Retention of interferent ions on the PUF minicolumn is dependent on pH as thiocyanate complex formation is affected by this parameter. This effect on the separation was studied in a pH range 0.5–6.0 (adjustments were made with nitric acid) with solutions containing Ni(II) 1 mg/ml, Fe(III), Cu(II) and Zn(II) 50 mg/ml each one. It was observed that signals were always constants for pH below 3.0.