Absorption and emission spectroscopy

AAS notes which includes introduction, principle, instrumentation and applications.

1. ATOMIC ABSORPTIOTION SPECTROSCOPY
 Atomic absorption spectroscopy is a method of elemental analysis.
 It is a powerful instrumental technique for the quantitative
determination of trace metals in liquids and is almost independent of
molecular form of the metal in sample.
 This method provides a total metal content of the sample.
 This method is very sensitive and can detect different metals in
concentrations as low as and frequently lower than 1ppm.
 Eg: determination of cadmium content of a water sample- it does not
consider whether cadmium exists as a chloride, nitrate, sulphate, etc.
PRINCIPLE:
 The absorption of energy by ground state atoms in the gaseous state
forms the atomic absorption spectroscopy.
 It is based on the principle that free gaseous atoms generated in
atomizer can absorb radiations at specific frequency.
 i.e, when a solution containing metallic species is introduced into a flame,
the vapour of metallic species will be obtained.
 Some of the atoms get excited and returned to the ground state with the
emission of radiation.
 But a large percentage of the metal atoms will remain in the non-emitting
ground state.
 These ground state atoms of a particular element absorb light radiation of
their own specific wavelength (in general, the same wavelength as they
would emit if excited).
 Thus, when a light of this wavelength is allowed to pass through a flame
having atoms of metallic species, part of that light will be absorbed.



 Once the value of absorption is known, the concentration of the metallic
element can be known.
 Mathematically , the total amount of light absorbed is give by the
expression as follows:




Where, e = electronic chare on the electron of mass m
ABSORPTION α density of the atoms in the flame
At ν the total amount of light absorbed = (πe
2
/mc) Nf

2. C = speed of the light
N = the total number of atoms that can absorb at frequency ν in the light path
f = the oscillator strength or ability for each atom to absorb at frequency, ν
 The extent to which radiation of a particular frequency is absorbed by
and atomic vapour is related to the length of the path transverse and to
the concentration absorbing atoms in the vapour.
 This is analogues to the Beer-Lamberts law relating to samples in
solution.
 Thus, for a collimated monochromatic beam of radiation of incident I0
passing through an atomic vapour of thickness t.
Log I0/It = A = εcl = -kt
Where, It = intensity of the transmitted radiation
K = absorption coefficient.
INSTRUMENTATION:



For all the type of atomic absorption spectrometer, the following components
are required:
1. Radiation Source:
The radiation source for atomic absorption spectrophotometer should
emit stable, intense radiation of the element to be determined, usually a
resonance line of the element.
Sharp line radiation source (hollow cathode lamp):
 Hollow cathode lamps are most commonly used radiation containing
tungsten anode and hollow cylindrical cathode made of element to be
determined.
 These are sealed with gas tube filled with an inert gas.

3.  When current flows between the anode and cathode in these lamps, metal
atoms are sputtered from the cathode cup, and collisions occur with the
filler gas.
 A number of metal atoms become excited and give off their
characteristics radiation.
Electrodeless Discharge Lamp:
 It is difficult to make stable hollow cathodes from certain elements,
particularly those that are volatile, such as arsenic, germanium or
selenium.
 The alternative light source is electrodeless discharge lamp (EDL).
 Where desired metals are get excited using argon gas in a sealed tube.


2. Atomisers:
 In order to achieve absorption of atoms, it becomes necessary to
reduce the sample to the atomic state. This is done by,
a) Flame atomisers
b) Non-flame atomiser
 Where the liquid samples are convert into gaseous state using
flame.
 Atomization is carried out by separation of particles into
individual molecules and breaking molecules into atoms.
 This is done by exposing analyte to high temperature in flame or
graphite furnace.
 Eg: carbon atomizers, L’ov Platform.
A solution Nebulizer and burner:
 Before the liquid sample enters the burner, it is first of all converted
into small droplets.
 This method of formation of small droplets from the liquid sample is
called nebulisation.
 Nebulizers suck up liquid samples at controlled rate and create a fine
aerosol spare for introduction into flame.
 Aerosol, fuel and oxidant are thoroughly mixed for introduction into
flame.
 In graphite furnace, samples are deposited in a small graphite coated
tube which can then be heated to vaporize and atomize the analyte.
 Graphite tubes are heated using high current power supply.

4. 3. Monochromator:
 They are used to select specific wavelength of light which is absorbed
by the sample, and to exclude other wavelengths.
 In AAS, the most common monochromators are prisms and gratings.
 When the cathode in the hollow cathode lamp is made up of
Transition metals, the emission spectrum from the hollow cathode is
so complicated that high dispersion is essential.
4. Detectors:
 For AAS, the photomultiplier tube is most suitable.
 PMT convert the light signal into electrical signal proportional to the
light intensity.
5. Amplifier:
 The electric current from the photomultiplier detector is fed to the
amplifier which amplifies the electric current many times.
 Generally, “Lock-in” amplifiers are preferred which provide a very
narrow frequency band pass and help to achieve an excellent signal-to-
noise ratio.
6. Recorder:
 In most of the AAS, chart recorders are used as read-out devices.
 A chart recorder is a potentiometer using a servomotor to move the
recording pen.
 The displacement is directly proportional to the input voltage.
APPLICATIONS:
AAS finds valid applications in every branch of chemical analysis.
1) Qualitative Analysis:
 In AAS, a different hollow cathode lamp is to be used for each
element to be tested.
 As qualitative analysis involves the checking of one element at a
time, it means that the process is very laborious.
2) Quantitative Analysis:
 The technique of quantitative analysis is based on the determination of
the amount of radiation absorbed by sample.
 If the value of radiation absorbed is substituted in equation, the value
of N, the number of absorbing atoms in the light path, can be
determined.
3) Determination of Metallic Elements in Biological Materials:
 AAS is becoming a very important tool for the determination of trace
metals in biological materials.

5. 4) Determination of Metallic Elements in Food Industry:
 Copper, zinc and nickel are the most common toxic elements of
interest to food analyst.
 For solid foodstuffs the most common procedure is to extract the trace
metals by digestion with the dilute sulphuric acid or with nitric acid.
5) Determination of calcium, magnesium, sodium and potassium in
Blood Serum:
 The determination of these elements in blood serum plays a vital
importance in diagnosing diabetes and primary aldosterionism.
 In the presence of lanthanum chloride, calcium and magnesium can be
estimated
 The determination of sodium and potassium in blood serum can also
measured by AAS by operating it in the emission mode.

6) Determination of Lead in Petrol:
 In petrol the two antiknocking additives are tetraethyl and tetramethyl
lead
 There are two methods are available they are,
a) Direct method
b) Indirect method
 Where it is determined by the destruction of tetraethyl and tetramethyl
lead followed by extraction into aqueous phase.
Advantages:
 The AAS technique is specific because the atoms of a particular
element can only absorb radiation of their own characteristic
wavelength.
 Small sample size is required
 Very little or no sample preparation is needed
 Sensitivity is enhanced
 Direct analysis of solid samples
Disadvantages:
 Analyte may be lost at the ashing stage
 The sample may not be completely atomized.
 Analytical range is relatively low.
 In aqueous solutions, the predominant anion affects the signal to a
negotiable degree.
 Sample must be in solution or at least volatile.
 Individual source lamps required for each element.

6. FLAME EMISSION SPECTROSCOPY
 Flame emission spectroscopy (FES) is a method of chemical
analysis that uses intensity of light emitted from flame, arc or spark
at particular wavelength to determine quantity of element in
sample.
 Flame photometry is based on the measurement of intensity of the
light emitted when a metal is introduced into a flame.
 In FES,
 Wavelength of spectral lines give identity of elements
 Intensity of emitted light is directly proportional to the
number of atoms present.
 Flame photometry is also named as flame emission spectroscopy
because of the use of a flame to provide the energy of excitation to
atoms introduced into the flame.
 In addition to the determination of metals, it can be applied to non-
metal analysis by utilizing the infrared region of the spectrum
 Flame photometry is a sample, rapid method for the routine
determination of elements that can be easily excited.
PRINCIPLE:

 Simplified version of the events in FES are given below:
 The solvent is vaporised, leaving particles of the solid salt.
 The salt is vaporised or converted into the gaseous state.
 The part or all of the gaseous molecules are progressively dissociated
to give free neutral atoms or radicals.
 These neutral toms are excited by the thermal energy of the flame.
 The excited atoms, which are unstable, quickly emit photons and
return to lower energy state, eventually reaching the unexcited state.
 The measurement of emitted photons, i.e., radiation, forms the basis of
flame photometry.
The following equations are applicable to the flame emission spectroscopy.
a) Bohrs Equation
b) Boltzmann Equation
Bohr’s equation:
 If we consider two quantized energy levels like higher energy as E2
and lower energy as E1.
 The radiation given out during the transition from E2 to E1 may be
expressed by the following equation.

7. Where, h = planks constant
ν = frequency of emitted light
Where ν = c / λ where, c = velocity of light
λ = wavelength of the absorbed radiation



 Wave length of the emitted radiation which is characteristic of the
atoms of the particular elements from which it was initially emitted
Wave length of radiation given out from a flame is indicative of the
elements that might be present in that flame.
Boltzmann equation:
 The fraction of the atoms which are excited thermally in other words
the relationship between the ground state and the excited state
quantum is exclusively represented by the Boltzmann equation.




Where,
N1 = number of atoms in the excited state
N0 = number of ground state atoms
g 1 / g0 = ratio of statistical weights for ground and excited states
E = energy of excitation (=hν)
K = Boltzmann constant
T = temperature (in Kelvin)
 Fraction of atoms excited solely depends upon the temperature of the
flame (T).
 Ratio N1 / N0 is dependent upon the excitation energy (∆E).
 Therefore the fraction of atoms excited critically depends on the
temperature of the flame there by emphazing the vital importance of
controlling the temperature in fame emission spectroscopy.




E2 –E1 = hν
E2 – E1 = hc/λ or λ = hc / E2 –E1
N1 / N0 = (g1 / g0) e
-∆E / KT

8. INSTRUMENTA TION:



 Flame photometers are the simplest type of atomic spectrometers.
 Solution is introduced into a fine spray.
 Solvent evaporates and leaving dehydrated salt.
 Certain fraction of atoms absorbs energy and is raised to excited state.
 These excited atoms on returning to ground state emit photons of certain
wavelength.
 Flame emission passes through monochromator which filters all emitted
light expect the wavelength of our interest.
 Photoelectric Detector measures the intensity of filtered light.



i. Atomizer / Nebulizer:
 The process of conversion of sample to a fine mist of finely
divided droplets using a jet of compressed gas.
 Pneumatic nebulizers
 Electro thermal vaporizer
 Ultrasound nebulizer
ii. Burner:
 Burners re used to spray the sample solution into fine droplets
mix with fuel and oxidants. So on the homogenous flame of
stable intensity is obtained.
Types of burners:
 Total consumption burner
 Laminar flow burner
 Mecker burner
 Shielded burner

9.  Lundergraph burner
iii. Fuel and oxidant:
 Ideal combination of oxidant and fuel which gives the desired
temperature in the flame photometry.
iv. Monochromator:
 They disperse radiation coming from the flame and falling on it.
 Dispersed radiation goes to detector from exit slit.
 Filter has chosen wavelength range transparent to emission from
element of interest.
 Monochromator are more efficient than filters in converting a
polychromatic light to monochromatic light.
 It consists of the following.
a) Entrance slit (to get narrow source)
b) Collimator (to render light parallel)
c) Grating or prisms (to disperse radiation)
d) Collimator (to reform the images of entrance slit)
e) Exit slit (to fall on sample cell)
v. Detectors:
 When a radiation is passed through a sample cell, part of its being
absorbed by the sample solution and rest is being transmitted.
 This transmitted radiation falls on the detectors and the intensity of
absorbed radiation can be determined.
 Detectors convert the light signal into electrical signal which can be
read or recorded.
Types:
 Barrier layer cell or photo voltaic cell
 Photo tubes or photo emissive cell
 Photo multiplier tubes

 The signal is amplified using amplifier and displayed for readout of
fed to data station for printout.
APPLICATIONS:
 Determination of metal at trace level in solution.
 Determination of purity
 Presence of heavy metal in body fluids
 Pollution of water by metals
 Food stuffs
 Soft drinks and beer

10.  Analysis of geochemical exploration for mineral
 Soils, crude oils, petroleum products, plastics.
 Qualitative analysis:
 It is used to detect elements of groups I and II of periodic table.
 These elements re Na, K, Li, Mg, Ca, Ba.
 These elements can be detected visually by colour.
 Eg: sodium which produces yellow flame.
 But this method is not very reliable
 Some of the elements can be identify by peak matching with
standard spectra.
 Quantitative analysis:
 Concentration of the sample can be detected by this method.
 It is used for the rapid quantitative determination of the elements in
groups I and II of the periodic table.
 Concentration of calcium in serum
 Concentration of sodium, potassium, calcium, present in urine
 Assay of potassium chloride in syrup can be determined.
 The amount or concentration of the sample can be calculated by
any of the following four methods.
a) Direct comparison method
b) Calibration curve method
c) Standard addition method
d) Internal standard method
LIMITATIONS:
 Non radiating elements cannot be detected.
 The number of excited atoms in flame is very small.
 It needs perfect control of flame temperature.
 Interference by other elements is not easy to be eliminated.
 Heavy and transition metal, the number of absorption and emission lines
is enormous and the spectra are complex.
 It does not provide information about the molecular form of the metal
present in the original sample
 Only liquid samples may be used. Sometimes lengthy steps are necessary
to prepare liquid samples.
 It cannot be used for the determination of the inert gases.

11. In FES, Beers law is not obeyed In AAS beers law is obeyed
Over a wide range of
Concentration.