Fundamental studies on pneumatic generation and aerosol transport in atomic spectrometry: effect of mineral acids on emission intensity in inductively coupled plasma atomic emission spectrometry

Abstract

The mechanism of the mineral acid interference has been studied in ICP-AES. For this study five mineral acids have been evaluated (HCl, HNO 3, HClO 4, H 2SO 4 and H 3PO 4) in four concentrations (0, 0.5, 5 and 30%). In order to investigate this interference emission signal, sample uptake rate, primary and tertiary drop size distributions, total analyte transport rate and excitation temperature have been measured. From the results obtained, it seems that this interference is contributed by a reduction of the analyte transport rate and, also, by a decrease in the plasma temperature. The degree of the contribution to the interference of each one of these causes depends on the type of acid and sample uptake mode. The physical properties of the acid solutions are in the origin of the interference. These physical properties modify the sample uptake rate and/or the primary drop size distribution of the aerosols. The acids evaluated can be classified in two groups. The first group would consist of HCl, HNO 3 and HClO 4, and the second one of H 2SO 4 and H 3PO 4. In natural uptake mode the interference is mainly due to changes in sample uptake rate, and in controlled uptake mode to changes in primary drop size distribution of the aerosols. In both sample uptake modes a density-effect may appear on increasing acid concentration. All these factors tend to decrease the analyte transport rate and, hence, the emission signal. Finally, a cooling effect of the plasma due to a higher load of acids is superimposed to these causes. We think that from this study the mineral acid interference in ICP-AES, with pneumatic nebulization, should be better understood.