Acid effects in inductively coupled plasma atomic emission spectrometry with different nebulizers operated at very low sample consumption rates

Abstract

The acid interference effects observed in inductively coupled plasma atomic emission spectrometry (ICP-AES) for three different nebulizers at sub-millilitre liquid flow rates were investigated. The nebulizers studied were a microconcentric nebulizer (MCN), a pneumatic concentric nebulizer and a conespray nebulizer. Five different matrices were studied: water and solutions containing 0.9 and 3.6 m nitric and sulfuric acid. Primary and tertiary aerosol drop size distributions and ICP-AES emission intensities were measured for all the nebulizers and solutions at liquid flow rates (Q_l) ranging from 0.03 to 0.6 ml min^–1. A study of the acid effect as a function of the tertiary aerosol drop size was carried out through a laboratory-made impactor device. The results indicate that finer primary and tertiary aerosols are generated as Q_l is reduced. Primary aerosols originating from nitric acid and water have similar drop size distributions, whereas they become coarser when sulfuric acid is used. In contrast, acids generate, under all the conditions studied, tertiary aerosols that have a higher proportion of small droplets than water. The results also demonstrate that the MCN gives rise to finer primary and tertiary aerosols than the other two nebulizers. The studies concerning the analytical signal reveal that, for all the acid solutions, a drop in the ICP-AES emission intensity is produced with respect to water. The magnitude of this signal depression depends on the liquid flow rate (i.e., the lower the Q_l the higher the signal drop) and on the acid solution employed. The measurements performed with the impactor device have demonstrated that, for the same nebulizer, the acid effects are more pronounced as the proportion of small droplets increases. Accordingly, it was observed that the drain analyte concentration is higher than that found in the tertiary aerosols. As a result, a phenomenon such as aerosol ionic redistribution is expected to contribute to the interferences when acid solutions are analyzed by ICP-AES.