3.4. Procedure
The linear working ranges were the following: 0.02–0.20 mg ly1 Co2q, 0.03–0.30 mg ly1 Mn2q and 0.05–0.50 Cu2q, Ni 2q and Zn2q. The composi- tions of the calibration mixtures were selected ac- 5y1 wx cording to a 2 fractional factorial design 26 . For model assessment, it was used a set of 12 test mix- tures, with concentrations randomly picked within the calibration range. As seen in Fig. 2b and c, the spec- tra of calibration and test mixtures display high collinearity even after the pre-selection procedure.