Determination of aliphatic amines in mineral flotation liquors and reagents by high-performance liquid chromatography after derivatization with 4-chloro-7-nitrobenzofurazan

The method described here fulfils the need for a suitable analytical method to determine the concentrations of single and mixed aliphatic amines in the range from hexylamine (C6) to octadecylamine (C18) in flotation test solutions and in commercial flotation collectors. Amines do not have a UV–vis spectrum in aqueous solution but by reacting an amine-containing solution with 4-chloro-7-nitrobenzofurazan solution (chloro-NBD), derivatized products (amino-NBDs) are formed which have absorbance maxima at 470 nm. Excess chloro-NBD and the amino-NBDs can be separated from each other by high-performance liquid chromatography (HPLC) and their concentrations measured with a UV–vis detector. Important variables in the derivatization stage are pH, temperature, chloro-NBD concentration, and reaction time, all of which interact with each other. A three-stage statistical procedure was used to determine the optimum conditions. In each stage, an 8-test design was used in which a high and low limit was set for each variable, and the chromatogram peak area of the derived amino-NBD was measured. The optimum derivatization conditions established were pH 8.9, chloro-NBD concentration 0.20% (w/v), temperature 70 °C, and reaction time 60 min. Optimum elution conditions for chromatography were an eluent containing 80% (v/v) acetonitrile in aqueous solution containing 40 mM acetic acid at pH 4.5. With a flow rate of 2.0 ml/min, dodecylamine had a retention time of about 3 min, whereas octadecylamine had a retention time of 44 min. Straight-line calibration curves were obtained up to at least 200 ppm of amine in solution. The lower limit of detection was estimated to be 0.05 μM (10 ppb) with a signal to noise ratio of 3. No interfering substances were found. The method was successfully applied to the analysis of solutions from an actual flotation test and to a solid commercial amine.