An effective and high-throughput sample preparation method involving demalonylation followed by an ultrahigh-performance liquid chromatography–charged aerosol detector for analyzing gypenoside XLIX and gypenoside A in Gynostemma longipes

Gypenosides (Gps) are the major bioactive components in Gynostemma species. They include neutral Gps and acidic malonylgypenosides (MGps). MGps are abundant in Gynostemma species and can be transformed into corresponding Gps via extraction, concentration, and drying. If only the Gps were quantified and MGps were ignored, the quality of Gynostemma species would be underestimated. This study aimed to develop a sample preparation method involving demalonylation and ultrahigh-performance liquid chromatography-charged aerosol detector (UHPLC-CAD) analysis to determine the contents of gypenoside XLIX (Gp XLIX) and gypenoside A (Gp A). First, the optimized ultrasonic extraction method was established to extract G. longipes powder ultrasonically. Then, the extracted solution was put into a closed container (centrifuge tube) and heated in a water bath at 95 °C. Then, MGps were converted into corresponding Gps. The proposed preparation method was compared with the other three methods, including water bath reflux heating, alkali hydrolysis, and extraction of heated powder, and was shown to exhibit higher conversion and better convenience. Subsequently, an UHPLC-CAD method was established and validated. Gp XLIX and Gp A showed excellent linear correlations between 15.55 and 248.8 μg/mL and 24.10-385.5 μg/mL, respectively (R2 > 0.999). The limit of detection was 1.40 ng (Gp XLIX) and 2.41 ng (Gp A), and the limit of quantification was 7.77 ng and 14.46 ng, respectively. The relative standard deviation for precision, stability, and repeatability was 0.63-3.15%. The average recovery of Gp XLIX and Gp A was 98.97% and 98.23%, respectively. The established method was applied for determining Gp XLIX and Gp A contents in wild or cultivated G. longipes samples collected from the Qinba Mountains area. The contents of Gp XLIX and Gp A were 5.16-23.02 mg/g and 15.78-54.55 mg/g, respectively. Conclusively, the proposed sample preparation and analysis method could be used for the quality control and evaluation of G. longipes.