Methodology for quantifying residues of chlorhexidine in raw dairy milk

A residue method was developed as part of a pharmacokinetics study to determine the elimination of chlorhexidine in raw milk after intramammary infusion into dairy cows affected with bovine mastitis. The developed liquid/liquid and solid-phase extraction procedures effectively reduced sources of milk product interferences in the final extract. By optimizing mobile-phase pH buffer/acetonitrile gradient conditions and employing an end-capped reverse-phase polar embedded-phase chromatographic column, excellent peak resolution was achieved without the additional need of mobile-phase amine modifiers or ion-pairing reagents. The combined cleanup and chromatographic method steps reported herein were sensitive and reliable for determining the pharmacokinetic elimination of chlorhexidine following intramammary infusion. The residue method was found to be rugged with a lower detection limit of 0.1 ppm.