Matrix effects of 28 pyrrolizidine alkaloids in Farfarae Flos and Lithospermi Radix; 1: Echimidine, 2: Echimidine-N-oxide, 3: Erucifoline, 4: Erucifoline-N-oxide, 5: Europine, 6: Europine-N-oxide, 7: Heliotrine, 8: Heliotrine-N-oxide, 9: Intermedine, 10: Intermedine-N-oxide,...