3. Atomic absorption flame spectrophotometry (AAS)
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.
Atomic absorption is a very common technique for
detecting metals and metalloids in environmental samples like
aluminum, Cu, lead, Li, Mg, Zn etc.
4. Atomic absorption flame spectrophotometry
(AAS)
Basic principle: Atomic absorption in which the element
is not excited in the flame, but is merely dissociated from its
chemical bond & placed in an unexcited (ground) state.
Thus the ground state atoms capable of absorbing radiation in
the flame, resulting in net ↓ in intensity of the beam from the
lamp, The analyte concentration is determined from the
amount of absorption.
5. Atomic absorption flame spectrophotometry
(AAS)
Concentration measurements are usually determined from a
working curve after calibrating the instrument with standards
of known concentration.
Absorption bands- .001 - .01 nm.
Entire absorption spectrum of atoms – line spectrum.
7. Light source
→
→
→
→
Quartz window
Pyrex body
Anode
Cathode
The light source is usually a
hollow cathode lamp.
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 ).
Neon: iron & lead(reddish-
orange)
Argon: lithium(blue to purple
glow)
Each element has its own unique
lamp which must be used for that
analysis.
Hollow Cathode Lamp
8. 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 .
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 for absorbtion by ground
state atoms in the flame.
9. Burner
Elements to be analyzed needs to be in
atomic sate.
Nebulization :Sample converted to aerosol
Atomization: flame, electrothermal (graphite tube) atomizers
Flame: it is oldest and most commonly used atomizers in
AAS, principally the air-acetylene flame with a temperature of
about 2300 °C and the nitrous oxide (N2O)-acetylene flame
with a temperature of about 2700 °C.
10. Stages in flame
Desolvation (drying) – the solvent is evaporated and the dry
sample nano-particles remain;
Vaporization (transfer to the gaseous phase) – the solid
particles are converted into gaseous molecules;
Atomization – the molecules are dissociated into free atoms.
11. Sample is
vaporized
in the flame.
Aspirator
tube sucks the
sample into the
flame in the
sample
compartment.
Light beam
12. Types of burner
I. Total Consumption
burner
Mixing of gas and
sample within
flame.
Flame is hot
enough for
molecular
dissociations
needed for some
chemical systems
II. Premix long path burner/
Laminar flow burner
- Gases are mixed and
sample is atomized
before being burned.
13. Advantages of long path burner
Larger droplets go waste
and only the fine mist
enters the flame thus
produces less noisy signal
Path length through the
flame of the burner is
longer then the total
consumption burner –
greater absorption and
increases sensitivity of
measurement.
Flame is not as hot as that
of total consumption
burner - cant dissociate
certain metal complexes in
flame- Ca- phosphate
complexes.
Disadvantages of long path burner
14. Monochromator
The monochromater in AAS is placed between flame and
detector
Used to select the specific wavelength of light which is
absorbed by the sample, and to exclude other wavelengths.
To allow the single line in the spectrum of analyte.
To minimize the emission from the flame itself because
detector detects photons over a wide wavelength range.
15. 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 signal could be
displayed for readout, or
further fed into a data station
for printout by the requested
format.
16. 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
17. Determining concentration from Calibration
Curve
A 1.0 - absorbance measured
b 0.9 -
S 0.8 - .
o 0.7 - .
r 0.6 - .
b 0.5 - . .
a 0.4 - .
n 0.3 - . concentration calculated
c 0.2 -
e 0.1 -
10 20 30 40 50 60 70 80 90 100
Concentration ( mg/l )
18. AAS applications
The are many applications for atomic absorption:
- Clinical analysis (blood samples: whole blood, plasma, serum;
Ca, Mg, Li, Na, K, Fe)
- Environmental analysis : Monitoring our environment – e g
finding out the levels of various elements in rivers, seawater,
drinking water, air, and petrol.
- Mining: By using AAS the amount of metals such as gold in
rocks can be determined to see whether it is worth mining the
rocks to extract the gold .
19. Advantages of flame AAS Disadvantages of flame AAS
•Inexpensive
•Easy to use
•High precision
-Only solution can be used
-Large samples are needed 1-2ml
-Less sensitive than graphite furnaces
standards are not to achieve due
to
-flame instability
- Variation in composition &
temperature.
20. Flame emission Spectroscopy
Flame emission spectroscopy is also an analytical technique that is
used to measure the concentrations of elements in samples
Principle: atoms of some metals,
when given sufficient heat energy (hot flame) become
excited & reemit this energy at wavelengths characteristic of the
element.
The intensity of radiant energy of characteristic wavelength produced
by the atoms in the flame is directly proportional to the number of
atoms excited in the flame ,which in turn is directly proportional to
the concentration of the alkali metal in the sample
21. Flame emission Spectroscopy
The excited atoms decay back to lower levels by emitting light
. Emissions are passed through monochromators or filters
prior to detection by photomultiplier tubes.
Alkali metals are easy to excite by flame
Li- red emission
Na – yellow emission
K- red violet emission
Rubidium- red emission
Mg- blue emission
22. Flame emission Spectroscopy
The instrumentation of flame emission spectroscopy is the same
as that of atomic absorption, but without the presence of a
radiation source .
In flame emission the sample is atomized and the analyte atoms
are excited to higher energy levels, all in the atomizer
23. Flame emission Spectroscopy
The source of energy in Atomic Emission could be a flame
like the one used in atomic absorption, or an inductively
coupled plasma ( ICP ) .
The flame ( 1700 – 3150 oC ) is most useful for
elements with relatively low excitation energies like
sodium, potassium and calcium.
The ICP ( 6000 – 8000 oC) has a very high
temperature and is useful for elements of high
excitation energies.
24. Application of flame emission
spectroscopy
1.Electrons of alkali metals like sodium, potassium, lithium
become easily excited hence preferentially analyzed by flame
photometry.
2.Used in clinical laboratory to determine concentrations of
sodium and potassium in biological fluids like serum, urine
and sweat.
3.Serum lithium levels – therapeutic monitoring.
25. Comparison Between Atomic Absorption and
Emission Spectroscopy
- Measure trace metal
concentrations in
complex matrices .
- Atomic absorption
depends upon the
number of ground state
atoms
- Measure trace metal
concentrations in
complex matrices .
- Atomic emission
depends upon the
number of excited
atoms .
Absorption Emission
26. Comparison Between Atomic Absorption and
Emission Spectroscopy
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.
27. Flameless atomic absorption
Here flame is not used, but high
temperature achieved by carbon
rod.
temp -2700C
100 times more sensitive than flame
methods and are highly specific for
the element measured.
28. Flameless atomic absorption
Atomization techniques- electrothermal
1. Drying : Sample is dried on carbon support by removing
solvent.
2. Pyrolysis- majority of matrix constituents are removed
3. Atomization: the analyte element is released to the gaseous
phase
4. Cleaning- residues in the graphite tube removed by high
temperatures
29. Flameless atomic absorption
Advantages
Solution and solid samples
can be used
Efficient atomization
Greater sensitivity
Small sample size 5 – 50
microlitres
Provides reducing
environment for easily
oxidised elements
Disadvantages
Expensive
Low precision
High operator skill
30. Interferences in atomic absorption
1.Spectral interference
2. Non-Spectral interference
Non-specific interference
Specific interference
31. Spectral interferences
1. Absorption by other closely absorbing atomic & molecular
species in the test sample.
2. Scattering by non volatile particles or oxides
3. Background emission
Absorption and scattering by molecular species – more
problematic at lower atomizing temperatures.
32. Non specteral interference
Non specific
1. Affects nebulization – 1. altering viscosity
2. density
3. surface tension of analyte &
consequently the sample flow rate.
2. Certain contaminants also decreases atomization by
decreasing atomizer temperature.
33. Specific/chemical interferences
Anions – form compounds that’s not completely dissociated
(decreasing the no of ground state atoms )
Eg : phosphate interference by calcium phosphate complexes.
Interference eliminated – Lanthanum, strontium (cation)
34. Ionization interference
Atoms are ionized instead of being in ground state- not absorb
incident light – apparent decrease in analyte concentration.
This interference is minimized by operating flame at low
temp.
35. Emission /excitation interference
Atoms excited by the flame, emit photons of same wavelength
as of incident light measured – enhances the signal received –
translated as ↓ A – low concentration of analyte.
Minimized by using a chopper or pulsing the light to the
hollow cathode lamp.
36. Burner problems
Essential to have a steady flame.
Burner head should be clean.
In flameless AA carbon rod should be changed after a no of
firings.
