Quantitative analysis and survey of 9,10-anthraquinone contaminant in Chinese Liupao tea

Recently, 9,10-anthraquinone (AQ) contamination in Chinese Liupao tea has attracted much attention because the tea for export must meet the EU limit (10 µg kg-1). In this study, a method was developed in which the sample was extracted with n-hexane-acetone solution, then purified with Florisil adsorbent, detected by GC-MS/MS and contamination levels of AQ determined using an internal standard. This method was found to be more suitable for Liupao tea and other dark tea complex substrates than the QuEChERS procedure. The sample pre-treatment method was optimized with respect to extraction reagent and clean-up column adsorbent and n-hexane-acetone selected as the optimal extraction solvent. When the content of Florisil in the column was 1.0 g, the optimum clean-up was achieved. The new method reduced the limit of quantification (LOQ) of AQ to 10 μg kg-1, and accuracy was also further improved. The recovery of AQ-fortified tea samples containing 20-100 µg·kg-1 was 94.5-100.4%, and the relative standard deviation (RSD) was less than 1.3%. In a small survey, 98 Liupao tea samples on the market were tested by the new method. It was found that 61 samples were positive (occurrence rate 63.3%), and thus exceeded the EU limit (10 µg kg-1). This study also found that the contamination of AQ in Liupao tea increased with the length of ageing. The source of AQ in the Liupao tea ageing process will be the focus of further research.