Preparation of stock standards and working solutions
The primary stock solution (1.00 mg/mL) of the four standard diterpenoids (AP1, AP3, AP4, and AP6) and an internal standard (digoxin) were prepared and then stored at − 20 °C. Briefly, each standard was accurately weighed and dissolved in the appropriate volume of methanol (HPLC grade). Working solutions containing the four diterpenoids were then prepared from the stock solutions by serial dilution with methanol to obtain the appropriate concentrations of these four analytes. For the internal standard (IS), a further dilution of the stock solution was prepared at a concentration of 50.00 ng/mL.