Characterisation of the abiotic degradation pathways of oxytetracyclines in soil interstitial water using LC–MS–MS

The fate of oxytetracyclines (OTCs) in soil interstitial water was investigated and the structure of a number of degradation products elucidated in a time-related experiment. A previously developed separation method for LC–MS–MS able to base separate and quantify OTC and three of its epimers and degradation products was applied. Compounds detected were 4-epi-oxytetracycline (EOTC) (tR=3.0 min), OTC (tR=4.4 min), α-apo-oxytetracycline (α-apo-OTC) (tR=11.4 min) and β-apo-oxytetracycline (β-apo-OTC) (tR=18.4 min). Furthermore, we tentatively identified 4-epi-N-desmethyl-oxytetracycline (E-N-DM-OTC) (tR=3.0 min), N-desmethyl-oxytetracycline (N-DM-OTC) (tR=3.5), N-didesmethyl-oxytetracycline (N-DDM-OTC), 4-epi-N-didesmethyl-oxytetracycline (E-N-DDM-OTC) (tR=3.7 and 4.7 min) and 2-acetyl-2-decarboxamido-oxytetracycline (tR=8.7) in all samples. Most compounds were only present in trace concentrations (less than 2%) relative to the parent OTC. EOTC was on the other hand formed up to a ratio of 0.6 relative to parent OTC concentration. Only EOTC, E-N-DM-OTC, N-DM-OTC, N-DDM-OTC and E-N-DDM-OTC were formed during the time-related experiment. All other compounds were probably only present as impurities in the spiked OTC formulation as they declined in concentration from the start of the experiment. Half-lives (T1/2, days) of the OTCs in soil interstitial water were in the order of 2 days (EOTC) to 270 days (β-apo-OTC).