ATOMIC ABSORPTION SPECTROSCOPY

Chetan Sharma m.Pharm (Analysis) I.s.f.c.p. Moga

Need of Atomic Absorption SpectroscopyGenerally flame photometry is used for detection of metallic elements but limited to only alkali metals and alkaline earth metals, because they require less energy for excitation.Other transition metals require high energy for their excitation which is not provided by the flame temperature. So a specific lamp of element to be determined is needed to provide a light of particular wavelength to cause of excitation of transition metallic elements.Ex-Na+ lamp, K+ lamp, Co lamp, Cr lamp etc….

PRINCIPLE Atomic absorption spectroscopy is a

quantitative method of analysis that is applicable to many metals and a few nonmetals.

It is the absorption phenomenon. When a sample solution is aspirated into the

flame, the solvent is vaporised leaving particles of the solid salt.

The salt burns and converted into the gasses form which is dissociated into free neutral atoms and some of them excited by the flame heat. Remaining atoms in the ground state absorbs light of specific wavelength emitted by the lamp of same element to be determined and the intensity of the light absorption is measured.

BOLTZMANN DISTRUBUTION EQUATION

N*/No = e - ∆E/kT

N* – number of excited atomsN

o – number of unexcited atoms

∆E – difference in energies of two levelsK – Boltzmann constantT – temperature of the flame

INSTRUMENTATION Radiation source Chopper Atomiser Monochromators Detector Amplifier Read out device

CONT….

Radiation source

1. Hollow Cathode Lamp

When a potential (300-500 V) is applied between two electrodes, The electric discharge ionizes the inert gas (Ne or Ar usually) atoms filled in the hollow tube, which in turn, are accelerated into the cathode with high velocity and sputter metal atoms into the gas phase. Metal atoms become excited and after returning to ground state give rise to metal emission spectrum.

2. Electrodeless Discharge Lamp Used for volatile elements Consist of evacuated tube in which metal of

interest is placed. Tube filled with argon gas at low pressure, sealed

and placed in a microwave discharge cavity & cause excitation of metal sealed inside the tube.

CHOPPER A rotating wheel is interposed between the hollow cathode lamp and flame .This rotating wheel is known as chopper.It is interposed to break the steady light coming from the lamp into pulsating light which is used to measure the intensity of light absorbed by elements without interference by radiation from the flame itself.

ATOMISERAtomiser converts the liquid into small droplets which are easily vaporised.Types of atomisers :- 1. Flame atomiser:- a.) Total consumption burner b.) premixed burner 2. Non-flame atomiser

Flame atomisera). Total consumption burner:- In this whole sample is atomised into the flame, hence named as total consumption burner. Fuel used – H2

Oxidant – O2

Disadvantages noisy and hard to use.

b). Premixed burner :- It is most widely used because of uniformity in flame intensity. In this the sample solution ,fuel and oxidant are mixed before they reach the burner tip.Advantages Non-turbulent Noiseless StableDisadvantagesOnly 5% sample reaches to the flame and rest 95% is wasted.

Temperature of some flames

Fuel oxidantTemperature (K)

H2 Air 2000-2100

C2H2 Air 2100-2400

H2 O2 2600-2700

C2H2 N2O 2600-2800For 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.

Non flame atomiserThe graphite furnace is an electro thermal atomiser system that can produce temperatures as high as 3,000°C. The heated graphite furnace provides the thermal energy to break chemical bonds within the sample held in a graphite tube, and produce free ground state atoms.The ground-state atoms are capable of absorbing energy, in the form of light, and are elevated to an excited state. The amount of light energy absorbed increases as the concentration of the selected element increases.

MONOCHROMATORMonochromators are used to disperse the radiation according to the wavelength.They are of two types:1) Prism 2) Grating 1). Prism monochromator :- Quartz material is used for making prism, as quartz is transparent over entire region.

2). Grating monochromator :- it consists of a series of parallel straight lines cut into a plane surface.

Diffraction by grating follows a law of diffraction:- n = d(sin I ± sin ø) where n – order λ – wavelength d – distance between grooves I – angle of incidence ø – angle of dispersion

DETECTORSThe role of the detector is to convert a light signal into an electrical signal.Generally in most of the instruments PMT is usedThe detector contains a photoemissive cathode and a series of dynodes. The number of electrons emitted from the cathode is directly proportional to the intensity of the light beam. Electrons emitted from the cathode are accelerated to the first dynode by a 90 volt potential where the electron impact dislodges several additional electrons which are accelerated to the next dynode by an additional 90V potential. After nine dynodes (each one at +90V with respect to the one before it), the number of electrons finally reaching the anode is in the order of ten million for each incident photon. The current measured at the anode collector is still proportional to the intensity of the light but it has been amplified over a million times.

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INTERFERENCE Interference is a phenomenon in which two waves superimpose to form a resultant wave of greater or lower amplitude. Interference decrease the intensity of absorption of light . Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency.Types of interferences1) Spectral interference2) Chemical interference3) Physical interference

Spectral interferences Spectral interferences arise when the absorption or

emission of an interfering species either overlaps or lies so close to the analyte absorption or emission that resolution by the monochromator becomes impossible. ex:-Manganese triplet (4031,4033,4035Å) overlapped by gallium line(4033Å).

This interference can be corrected by amplitude modulation of the source.

Chemical interference Occurs due to incomplete dissociation of compounds in

the flame when the concentration of compound is more. Removed by use of higher flame temperature. Chemically –by addition of more thermally stable

compound. ex-addition of lanthanum to the aluminium and magnesium for detection of magnesium

Physical interference :-viscosity -solvent -ionization1) Viscosity – viscosity is invertionaly proportional to the

intensity of absorption.2) Solvent – organic solvent increases the intensity and

aqueous solvent decreases the intensity of absorption.3) Ionization – occurs due to high flame temperature. A

number of vaporized atoms become ionized by the flame. Resulting ions absorb at a different wavelength than the vaporized atoms the new wavelength will not be selected by the monochromator and low results occurs.

Na Na+ + e-

Overcome by addition of more easily ionizable element Ex- ionization interference of Na is corrected by the addition of Potassium to the Sodium .

APPLICATIONS OF AAS Qualitative analysis of elements. Quantitative analysis of elements. Simultaneous elemental analysis. Clinical analysis of biological fluids such as urine and blood Environmental analysis – analysis of river water, industrial

wastes, seawater, drinking water, air etc…. Pharmaceutical analysis Determination of metallic elements( Cu,Zn,Ni) in food

industry. Determination of lead in petrol (diethyl lead and tetraethyl

lead)

Difference between AAS and Flame Photometry

Flame photometry Atomic absorption spectroscopy

Emission phenomenon Absorption phenomenon

Limited to only alkali and alkaline earth metals

used for approx. 81 metals

Dependent upon flame temperature Independent upon flame temperature

Dependent upon number of excited atoms Dependent upon unexcited atoms