Background: Antibiotics are an emerging class of aquatic contaminants whose occurrence at environmentally relevant levels is especially concerning due to the proliferation of antibiotic resistance in microbial flora . Identifying antibiotic degradants has been identified as a key research gap necessary to characterise the risk of environmental antibiotic exposure . It is therefore of crucial importance to characterise the lifecycles of antibiotics in the aquatic environment. Photochemical degradation is one of the primary pathways by which antibiotics can break down in nature . However, our current knowledge of antibiotic photolysis breakdown products is unreliable, particularly in relation to identifying toxic end products [6,7].
Objectives: To obtain a better understanding of the identity of antibiotic photoproducts, the following goals are necessary:
• Develop a new methodology for simplifying the identification of the direct photolysis products of an antibiotic. The method should at least be able to provide structural identification equivalent to that currently obtainable by MS, but more definitive structural identification (e.g. spectroscopic) of photoproducts is highly desirable.
• Develop a new methodology for performing “on-line” analysis of a photolysed antibiotic in solution, to allow for continuous sampling so that time-dependent changes in the solution composition can be followed, as well as identification of short-lived intermediates. Again, structural identification equivalent to that obtainable by MS is essential, with more definitive structural spectroscopic identification being highly desirable.
Experimental Approach and Novelty: We propose to meet these objectives by:
1. Photolysing key antibiotics in the gas-phase using laser-interfaced mass spectrometry
Within solution, photolysis pathways are affected by many variables , so we propose to dramatically reduce this complexity by taking the antibiotic out of solution and studying its breakdown pathways in the gas-phase for the first time. These experiments will use the Dessent group’s novel laser-interfaced mass spectrometer (LIMS), which allows the gas-phase UV-VIS absorption spectrum to be recorded, along with MS identification of primary and secondary photoproducts [10, 11]. A range of key antibiotics will be studied.
2. Developing full “on-line” analysis of photolysis of pharmaceuticals in solution
In the next stage, we will develop on-line solution photolysis detection, linked to the LIMS instrument. This will allow us to use MS to characterise any photoproducts/reaction intermediates, and also perform UV/VIS spectroscopy on them within the LIMS machine, providing definitive structural identification. This would be the first such experiment developed internationally.
3. Comparative assessment of the results and application to real world samples
In stage 3, we will compare the results obtained in Steps 1 and 2 against off-line photolysis (manual sampling of photolysed solutions). It will also be important to perform additional analysis of photolysed solutions using HPLC-MS/MS (in collaboration with JETO) . Stable photoproducts identified in stages 1 & 2 whose detectability is confirmed by HPLC will be targeted for HPLC analysis in surface water samples collected globally. With access to a huge repository of water samples from rivers in nearly 100 countries (BS group) which have been analysed for 14 antibiotic compounds, samples with detectable levels of the parent compounds will be re-analysed to target these photoproducts. This will not only confirm the utility of the proposed technique, but will also provide new insights into the distribution of antibiotic photoproduct occurrence in surface waters globally.
Training: Full training in MS and (soft ionization techniques and HPLC) and laser photochemistry (use of class 4 laser systems and diode lasers) will be provided, along with training in data analysis and storage. The student will be strongly encouraged to participate in national and international conferences, allowing them to develop oral and presentation skills.
All Chemistry research students have access to our innovative Doctoral Training in Chemistry (iDTC): cohort-based training to support the development of scientific, transferable and employability skills: https://www.york.ac.uk/chemistry/postgraduate/idtc/
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. This PhD project is available to study full-time or part-time (50%).
This PhD will formally start on 1 October 2020. Induction activities will start on 28 September.