Simultaneous determination of Fe and Ni in guarana (Paullinia cupana Kunth) by HR-CS GF AAS: Comparison of direct solid analysis and wet acid digestion procedures

[Display omitted] •Simultaneous determination of Fe and Ni in guarana by HR-CS GF AAS.•Direct solid sample analysis with calibration against aqueous standard solutions.•Comparison with four acid digestion procedures.•Method simpler and faster than conventional methods of wet digestion of samples.•Better recovery and reproducibility and more precise and accurate results. Direct solid sampling method was compared with the conventional wet acid digestion method to simultaneously assay iron and nickel in guarana by high-resolution continuum source graphite furnace atomic absorption spectrometry. Measurements were done with the secondary lines of Fe and Ni. Four digestion procedures using mixtures of HNO3 and H2O2 (procedure A), HNO3 and HCl (procedure B), HNO3, HCl and H2O2 (procedure C) and HNO3, H2SO4 and H2O2 (procedure D) were evaluated. All tested procedures provided quantitative recoveries for Ni, whereas a good recovery for Fe was obtained only with procedure D. Procedure D was chosen for the digestion of all guarana samples. The limit of detection for the direct solid sampling method was 1.004 μg g−1 for Fe and 0.022 μg g−1 for Ni, and the precision ranged from 3.5–20.0% and 2.8–8.0% for Fe and Ni, respectively. Method accuracy was evaluated by statistical comparison between analyte concentrations, obtained by measurements in the solid samples by the proposed method and after the digestion of the samples by procedure D. The validation of the analytical results obtained for the solid and the digested sample was performed by Energy Dispersive X-Ray Fluorescence.