pre-treated with 25 ml of glacial acidic acid.
The desorption was performed by eluting with 25 ml/100 ml aqueous ammonia solution until the full discoloration of the adsorbent in a cartridge was achieved.
The eluate was collected and evaporated to dryness in a water bath at 80-90 C, cooled and dissolved in water to give the sample solution for further analysis.