Synthesis of value-added MIL-53(Cr) from waste polyethylene terephthalate bottles for the high-performance liquid chromatographic determination of methylxanthines in tea

In this work, which was based on the concepts of green chemistry, polyethylene terephthalate (PET) waste bottles were employed as the source of a benzene-1,4-dicarboxylate linker for the synthesis of MIL-53(Cr). Subsequently, the synthesized MIL-53(Cr) was characterized and employed for the first time as a stationary phase for the HPLC determinations of different methylxanthines in black, green, and white tea samples, respectively. As observed, the main working mechanism was the reversed-phase; however, size-exclusion effects were noticeable on catechins, which exceeded the dynamic diameter of MIL-53(Cr) (similar to 8.5 angstrom). Under optimal conditions, the MIL-53(Cr) column exhibited the best efficiency for caffeine with 35,300 plates m(-1) and a chromatographic resolution of >= 1.72 for all the detected compounds in the tea samples. The developed method was linear (0.183-200 mu g mL(-1)) and the obtained LOD values were 0.055-0.072 mu g mL(-1). The obtained RSDs, which were determined employing different chromatographic parameters indicated the feasible reproducibility of the columns and verified the ruggedness of the method. The results confirmed that the methylxanthine contents of the black teas were the highest (51.28-60.02 mg per tea bag; 2.0 g), followed by those of the green teas (26.72-43.68 mg per tea bag), before those of the white teas (20.44-25.36 mg per tea bag). Furthermore, the separation performances of the MIL-53(Cr) column and two silica-based C-18 commercially columns (3 and 5 mu m-sized particles) were compared. The structure of the MIL-53(Cr) afforded higher retention factors in the methylxanthines because of the hydrophobic pi-pi interactions between the solutes and aromatic ring of the organic linker. Conclusively, the MIL-53(Cr) column, which was prepared from PET bottles, could be applied to the routine analyses of methylxanthines in tea samples.