Determination of 5-nitroimidazole residues in milk by capillary electrochromatography with packed C18 silica beds

This work presents a novel methodology for analysing 5-nitroimidazole residues in milk samples by capillary electrochromatography using lab-made packed columns, produced by carrying out a high pressure packing procedure using acetone as driving solvent and C-18 silica uncapped particles (5 mu m particle size) as packing material. Column frits resulted from sintering the proper stationary phase by heating the packed material for 20 s with a nichrome ribbon (80% Ni-20% Cr, 28 cm x 2 mm x 0.2 mm, electric resistance 1.3 Omega) connected to a 7 V AC power supply. Lab-made C-18 silica packed capillaries (40 cmx50 mu m i.d.) were employed for the determination of 5-nitroimidazole drugs. Milk samples were treated by a salting-out assisted liquid-liquid extraction followed by a solid phase extraction with Oasis (R) HLB cartridges prior to their injection. Samples were hydrodynamically injected into the column for 120 s at 11.5 bar. Afterwards eight 5-nitroimidazole compounds were separated in isocratic mode under an applied voltage of 27 kV and a temperature of 30 degrees C. The selected mobile phase consisted of a mixture 60:40 acetonitrile:ammonium acetate (2.5 mM, pH=5). Separation was monitored at 320 nm and it was performed in less than 15 min. The method was characterized in terms of linearity (R-2 >= 0.993) and precision (repeatability, RSD <= 12.2% and reproducibility, RSD <= 14.5%), obtaining detection limits lower than 29 mu g/L for all compounds under study. (C) 2015 Elsevier B.V. All rights reserved.