Determination of Eight Catechins and Four Theaflavins in Qingzhuan Dark Tea of Different Years (1, 5, 9, and 13 Years)

Catechins and theaflavins are the key polyphenolic compounds in Qingzhuan dark tea (QDT). They not only give QDT unique taste and color but also have significant health benefits such as antioxidant, anti-inflammatory, and cardiovascular protection, which are the core of the quality and healthcare value of QDT. However, the variation of its content is closely related to the vintage of the tea. The aim of this study was to develop a rapid, sensitive, and accurate high-performance liquid chromatography–tandem mass spectrometry (HPLC-MS/MS) method for quantifying eight catechins and four theaflavins in QDT of different years (1, 5, 9, and 13 years). An orthogonal experimental design was employed to determine the optimal extraction conditions. Under these conditions, QDT samples from different years (1, 5, 9, and 13 years) were extracted, and eight catechins and four theaflavins were quantified using the established HPLC-MS/MS method. The results showed that the optimal extraction conditions for methanol were determined as follows: methanol concentration of 60%, temperature of 90°C, and extraction time of 15 min, while for water extraction, the optimal conditions were temperature of 90°C and extraction time of 30 min. Methodological validation of the eight catechins and four theaflavins was conducted, including specificity, linearity, lower limit of quantitation, precision, accuracy, stability, and reproducibility. Different extraction methods (methanol extraction and water extraction) significantly influenced the content of eight catechins and four theaflavins, with higher content observed with methanol extraction compared to water extraction. Significant differences were also observed in the content of eight catechins and four theaflavins among QDT samples of different years (1, 5, 9, and 13 years), with higher years associated with lower content. Conclusion: The method established in this study is reliable, accurate, and reproducible, and could be used to evaluate QDT of different years by quantifying the content of eight catechins and four theaflavins.