Electrochemical approach for improed analysis of tea leaves with thin layer chromatography

Abstract

Tea is one of the most consumed beverages worldwide with potential benefits derived mainly from antioxidant properties. During fermentation and preservation, tea leaves undergo oxidation and hydrolysis leading to formation and degradation of various tea constituents and affecting the antioxidant capacity. This study developed an analytical approach to compare chemical fingerprints of various types of tea and the oxidation/reduction products by using electrolysis combined with thin layer chromatography (TLC). Electrolysis approach was performed on TLC plates followed by the TLC separation using the selected mobile phase system of toluene, ethyl acetate and formic acid (2:9:1). Effects of electrolysis voltage and time were investigated in details with the suitable conditions of 1V and ≥ 1 min. The TLC fingerprints after the electrolysis revealed the increasing amount of less polar nonvolatile components which improved the tea fingerprints compared with the sample without electrolysis. The approach may also reveal greater antioxidant properties of Earl Grey tea than Oolong Tea as indicated by the longer oxidation time required to change the Earl Grey tea fingerprint. The additional TLC spots were also selectively cut and desorbed at 80°C and the volatile compounds in the headspace were sampled with solid phase microextraction (SPME) and analyzed with GC-MS. This could detect several peaks albeit with their small areas which could not be identifiable. Electrolysis of the tea solution was then performed by using stainless steel spring coils as the electrodes at 9V for 5 min. The SPME GC-MS analysis revealed degradation of several volatiles with the increasing amounts of benzaldehyde (in both teas), 1,2 dihydro-linalool (in Earl Grey Tea) and alkanes (in Oolong Tea) after the electrolysis. With further development, the developed on-TLC plate electrolysis technique will allow assessment of tea quality in terms of antioxidants on one hand and quality control for improved processing, manufacturing, storage, and preparation in order to maximize the nutritional value on the other.