Simultaneous determination of chromium and manganese in alumina by slurry sampling graphite furnace atomic absorption spectrometry using NbC and NaF as modifiers

Abstract

This paper reports a method for the direct and simultaneous determination of Cr and Mn in alumina by slurry sampling graphite furnace atomic absorption spectrometry (SiS-SIMAAS) using niobium carbide (NbC) as a graphite platform modifier and sodium fluoride (NaF) as a matrix modifier. 350 μg of Nb were thermally deposited on the platform surface allowing the formation of NbC (mp 3500 °C) to minimize the reaction between aluminium and carbon of the pyrolytic platform, improving the graphite tube lifetime up to 150 heating cycles. A solution of 0.2 mol L⁻¹ NaF was used as matrix modifier for alumina dissolution as cryolite-based melt, allowing volatilization during pyrolysis step. Masses (c.a. 50 mg) of sample were suspended in 30 ml of 2.0% (v/v) of HNO₃. Slurry was manually homogenized before sampling. Aliquots of 20 μl of analytical solutions and slurry samples were co-injected into the graphite tube with 20 μl of the matrix modifier. In the best conditions of the heating program, pyrolysis and atomization temperatures were 1300 °C and 2400 °C, respectively. A step of 1000 °C was optimized allowing the alumina dissolution to form cryolite. The accuracy of the proposed method has been evaluated by the analysis of standard reference materials. The found concentrations presented no statistical differences compared to the certified values at 95% of the confidence level. Limits of detection were 66 ng g⁻¹ for Cr and 102 ng g⁻¹ for Mn and the characteristic masses were 10 and 13 pg for Cr and Mn, respectively.