Systematic investigation of aluminium interferences with the alkaline earths in flame atomic absorption spectrometry

Abstract

Aluminium interferes with the absorption of Mg and Ca in the air-acetylene flame to such an extent that the corresponding absorbances may fall even to zero. This well-known chemical interference can be overcome with the nitrous oxide-acetylene flame, completely in the case of Mg, however only to a restricted extent in the case of Ca. Mg and Ca with concentrations of the AAS-working range in aqueous solutions and Cl− or NO 3 − as anions (in an aqueous HCl or HNO3 matrix, respectively), were determined in the air-acetylene flame with continuously rising Al portions and with (or without) 0.25% Cs as radiation buffer, in order to quantify the degree of these interferences. The same was done to evaluate the extent of the suppression of those interference when using a releaser or protector reagent in both the air-acetylene and the nitrous oxide-acetylene flame. After the decrease of absorption in the air-acetylene flame by forming thermally stable Mg or Ca aluminates, a rapid increase (positive interference) occurs unexpectedly in the presence of Cl−±Cs and with further rising Al contents. This effect still appears for Ca also in the hotter nitrous oxideacetylene flame, however, only in a restricted extent. In the air-acetylene flame the undisturbed absorptions for Mg and Ca (i.e. the starting data without Al) are nearly reached again within the range of the positive interference. This supports the assumption that in consequence of a continuous equilibrium change in the flame because of the rising Al content and in the presence of Cl− and ±Cs the formation of only pure Al oxides now generates the release of Mg and Ca (instead of the thermally stable aluminates in the beginning). In the air-acetylene flame interferences of 1000 mg/l Al are completely removed by an addition of 1% releaser-La, when measuring up to 0.2 mg/l Mg and up to 4 mg/l Ca. The extent of releasing Mg and Ca is effective only up to that Al concentration range which leads to the absorption maximum of Mg and Ca. In the nitrous oxideacetylene flame 5000 mg/l Al are compensated when determining up to 1 mg/l Mg. In the case of Ca, which is detected up to 4 mg/l, interferences of 1000 mg/l Al are only avoided by using the nitrous oxide acetylene flame together with 1% releaser-La. The excellent sensitivity of Ca in this flame (in contrast to the air-acetylene flame) permits a strong dilution, lowering thereby the interfering Al concentration, too. For Mg the same option is valid because of its high sensitivity in the air-acetylene flame.