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Study Notes: Chemical Interferences

Interferences lead to a loss of absorbance signal or a change in absorbance signal that is due to the interfering factor and not the sample itself. There are two broad types of interferences - spectral and chemical. This Study Note looks at Chemical Interferences.

Chemical Interferences arise from various chemical processes that occur during atomisation and they alter the absorption characteristics of the analyte. Often the compounds are excessively volatile or excessively stable. These can often be minimised by choice of operating conditions.

For example, interferences due to species of low volatility can sometimes be eliminated or reduced by the use of higher temperatures. Alternatively, releasing agents, which are cations that react preferentially with the interference and prevent its interaction with the analyte, can be added. Anions that form compounds of low volatility with the analyte and decrease its atomisation rate are a potential problem.

For example, decrease in Calcium (Ca) absorbance observed with increasing concentrations of sulphate or phosphate ions. Higher temperatures will often alleviate these effects or releasing agents, which are cations that react preferentiality with the interfering species and prevent interaction with the analyte, can be used. For example, the addition of excess Strontium (Sr) or Lanthanum (La) ions minimises interference by phosphate in the analysis of Ca.

Protective agents prevent interference by preferentially forming stable but volatile species with the analyte. For example, EDTA will eliminate interferences by Si, phosphate and sulphate in the analysis of Ca.

In high temperature flames ionisation of the atomic species becomes apparent and this leads to low results. The addition of an ionisation suppressor which provides relatively high numbers of electrons in the flame will inhibit ion formation of the analyte by mass action. Potassium (K) salts are often used as ionisation suppressors due to the low ionisation energy of this element.

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