Abstract:

Antibiotics are emerging contaminants that have recently attracted attention. They have been detected in natural water and pose health concerns owing to potential antibiotic resistance. Antibiotics are ubiquitous in aquatic environments, with a wide spectrum and trace levels. It is difficult to detect all types of antibiotics with completely different physicochemical properties. Solid phase extraction (SPE) is a common sample preparation procedure. For a fast and high-throughput continuous on-line analysis of these emerging contaminants, a method for the determination of 42 antibiotics (grouped into seven categories: sulfonamides, fluoroquinolones, lincosamides, macrolides, tetracyclines, cephalosporins, and chloramphenicols) in environmental water was developed based on ultra high performance liquid chromatography combined with tandem mass spectrometry (UHPLC-MS/MS) involving large volume direct injection without sample enrichment and cleanup.
The collected water samples were filtered through a 0.22-μm filter membrane, their pH levels were adjusted to 6.0-8.0 after adding Na₂EDTA, and then the solutions were mixed with an internal standard. The addition of Na₂EDTA contributed to the release of tetracyclines and fluoroquinolones from the metal chelate. Improved recoveries were observed for all the compounds when the pH of the aqueous solution was set at 6.0-8.0. The optimized UHPLC conditions were as follows: chromatographic column, Phenomenex Kinetex C18 column (50 mm×30 mm, 2.6 μm); mobile phase, acetonitrile and 0.1% (v/v) formic acid aqueous solution; flow rate, 0.4 mL/min; injection volume, 100 μL. In the UHPLC-MS/MS experiment, chloramphenicol, thiamphenicol, and florfenicol were analyzed in the negative ionization scheduled multiple reaction monitoring mode (scheduled-MRM), while the other 39 antibiotics were analyzed in the positive scheduled-MRM mode. This acquisition method improved the response of each target compound by dividing the time of the analysis test cycle and scanning the ion channels of chromatographic peaks at different time periods. The ionspray voltage was set at 5500 and -4500 V in positive and negative modes, respectively. The source temperature for both ionization modes was set at 500 ℃, which was optimized to improve the sensitivity. Instrumental parameters like collision energy and declustering potential were also optimized.
Good linearity was observed for all the tested antibiotics, with a correlation coefficient (r) greater than 0.995. The method detection limits (MDLs) were 0.015-3.561 ng/L. The average recoveries ranged from 80.1% to 125%, while the relative standard deviations (RSDs) were between 0.8% and 12.2%. The method was successfully applied to the determination of 10 source water samples and 5 tap water samples. Twelve antibiotics, viz. sulfachloropyridazine, sulfadiazine, sulfamethazine, sulfamethoxazole, sulfisomidine, clindamycin, lincomycin, roxithromycin, clarithromycin, erythromycin, thiamphenicol, and forfenicol, were detected in the 10 water samples with a detection frequency of 100%. The total antibiotic content in each sample ranged from not detected to 80.3 ng/L. Lincosamides and chloramphenicols were the predominant antibiotics in the water samples, with contents in the ranges of 3.83-13.7 and 4.23-33.6 ng/L, respectively. Therefore, the large volume direct injection method exhibited good performance in terms of MDL and recovery compared to standard methods and those reported previously.
Compared with traditional pretreatment methods, the large volume direct injection method is simpler, more rapid, more precise, and more accurate. It is a viable alternative to SPE, and can be used for the determination of the 42 antibiotics at trace levels in cleaner water bodies, such as surface water, groundwater, and tap water.

Key words: large volume direct injection, ultra high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS), antibiotics, water body