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Atomic absorption spectroscopy ppt

Atomic absorption spectroscopy is a technique used to determine the concentration of trace metals in liquid samples. It works by measuring the absorption of light by ground state atoms in a flame. The sample is atomized in the flame and specific elements absorb light from a hollow cathode lamp emitting their characteristic wavelength. The amount of light absorbed is measured and the concentration of the element determined using a calibration curve. AAS is a sensitive and selective method used widely in fields like environmental analysis, food testing, and clinical analysis to determine concentrations of metals like sodium, calcium, lead and zinc.

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-R DEEPTHI Asst. Professor, Vignan Institute of Pharmaceutical Technology, Visakhapatnam
 Atomic Absorption Spectroscopy (AAS) is one of the most popular and useful methods of elemental analysis that measures radiations absorbed by neutral (unexcited) atoms in the gaseous state.  This technique was developed by Alan Walsh in 1956. It is a powerful instrumental technique which is used for quantitative estimation of trace metals in liquid samples.  The technique which is used for quantitative estimation of trace metals in liquid samples.  The technique is very versatile as it has helped in determining 60-70 trace metals whose concentration ranged from trace to macromoles.
 It can be used in for the estimation of both aqueous and non-aqueous solutions. Moreover, the technique is also helpful in estimating a particular element even in the presence of several other elements.  This implies that it does not require separation of the test element from the rest which saves a great deal of time also helps to overcome the possibility of errors.
Characteristics of AAS  It is a highly sensitive and error free technique.  It is most suited method for the analysis of metals like Al, As, Ba, B, Ca, Cu, Pb, Ni, K, Na, Se, Sn, Te, Zn etc.  It is applicable to aqueous as well as non-aqueous solutions.  It requires less time.  It does not require sample preparation.  It requires very little amount of sample.  It is independent of the molecular form of the metal in the sample. For example, the total sodium content of water sample can be determined irrespective of the molecular form (halide, sulphate, hydride) in which the sodium exists.
Advantages of AAS over Flame Photometry  It is less liable to spectral interferences when compared to flame photometry.  It is a more reliable method.  It is relatively more sensitive than flame photometry in the detection of Ag, As, Au, B, Bi, Cd, Co and Fe.  Since large no. of atoms produce atomic absorption signal and as only thermal energy is utilized to atomize the sample, AAS is independent of flame temperature.
 It is a more specific technique because the atoms of a particular element can absorb the radiation of their own characteristic wavelength.  It obeys Beer’s law over a wide concentration range which is not true in case of flame photometry.  A wide range of elements can be quantitatively determined by this technique.  It is a more specific process because wavelength of absorbed radiation is characteristic of elements.  It requires only small amounts of sample.
Disadvantages:  A separate lamp is required for the determination of each element. This difficulty can be overcome either by using a continuous source of Electro Magnetic Radiation (EMR) with a high resolution monochromator or by using a line source of EMR with the help of flame.  Elements like aluminium, molybdenum, vanadium, titanium and silicon etc., form red metallic oxides in the flame and hence cannot be detected using AAS, except under modified conditions.  When aqueous solutions are used, the predominant anion affects the absorption signal to a high degree.  This technique cannot be used for the detection of non metals due to the strong absorption of light by flame gases.
Principle  The basic principle of atomic absorption spectroscopy (AAS) is the absorption of energy by the ground state atoms in the gaseous form.  When a solution containing a specific metal is introduced into a flame, it results in the formation of vapourized forms of the metallic species.  Only some of the metal atoms from the ground state undergo transition to such an energy level that they are able to emit the radiation which is characteristic to the metal.
However, most of the metal atoms remain in the non emitting ground state. These non-emitting ground state atoms absorb that light radiation which has the wavelength specific to them. Therefore, when light of this specific wavelength is passed through a flame which contains the atoms of the metallic species, then some of the light will be absorbed and the absorption is directly proportional to the density of the atoms of metallic species present in the flame. Hence, in AAS the concentration of the test metal is directly determined from the amount of light absorbed.
The total amount of light absorbed can be mathematically expressed as, The total amount of light absorbed(at ύ)= πe2/mc X Nf 1 Where, ύ = Frequency of the light e = Charge on electron m= Mass of electron c = Speed of light N = Total no. Of atoms that can absorb at ύ f = Oscillator strength (Ability of an atom to absorb at ύ) Since, π, e, m and c are constants, the above equation gets simplified as, Total amount of light absorbed = KNf 2 From equation 2, it is clear that the total amount of light absorbed is independent of wavelength of absorption and temperature.
Instrumentation
Instrumentation
Components of Atomic Absorption Spectrophotometer Radiation Source  AAS is concerned with the transition of free atoms of the test metal from their ground state to the excited state.  The transition from the ground state to the first excited state is known as the first resonance line.  It is the line with strongest absorptivity.  The absorptivity of a particular element decreases as the energy difference between the ground state and the excited state increases.
 When high sensitivity is required, the first resonance line of the test element is used.  The wavelength of first resonance line for all metals and metalloids is longer than 200nm and for non-metals it lies in vacuum UV region below 185nm and hence cannot be measured by conventional spectrophotometers.  Atomic absorption spectrophotometers are the widely used for analysis of metals and metalloids and the non- metals can be detected by modifying the optical system of the spectrophotometer.  However, such modifications make the instrument expensive and hence these are not widely used.
For effective quantitative estimation by AAS, the radiation source should emit the resonance spectral lines of the test metal. The bandwidth of the incident light from the radiation source should be narrower than the absorption line of the sample. This implies that the bandwidth of the radiation source should be less than 0.001nm.  Moreover, there should not arise any extraneous line emitting within the band pass of the monochromator.  The problem of narrow resonance spectral lines can be overcome by the use of hallow cathode lamp.
Hollow cathode lamp:  It is the most commonly used radiation source for atomic measurements. It consists of an evacuated glass tube with a tungsten anode and a cylindrical cathode.  The hollow cathode is made up of the element to be analyzed or its alloy and is enclosed by porcelain or glass to avoid diffusion of the atoms produced.
The glass tube is filled with an inert gas. Application of a potential of about 100-200 volts across the electrodes causes the ionization of inert gas.  The charged gas then migrates at high velocity towards the cathode and collides with the metal atoms from its surface. The metal atoms absorb energy and get excited to higher energy levels. The excited atoms then transit to the lower energy states and emit radiations characteristic of metal used in cathode.
 Therefore, the emission spectrum given by the hollow cathode lamp is actually a sharp line spectrum of the cathode metal and inert gas.  The spectral lines produced by the hollow cathode lamp are narrower than the absorption line of the source.  For example, a zinc cathode emits a zinc spectrum and a copper cathode emits a copper spectrum.
Disadvantages  Different hollow cathode lamps are required for the determination of different elements.  Attempts to increase the intensity of emitted radiation results in increased sputtering, resonance broadening and self-reversal phenomenon (absorption of emitted spectrum by similar atoms in lamp)  It restricts the analysis of multiple elements.
Electrodeless Discharge lamps  Stable hollow cathode lamps are difficult to construct from volatile elements like selenium, germanium and arsenic.  Hence for the determination of such volatile elements, electrodeless discharge lamps have been developed.  Each lamp consists of a sealed quartz tube filled with an inert gas (argon) and in which the test metal is placed.
The lamp is placed in a microwave radiation field so that the sample element vaporizes and transits to higher energy levels. As the excited atoms return to lower or ground state, radiation that is characteristic of the metal element is emitted.  Intensity of the emitted radiation is 10 times more than that obtained using cathode lamp.
Atomizers  Flame atomizers like total consumption burner and premix burners are used for converting the liquid sample into gaseous form and also for converting the sample molecules into atomic vapours.
Total consumption burner/Pneumatic Nebulizer  This burner uses hydrogen (fuel) along with oxygen (oxidant) to produce flame.  In it, the liquid sample, fuel and oxidant are introduced through different inlets.  When the liquid sample reaches the base of the flame, it gets converted into fine droplets and then gets burnt or evaporated.  The residue left is further reduced into atoms which are then excited.
Advantages  The total amount of sample entering the inlet aspirates completely, irrespective of its droplet size.  Simple in design. Disadvantages  Burner produces a lot of noise.  Flame is not homogeneous which results in variation in the intensity of the flame.  Less efficient.  Rate of introduction of sample is dependent on its viscosity.
Premix Burner/Laminar Flow Burner  Premix burner is also called Laminar flow burner because the gases move in laminar (parallel) or non-turbulent fashion.  In this, the liquid sample is initially aspirated with air and then thoroughly mixed with fuel and oxidant.  The mixture then reaches the opening of the burner and enters into the flame.  Only about 5% of the sample in the form of fine droplets reaches the flame whereas large droplets i.e., 95% of the sample drains out from the side of the spray chamber (burner).  This small portion of the sample is easily decomposed in the flame and helps in effective atomization of the sample.
Fig: Premix Burner
Advantages:  Flame produced is stable.  It is a quite process i.e., the burner does not produce noise.  Non-turbulent flame is produced.  Effective atomization of the sample takes place.  Clogging of the sample solution does not occur. Disadvantages  Chances of explosion in the mixing chamber of the burner.  Sensitivity is low because most of the sample is wasted.
Fuel and oxidants  Air-acetylene or nitrous oxide-acetylene are the most commonly used combination of fuels and oxidants.  Temperature of the fuel affects the absorption spectra in a complex manner.  Lower temperatures prove to be inefficient in elevating atoms to an excited state while higher temperatures tend to cause ionization of the atoms.  Thus, the desired no. Of may not be achieved in both the cases.  Hence, it is necessary to choose an ideal combination of fuel and oxidant that gives the required temperature.
Mixture of Fuel and Oxidant Temperature (0C) Hydrogen + Oxygen 2800 Hydrogen + Air 2100 Acetylene + Oxygen 3000 Acetylene + Air 2200 Propane + Oxygen 2800 Propane + Air 1900 Butane + Oxygen 2800 Butane + Air 1900 Cyanogen gas + Oxygen 4500 Table: Fuel and Oxidant Mixtures
4.Monochromator 5. Detector Photomultiplier tube is the most suitable detector for atomic absorption spectroscopy.
6.Amplifier  Lock-in amplifiers are suitable for atomic absorption spectroscopy.  An AC- amplifier modulates the signal from reference source at the same frequency and directs each of them to the synchronous detector (via a phase shifter) where in the two signals combine and generate sum and difference frequencies.  The signal then enters into the low pass RC filter that eliminates the unwanted signals and directs the signal of interest to DC amplifier from where it enters into the read out device.
Block diagram of Lock-in amplifier
7.Read out device  Chart recorders are mostly used as readout devices in atomic absorption spectroscopy. Digital readout devices are also available. Working of Atomic Absorption Spectrophotometer  Initially the detector-amplifier unit is adjusted to read 100% transmittance or zero absorbance.  A blank solution is then sprayed into the flame and simultaneously the uninterrupted beam of light from the lamp is allowed to fall onto the Photomultiplier tube.  When the solution containing test material is introduced into the flame and illuminated, a portion of the radiation is absorbed by the excited atoms.
 This causes a decrease in intensity of light falling onto the photomultiplier tube which produces a deflection and yields the value of absorption.  A calibration curve is drawn by preparing standard solutions of the element to be determined and determining their absorbance.  The concentration of test metal in the sample can be measured by extrapolation of the calibration curve.
Single Beam Atomic Absorption Spectrophotometer  In single-beam atomic absorption spectrophotometer, chopper is placed between the hollow cathode lamp and flame.  When a blank solution is sprayed into the flame and radiation from the lamp is allowed to fall onto the detector, the detector- amplifier unit is adjusted to give zero absorbance or full transmittance.  When the sample solution is sprayed into the flame, there occurs a decrease in the intensity of light and the output of detector-amplifier unit is recorded.  One of the demerit of single beam equipment is its low stability.
Double beam Atomic Absorption Spectrophotometer  The Double beam Atomic Absorption Spectrophotometer overcomes the demerit of single beam equipment.  In this, beam of light from the source is split into two, one portion is allowed to pass through the flame while the other portion is not allowed.  The two beams of light are recombined and made to pass through the monochromator and finally to the read out device.
Applications: Qualitative Analysis  AAS is not used for Qualitative analysis since separate lamps are required for the determination of each element. Quantitative Analysis  Quantitative Analysis of various samples by AAS is achieved by Calibration curve method. In this method, a series of standard solutions of known concentration of the element to be analyzed is prepared.  The concentrations of standard solution are plotted on X-axis and their respective absorbance on Y-axis. The absorbance of sample solution is measure, plotted on graph and then extrapolated on X-axis to get the concentration of the element being analyzed.  Concentration up to 0.001ppm can be determined using AAS.
 It is the most suitable method for the estimation of sodium, potassium, calcium, magnesium and lithium in serum and plasma and lead, iron and zinc in whole blood. Determination of these elements helps in diagnosing pathological states.  It is an important tool for the estimation of elements like copper, nickel and zinc in food products.  It is an ideal method for the analysis of alloys, minerals, ores etc., and hence is widely used in metallurgy and inorganic analysis.
Helps to estimate pollutants like cadmium, chromium, copper, lead, mercury, nickel and zinc in water, soil and environmental samples.
 Estimation of Zinc in zinc insulin injection, oils, creams, calamine lotion, calcium in calcium preparations, lead in CaCo3 and mercury in thiomersal solution is possible with AAS.  AAs is also helpful in estimating the levels of Na+, K+, Ca2+ in saline and Ringer’s solution.  It has been used for the assay of cisplatin (for Ag), intraperitoneal dialysis fluid (for Ca, Mg), activated charcoal (for Zn) etc.
In cement industry, AAS is used to determine the quality of cement as it helps in estimating sodium, potassium, magnesium and calcium content of cement.
The boiler ash is analyzed for sodium, potassium, calcium, magnesium and iron by AAS.