Addressing the challenges of tetracycline analysis in soil: extraction, clean-up, and matrix effects in LC-MS

An optimized extraction and clean-up method for the analysis of chlortetracycline, doxycycline, oxytetracycline, and tetracycline antibiotics in soil is presented in this work. Soil extraction using different solvents was performed, but the use of a 50 : 50 (v/v) methanol : acetate buffer (pH 8) solvent mixture in a pressurized liquid extraction (PLE) system proved to give the best extraction efficiency and reproducibility. The effect of soil composition on the PLE extraction efficiency was also examined, and results indicated that recovery data for one soil is not necessarily consistent with other soil types containing different compositions of clay and organic matter content. The percent recoveries of the optimized PLE method varied between the soils and ranged from 22-99%, depending on soil type, and more specifically clay content. In addition, the extent of ionization suppression caused by co-extracted humic acids was examined in an ion trap mass spectrometer (MS), and a single quadrupole MS. It was found that under positive electrospray ionization, the single quadrupole MS was less susceptible to ionization suppression than the ion trap MS. Therefore, various sample clean-up procedures were evaluated to selectively reduce the amount of co-extracted humic acids in the soil extracts. The most effective clean-up was obtained from the use of StrataX sorbent in combination with a strong anion exchange cartridge.