pathlength, b is width of the flame
A = abc
49. Calibration Curve Method
• A general method for determining the concentration of a
substance in an unknown sample by comparing the
unknown to a set of std sample of known concentration.
• Plot is linear over a significant concentration range (the
dynamic range).
• Analysis should never be based on the measurement of
a single standard with assumption that Beer‟s law is
being followed.
50. Standard Addition Method
• Extensively used in AAS
• Compensate for variation caused by physical & chemical
interferences in the analyte solution
• The most common way involves adding one or more
increments of a standard solution to sample aliquots of
the same size.
• This process is often called spiking the sample.
• Each solution is then diluted to a fixed volume before
measurement.
51. Example: Standard Addition Method
The chromium in an aqueous sample was determined by
pipetting 10.0 mL of the unknown into each of five 50.0 mL
volumetric flasks. Various volumes of a standard containing
12.2 ppm Cr were added to the flasks, following which the
solutions were diluted to the volume.
Volume of Volume of Absorbance
unknown (mL) standard (mL)
10.0 0.0 0.201
10.0 10.0 0.292
10.0 20.0 0.378
10.0 30.0 0.467
10.0 40.0 0.554
1. Plot the data.
2. Calculate the concentration of Cr in the sample.
52. Standard Addition
- if Beer‟s law is obeyed,
A= bVstdCstd + bVxCx
Vt Vt
= kVstdCstd + kVxCx
k is a constant equal to b
Vt
53. • Standard Addition
- Plot a graph: A vs Vstd (should yield a straight line)
A = mVstd + b slope; m = kCstd
intercept; b = kVxCx
Concentration of unknown:
Cx = bCstd
mVx
54. • Detection Limits (ng/mL)
Element AAS Flame Element AAS Flame
Al 30 Cu 2
As 100 Fe 5
Ca 1 Hg 500
Cd 1 Pb 10
Cr 3 Zn 2
Nanogram/mililiter = 10-3 g/mL = 10-3 ppm
55. Accuracy
– Relative error associated with a flame
absorption analysis is the order of 1% to 2%
– With special precautions, this figure can be
lowered to a few tenths of one percent.
56. Standard addition method
- used to overcome matrix effect
- involves adding one or more increments
of a standard solution to sample aliquots
of the same size.
- Each solution is diluted to a fixed volume
before measuring its absorbance.
58. How to produce standard addition
curve?
1. Add same quantity of unknown sample to a series of flasks.
2. Add varying amounts of standard (made in solvent) to each
flasks, e.g. 0, 5, 10, 15 mL).
3. Fill each flask to line, mix and measure.
59. Standard Addition
Methods
Single-point standard Multiple standard
addition method addition method
60. Standard addition
- if Beer‟s Law is obeyed,
A = bVstdCstd + bVxCx
Vt Vt
= kVstdCstd + kVxCx
k is a constant equal to b
Vt
61. Standard Addition
- Plot a graph: A vs Vstd
A = mVstd + b
where the slope m and intercept b are:
m = kCstd ; b = kVxCx
62. • Cx can be obtained from the ratio of these
two quantities: m and b
b = kVxCx
m kCstd
Cx = bCstd
mVx
63. Example:
• 10 ml aliquots of raw-water sample were pipetted into
50.0 ml volumetric flasks.
Then, 0.00, 5.00, 10.00, 15.00 and 20.00 ml
respectively of a standard solution containing 10 ppm
of Fe3+ were added to the flasks, followed by an excess
of aqueous potassium thiocyanate in order to produce
the red iron-thiocyanate complex. All the resultant
solutions were diluted to volume and the absorbance of
each solution was measured at the same.
64. The results obtained:
Vol. of std added Absorbance
(ml) (A)
0 0.215
5.00 0.424
10.00 0.625
15.00 0.836
20.00 1.040
Calculate the concentration of Fe3+ (in ppm) in
the raw-water sample
65. Absorbance vs Vol. of std added
1.2
1
0.8
Absorbance
b = 0.24
0.6
Slope, m = 0.0382
(Vstd)0 = -6.31 ml 0.4
0.2
0
-10 -5 0 5 10 15 20 25
Vol. of std
Note: From the graph, extrapolated value represents
the volume of reagent corresponding to zero
instrument response.
66. • The unknown concentration of the analyte
in the solution is then calculated:
Csample = -(Vstd)0Cstd
Vsample
Cx = bCstd
mVx
67. SELF-EXERCISE
The chromium in an aqueous sample was determined by pipetting
10.0 ml of the unknown into each of 50.0 mL volumetric flasks.
Various volumes of a standard containing 12.2 ppm Cr were added
to the flasks, following which the solutions were diluted to the mark.
Volume of Volume of Absorbance
unknown (mL) standard (mL)
10.0 0.0 0.201
10.0 10.0 0.292
10.0 20.0 0.378
10.0 30.0 0.467
10.0 40.0 0.554
i) Plot a suitable graph to determine the concentration
of Cr in the aqueous sample.
