ONLINE ION-EXCHANGE FOR THE REMOVAL OF SULFUR ANION INTERFERENCE ON THE DETERMINATION OF MANGANESE IN GEOTHERMAL FLUIDS BY FLOW-INJECTION ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY

An efficient flow injection system with an on-line anion-exchange resin column (Dowex-1) incorporated into a time-based injector is described for the removal of sulfur anion spectral interference for the determination of manganese by electrothermal atomic absorption spectrometry. The positive interference effect caused by high concentrations of sulfur ions could not be reduced by using different matrix modifiers. However, the elimination of this effect was possible by retaining the interferent species on the resin column and eluting the analyte with dilute nitric acid. The analyte plug was introduced into the graphite tube atomizer with a sampling arm assembly by means of positive displacement with air through a time-based solenoid valve. The entire system was controlled by a computer, independent of the spectrometer, This approach to the elimination of sulfur anion interference has been applied to geothermal fluids with a high content of these species. The linear range was from 0 to 15 mu g l(-1) and the detection limit was 0.2 mu g l(-1) (4 pg) of manganese. The results were precise (2.8-3.2% RSD) and were in good agreement with those obtained by atomic absorption chelation-extraction, differences between mean values were <7%.