| Summary: | Phenolic composition of green tea (<i>Camellia sinensis</i>) varies according to manufacturing processes that result in deglycosylation of glycosylated phenolics and condensation, epimerization, and degalloylation of flavan-3-ols (catechins). Ambiguous phenolic assignments based on UV absorbance alone can occur when the chromatographic peaks overlapped slightly. We established an improved method using an HPLC–UV coupled with a single-quadrupole MS detector (MS<sup>1</sup>) that can reject false UV peaks after checking the preceding MS<sup>1</sup> peaks. Adjusted UV data coded by the Python algorithm were deployed to compare tea phenolics. Performance validation of the MS<sup>1</sup> and UV analysis methods for 19 phenolics revealed a sensitivity of 0.17 and 0.47 pmol/injection, limit of detection of 15 and 33 μg/L, limit of quantification of 50 and 110 μg/L, intra-day precision of 5% and 1% relative standard deviation, and trueness of 83–135% and 97–100%, respectively. Our results suggest that the HPLC–UV–MS<sup>1</sup> method, which is a low operational cost method, potentially provides the precise phenolic composition of teas.
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