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Atomic Absorption Spectroscopy

Lecture 17

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Chemical Interferences

These are interferences resulting from chemical processes occurring in flames and electrothermal atomizers and affect the absorption signal. To quantitatively assess the effects of the different chemical processes occurring in flames, one should regard the burnt gases as behaving like a solvent. This is necessary since our knowledge of gaseous state reaction equilibria is rather limited. Chemical interferences include three major processes:

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1 .Formation of Compounds of Low Volatility

Anionic species forming compounds of low volatility are the most important. The formation of low volatility species will result in a negative error or at least will decrease the sensitivity. For example, the absorption signal of calcium will be decreased as higher concentrations of sulfate or phosphate are introduced. Cations forming combined products with the analyte will also decrease the signal obtained for the analyte. For example aluminum forms a heat stable compound with magnesium.

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Elimination of Low Volatility Compounds

1. Addition of a releasing agent: cations that can replace the analyte (preferentially react with the anion) are called releasing agents. In this case the analyte is released from the compound of low volatility and replaced by the releaseing agent. Lanthanum or strontium are good releasing agents in the determination of calcium in presence of phosphate or sulfate. Also, lanthanum or strontium are good releasing agents in the determination of magnesium in presence of aluminum since both can replace magnesium.

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2. Addition of a protective agent: organic ligands that form stable volatile species with analytes are called protective agents. An example is EDTA and 8-hydroxyquinoline which will form complexes with calcium even in presence of sulfate and phosphate or aluminum.

3. Use  of higher temperature is the simplest procedure to try if it is possible

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2. Dissociation Equilibria

Dissociation reactions occur in flames where the outcome of the process is desired to produce the atoms of analyte. For example, metal oxides and hydroxides will dissociate in flames to produce the atoms as in the equations

MO = M + O

M(OH)2 = M + 2 OH

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Remember that we are not working in solution to dissociate the compounds into ionic species. In fact, not much is known about equilibrium reactions in flames. It should also be remembered that alkaline earth oxides and hydroxides are relatively stable and will definitely show characteristic broad band spectra (more intense than line spectra), except at very high temperatures. The opposite behavior is observed fro alkali metals oxides and hydroxides which are instable even at lower flame temperatures and thus produce line spectra.

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An equilibrium can be established for the dissociation of compounds containing atoms other than oxygen, like NaCl where:

NaCl = Na + Cl

Now, if the signal from a solution of NaCl was studied in presence of variable amounts of Cl (from HCl, as an example), the signal will be observed to decrease as the concentration of Cl is increased; a behavior predicted by the Le Chatelier principle in solutions.

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The same phenomenon is observed when a metal oxide is analyzed using a fuel rich flame or a lean flame. Signal will be increased in fuel rich flames since the dissociation of metal oxides is easier due to less oxygen while the opposite takes place in lean flames (oxygen rich).

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A good example on dissociation equilibria can be presented for the analysis of vanadium in presence of aluminum and titanium, fuel rich flames result in higher absorbance signal for vanadium since the little oxygen present in flames will be mainly captured by Al and Ti, thus more V atoms are available. However, in lean flames, excess oxygen is present and thus vanadium will form the oxide and addition of extra Ti and Al will not affect the signal. 

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3. Ionization Equilibria

 Ionization in fuel/air flames is very limited due to relatively low temperatures. However, in fuel/nitrous oxide or fuel/oxygen mixtures, ionization is significant. Therefore, at higher temperatures an important portion of atoms can be converted to ions:

M = M+ + e

K = [M+][e]/[M]

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Ionization in flames may explain the decrease in absorption signal for alkali metals at very high temperatures where as the temperature is increased signal will increase till an extent at some temperature where it starts to decrease as temperature is further increased; a consequence of ionization. Therefore, usually lower flame temperatures are used for determination of alkali metals. A material that is added to samples in order to produce large number of electrons is referred to as an ionization suppressor, the addition of which results in higher sensitivities.

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Practical Details in AAS

Sample PreparationThe most unfortunate requirement of AAS may

be the need for introduction of samples in the solution form. This necessitates the dissolution of the sample where in many cases the procedure is lengthy and requires very good experience. Care should be particularly taken in order not to lose any portion of the analyte and to make sure that the reagents, acids, etc. used in the dissolution and pretreatment of the sample are free from analyte impurities.

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I suggest that you follow exact procedures for preparation of specific samples for analysis by AAS. In some cases where the sample can be introduced directly to an electrothermal atomizer without pretreatment (like serum samples), definitely, electrothermal atomizers will have an obvious advantage over flame methods which require nebulization.

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Organic Solvents

1. Increased nebulization rate due to lower surface tension of organic solvents which produces smaller droplets as well as faster evaporation of solvents in flames will result in better sensitivities.

2. Immiscible organic solvents containing organic ligands are used to extract metal ions of interest and thus concentrate them in a small volume (thus increasing sensitivity) and excluding possible interferences due to matrix components.

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 Calibration Curves

The absorbance of a solution is directly proportional to its concentration but due to the large number of variables in AAS, usually this direct relationship may slightly deviate from linearity. The standard procedure to do is to construct a relation between the absorbance and concentration for a series of solutions of different concentrations. The thus constructed graph is called a calibration curve.

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The unknown analyte absorbance is found and the concentration is calculated or located on the curve. Neither interpolation nor extrapolation is permitted to the calibration curve. A sample can be diluted or the calibration curve may be extended but always the analyte absorbance should be within the standard absorbance range recorded. Usually, the concentration axis has the ppm or ppb units.

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Standard Addition method

Chemical and spectral interferences can be partially or wholly overcome by the use of a special technique of calibration called the method of standard addition. In addition, the use of this method provides better correlations between standards and sample results due to constant nebulization rates. The method involves addition of the same sample volume to a set of tubes or containers.

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Variable volumes of a standard are added to the tube set followed by completion to a specific volume. Now, all tubes contain the same amount of sample but different concentrations of analyte. A plot is then made for the volume of standard and absorbance. This plot will have an intercept (b) with the y axis and a slope equals m.

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The concentration of the analyte can be determined by the relation:

Cx = bCs/mVx

Where, Cx and Vx are concentration and volume of analyte and Cs is the concentration of standard.

One can only use two points to get the analyte concentration using the relation:

Cx = AxCsVs/(At –Ax)Vx