Principle

if the absorption process takes place in the flame under REPRODUCIBLE Conditions,the absorption is propotional to the number of absorbing ions/atoms.


This technique is based on the fact that when atoms ,ions or ion complexes of an element in the ground state are atomised in a flame.


They absorb light at characteristic wavelength of that element.




Basis of AAS

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•Measurement of absorption of radiation by the atomic vapours of the element generated from a solution of that element is the basis of AAS.

•Determination is carried out at the wavelength of one of the absorption line of the element concerned.

•Comparison of Emission intensities of unknown samples to either that of standard solutions by plotting calibration curve or to those of an internal standard (standard addion method) helps in quantitative analysis of the analyte metal in the sample solution.


Instrumentation

•Emission source: that provides characteristic spectral line of element such as hollow cathode discharge lamp.

•Monochromator:To select Required resonance/wavelength.

• Atomizer: for introducing the sample solution into a flame.

•Detector: To detect Atoms/ions present in sample ion.

•Atomic Vapour may also be generated outside the Spectrophotometer as in the case of mercury vapour generator or hydride vapour generator.


Calibration of AAS

•Calibration of AAS instument is done with Cu standards.

•Usually we prepare five standard dilutions with different concentrations as 0.5ppm,1.0ppm, 1.5ppm, 2.0ppm, 2.5ppm starting with 1000ppm standard stock solution.

•Now we record absorption in each case and draw calibration curve with coefficient of linearity not less than 0.99 .

Note:-

Calibration may also include repeatability of observed absorption for particular concentration.

Why Cu is Used for calibration ?
Cu standard solutions are most stable one and their absorption is free from noise i.e. Cu has minimum RSD(relative standard deviation).

General method of Sample Prepration

Digestion:-

first method involes digestion of substance under examination in concentrated acids such as HCl ,HNO3.

•Mainly this method is used for Solid substances

Ashing:-

this method involes ashing of substance under examination at very high temperature(550℃-1200℃).

•This method is used only when sample is sticky(like syrups) or presence of metal ion under observation is very low in that substance.

Gases Used :

The two commonly used gas combinations are:

1.Air – acetylene

2.Nitrous oxide – acetylene

Air – Acetylene flames have temperatures ranging from 2150°C – 2300°C which is sufficient to break down molecules of most elements to ground state atoms during the time the sample resides in the flame. Examples of such elements which can be analyzed using this gas combination are salts of Ca, Mg, K, Na, Fe and other transition elements.

Nitrous oxide – acetylene

Some elements such as Ti, V, Cr, Os, Rh, Al, etc have high melting points and their oxides also dissociate at high temperatures. Such elements are referred to as refractory elements. Nitrous oxide can be used as an oxidant gas when hotter flames are required. It has no explosive hazard and temperatures in the range of 3000°C are reached.At such temperatures molecular dissociation of compounds of such elements takes place easily. At the same time it is important to realize that too high temperatures do not serve any benefit as a fraction of atoms would be ionized at such temperatures and such ionized atoms will not absorb at same wavelength as ground state atoms thereby leading to a drop in the absorption measurement.

Preventive Maintenance of AAS

•FirstPreventive Maintenance of AAS involves proper handling of Lamps.

•Cleaning of nebulizer capillary: cleaning of capillary can be done with metallic wire followed by sonication for about 10mins in methanol, Similarly plastic capillary tubing can be cleaned.


Check for any blockages in the drain pipe and empty out the waste drain regularly.


Clean Burner regularly with metal strip by swiping it in burner slit followed by aspirating with 5% nitric acid.

Note:

it is advised to aspirate with 5%nitric acid for 5-10min at the end of each analysis.

Advantages of AAS

• Wide range of metal ions that can be analysed.

•More sophisticated and accurate than flame photometer.

•Easy to use & fast analysis.

• Selective and sensitive to even parts per million (ppm) to part per billion (ppb).

•There is no interference of presence of other ions present in sample solution.

•Organic solvents such as methyl isobutyl ketone (MIBK) can be used.

Disadvantages of AAS

• Information about the molecular structure of the compound present in the sample cannot be determined.

•Elements such as carbon ,hydrogen ,halides cannot be detected due to their non-radiative nature.


PREPARATION OF 1000PM STOCK AA STANDARDS

Aluminium

Dissolve 1.000 g of Aluminium metal in 25ml. of hydrochloric acid (S.G. 1.18) and a few drops of nitric acid (S.G. 1.42) Dilute to 1 litre in a volumetric flask with deionised water.

Antimony

Dissolve 1.000g antimony in 10ml. conc. hydrochloric acid, a few drops of conc. nitric acid, and 10g of tartaric acid. Dilute to 1 litre with deionised water.

Arsenic

Dissolve 1.000g of arsenic powder in 50ml. conc. nitric acid. Dilute to 1 litre with deionised water or

Dissolve 1.3200g arsenous oxide (As2O3) , dried to 110°C, in 50ml. of conc. hydrochloric acid. Dilute to 1 litre with deionised water.

Barium

Dissolve 1.4380g.of barium carbonate (BaCO3) in 20ml. 1M hydrochloric acid. Dilute to 1litre in a volumetric flask with deionised water.

Beryllium

Dissolve 19.639g. of beryllium sulphate (BeSO4) in 250 deionised water. Dilute to 1 litre in a volumetric flask with deionised water.

Bismuth

Dissolve 1.000g. of bismuth metal in 50ml. conc. nitric acid Dilute to 1litre in a volumetric flask with deionised water.

Dissolve 1.1148g. of bismuth trioxide (Bi2O3) in the minimum quantity of comc. hydrochloric acid. Dilute to 1litre in a volumetric flask with deionised water.

Boron

Dissolve 5.1744g. of boric acid (H3BO3) in 500ml. of deionised water. Dilute to 1litre in a volumetric flask with deionised water..

Cadmium

Dissolve 1.000g. of cadmium metal in 20ml. of 5M.hydrochloric acid and 2 drops of conc. nitric acid. Dilute to 1litre with deionised water.

Dissolve 2.0360g. of cadmium chloride in 250 ml deionised water.Dilute to 1 litre in a volumetric flask.

Dissolve 2.1032g. of cadmium nitrate in 250ml.of deionised water. Diluteto1 litre in a volumetric flask.

Caesium

Dissolve 1.2670g. of caesium chloride (CsCl) in 100ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water.

Calcium

Dissolve 2.4973g. of calcium carbonate (CaCO3) in 25ml. of 1M hydrochloric acid. This should be added dropwise to avoid losses during the vigorous effervescence. Dilute to 1litre in a volumetric flask with deionised water.

Dissolve 2.7693g. of calcium chloride (CaCl2) in 100ml. deionised water. Dilute to 1litre in a volumetric flask with deionised water.

Chromium

Dissolve 7.6960g. of chromium nitrate (Cr(NO3)3.9H2O) in 250ml.of deionised water. Dilute with more deionised water in 1 litre vol. Flask.

Cobalt

Dissolve 1.000g. of cobalt metal in 50ml. of 5M hydrochloric acid. Dilute to 1litre in a volumetric flask with deionised water.or

Dissolve 4.9379g. of cobalt nitrate (Co(NO3)2.6H2O in 250 ml. deionised water. Dilute to 1litre in a volumetric flask with deionised water.

Dissolve 4.0380g. of cobalt (11) chloride (CoCl2.6H2O) in 200ml. deionised water. Dilute to 1litre in a volumetric flask with deionised water.

Copper

Dissolve 1.000g. of copper metal in 50ml. of 5M nitric acid. Dilute to 1 litre in a volumetric flask with deionised water. or

Dissolve 3.7980g. of (Cu(NO3)2.3H2O in 250ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water.

Gold

Dissolve 1.000g. of gold metal in 15ml. of conc. hydrochloric acid and 5ml. of conc. nitric acid. Dilute to 1 litre in a volumetric flask with deionised water.or

Dissolve 1.8110g. of ammonium chloroaurate (NH4AuCl4) in 100ml.of deionised water. Dilute to 1 litre in a volumetric flask with deionised water.

Iron

Dissolve 1.000g. of iron wire or granules in 20ml. of 5M hydrochloric acid Dilute to 1 litre in a volumetric flask with deionised water.or

Dissolve 4.8400g. of iron (111) chloride (FeCl3.6H2O) in 200ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised.water

Lanthanum

Dissolve 1.1782g. of lanthanum oxide (La2O3) in 20ml. of 5M hydrochloric acid. Dilute to 1 litre in a volumetric flask with deionised water.

Lead

Dissolve 1.000g.of lead metal in 50ml. of 2M nitric acid. Dilute to 1 litre in a volumetric flask with deionised water.

Dissolve1.5980g. of lead nitrate ( Pb(NO3)2 ) in 100ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water.

Magnesium

Dissolve 1.000g. of magnesium metal in 50ml. of 5M hydrochloric acid. Dilute to 1 litre in a volumetric flask with deionised water.or

Dissolve 3.9160g. of magnesium chloride (MgCl2.6H2O) in 200ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water.

Manganese

Dissolve 1.000g. of manganese metal in50ml. of conc. hydrochloric acid. Dilute to 1 litre in a volumetric flask with deionised water.

Dissolve 3.6077g. of manganesse chloride (MnCl2.4H20) in 50ml. conc. hydrochloric acid. Dilute to 1 litre in a volumetric flask with deionised water.

Mercury

Dissolve 1.000g. of mercury metal in20ml. of 5M nitric acid. Dilute to 1 litre in a volumetric flask with deionised water.or

Dissolve 1.3520g. of mercury (11) chloride (HgCl2) in 250ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water. or

Dissolve 1 .0800g. of mercury (11) oxide (HgO) in 20ml. of 5M hydrochloric acid. Dilute to 1 litre in a volumetric flask with deionised water.

Molybdenum

Dissolve 1.000g. of molybdenum powder in 10 ml. of conc. hydrochloric acid and 10ml. of deionised water. Add 1ml. of conc.nitric acid. Use a magnetic stirrer to completely dissolve the molybdenum. Dilute to 1 litre in a volumetric flask with deionised water. or

Dissolve 1.8403g. of ammonium molybdate (NH4)6MO7O22.4H2O in 500ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water. or

Dissolve 1.5000g. of molybdenum trioxide (MoO3) in 25ml. conc. hydrochloric acid. Dilute to 1 litre in a volumetric flask with deionised water.

Nickel

Dissolve 1.000g. of nickel in 20ml. of conc. nitric acid. Dilute to 1 litre in a volumetric flask with deionised water.or

Dissolve 4.9530g. of nickel nitrate (Ni(NO3)2.6H2O) in 1 litre volumetric flask with deionised water.

Potassium

Dissolve1 1.9070 g. of dry potassium chloride in 250 ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water.

Silver

Dissolve 1.5750 of silver nitrate in 200ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water.

Sodium

Dissolve 2.5420g. of sodium chloride (NaCl) in 200ml. of deionised water. Dilute with deionised water to 1 litre in a volumetric flask.

Strontium

Dissolve 2.4150g. of strontium nitrate (Sr(NO3)2 in 200ml. of deionised water. Dilute to 1 litre in a volumetric flask with water.

Tin

Dissolve 1.000g. of tin metal in 200ml. of conc. hydrochloric acid and 5ml. of conc.nitric acid. Dilute to 1 litre in a volumetric flask with deionised water. or

Dissolve 2.9540 of tin (1V) chloride pentachloride (SnCl4.5H2O) in 500ml. deionised water.If precipitate forms, redissolve by adding 6M hydrochloric acid. Dilute to 1 litre in a volumetric flask with deionised water

Uranium

Dissolve 21.100g. of uranyl nitrate (UO2(NO3)26H2O) in 250ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water.or

Dissolve 14.329g. of uranyl chloride (UO2Cl2) in 250ml. of deionised water. Dilute to 1 litre in a volumetric flask with deionised water.

Vanadium

Dissolve 1.000g. of vanadium metal in 25ml.conc.nitric acid. Dilute to 1 litre.

Dissolve 2.2960g. of ammonium metavanadate (NH4VO3) in 20ml. of 100 volume hydrogen peroxide. Dilute to 1 litre in a volumetric flask with deionised water.

Zinc

Dissolve 1.000g. of zinc metal in 30ml. of 5M hydrochloric acid. Dilute to 1 litre in a volumetric flask with deionised water.or

Dissolve 1.2450g. of zinc oxide (ZnO) in 5ml of deionised water followed by 25ml. of 5M hydrochloric acid. Dilute to 1 litre in a volumetric flask with deionised water

Zirconium

Dissolve 1.000g. of zirconium metal in 25ml. of hydrofluoric acid (40% w / v) in a P.T.F.E. beaker. Dilute to 1 litre in a polpropylene volumetric flask with deionised water, or

Dissolve 3.7174g. of dry zirconium nitrate in 1 litre deionised water