On-line Identification of chiral ofloxacin in milk with an extraction/ionization device coupled to Electrospray Mass Spectrometry

The direct separation and analysis of chiral drugs in the complex matrix systems are meaningful and challenging. As the most common broad-spectrum antibiotic, levofloxacin has a strong antibacterial ability, but its enantiomer , dextrofloxacin can cause serious harm to human health. In this work, we reported a rapid on-line extraction/ionization device coupled with Electrospray Mass Spectrometry (ESI-MS) for chiral analysis of ofloxacin enantiomers in complex matrix of milk. Since ofloxacin is difficult to dissolve in water and most organic solvents, the procedure of separating ofloxacin in complex system is often complicated. Using the homemade apparatus, the sample pretreatment process was greatly simplified. Milk sample was directly injected and chiral ofloxacin in the sample was extracted at PTFE membrane for further ionization. It took less than 10   s to finish all the procedures including sampling, extraction, reagents mixing, ionization and mass analysis. Utilizing reaction thermodynamics method , trimeric cluster ion [Ni ΙΙ (ref) 2 Ofloxacin-H] + was formed and collisionally dissociated to get chiral resolution of levofloxacin and dextrofloxacin due to the different relative stabilities of the two diastereomeric clusters produced through the dissociation of Ni ΙΙ bound trimeric clusters. With the proposed method, qualitative and quantitative chiral analysis of ofloxacin in milk was successfully achieved in a simple and fast way.