Enhancement of evaporative light scattering detection in high-performance liquid chromatographic determination of neomycin based on highly volatile mobile phase, high-molecular-mass ion-pairing reagents and controlled peak shape

In the frame of the development of a novel HPLC–ELSD (evaporative light scattering detection) method for the determination of the aminoglycoside antibiotic neomycin sulfate, the influence of mobile phase composition and peak broadening on ELSD response was evaluated. ELSD response was enhanced by: (a) increase of mobile phase volatility (solvents examined: water, acetonitrile, methanol and acetone), (b) increase of molecular mass of ion-pairing species [acidic reagents tested: formic, acetic, trifluoroacetic, trichloroacetic and heptafluorobutyric acid (HFBA)], and (c) decrease of peak width and asymmetry obtained by controlling the concentration of the ion-pairing acidic reagent (HFBA). Utilizing a Waters ODS-2 C18 Spherisorb column, evaporation temperature of 45 °C and nitrogen pressure of 3.5 bar, the optimized mobile phase was water–acetone (50:50), containing 11.6 mM HFBA, in an isocratic mode at a rate of 1.0 ml/min. Neomycin was eluted at 4.9 min, with asymmetry factor 1.3. Logarithmic calibration curve was obtained from 2 to 50 μg/ml (r > 0.9997). Limit of detection (LOD) was 0.6 μg/ml and R.S.D. = 1.7% (n = 3, 3.3 μg/ml). In raw materials, the simultaneous determination of sulfate (LOD = 3 μg/ml, R.S.D. = 1.7%, r > 0.9998) and of minor impurities was feasible. The developed method was also applied for the determination of neomycin in pharmaceutical formulations (powder, aerosol and cream) without any interference from excipients (recovery from spiked samples ranged from 99 to 102%) and a %R.S.D. of <2.1 (n = 3). The HPLC–ELSD method was also found applicable in the determination of neomycin in animal feeds (LOQ = 0.2%) without any interference from the feed matrices.