Application of factorial design and Doehlert matrix in the optimisation of instrumental parameters for direct determination of silicon in naphtha using graphite furnace atomic absorption spectrometry

Abstract

A method for direct determination of silicon in naphtha samples using graphite furnace atomic absorption spectrometry (GFAAS) is proposed. The optimisation of the instrumental conditions was multivariate using a fractional factorial design and Doehlert matrix. Firstly, the fractional factorial design was performed for preliminary evaluation of the significance of the factors, the factors chosen being: sample volume, atomisation temperature, pyrolysis time and pyrolysis temperature. The results demonstrated that the variables sample volume, varying from 10 to 30 µL, and pyrolysis time, from 10 to 40 s, were not significant. However, the variables pyrolysis temperature and atomisation temperature in the levels studied were statistically significant. Because of this, a Doehlert design was performed in order to determine the optimum conditions for pyrolysis and atomisation temperature. The response surface obtained indicated a maximum at 728 °C for the pyrolysis temperature and at 2696 °C for the atomisation temperature. The proposed method allows silicon determination in naphtha with a detection limit of 15 µg L⁻¹ and a characteristic mass of 143 pg. The precision, expressed as relative standard deviation, was 8 and 11.4% for silicon concentrations of 100 and 50 µg L⁻¹, respectively. Addition/recovery tests of silicon in several naphtha samples proved that this procedure could be applied satisfactorily for analysis of this matrix. The proposed method has been used for silicon determination in naphtha samples processed at Petrobras (São Francisco do Conde, Bahia, Brazil).