*2.4. Quality Assurance and Quality Control (QA/QC)*

Fity-four PA standard compounds were obtained from the following sources: echimidine, indicine, indicine *N-*oxide, intermedine, intermedine *N-*oxide, lycopsamine, lycopsamine *N-*oxide, monocrotaline, monocrotaline *N-*oxide and otosenine from Phytolab (Vestenbergsgreuth, Germany); heliotrine and trichodesmine from Latoxan (Valence, France); usaramine from BOC Sciences (Shirley, Suffolk, NY, USA); florosenine from PRISNA (Leiden, the Netherlands); and Usaramine *N-*oxide and trichodesmine *N-*oxide were in-lab synthesized according to [22] and the rest from Phytoplan (Heidelberg, Germany). An analytical grade of formic acid and ammonium carbonate (Energy Chemical) and a LC-MS grade of acetonitrile and methanol (Sinopharm) were purchased from Shanghai, China.

Quality assurance and quality control (QA/QC) procedures were performed as follows: a procedural blank, a spike blank, a mixed PA sample (1 μg/mL of external standards in methanol, used to spike the (herbal) teas in 25 ng/mL and 100 ng/mL) and a duplicate were run for each batch of 20 samples to check for cross-contamination and instrumental reliability as well as to indicate recoveries. No PAs in the blanks were detected. The standard deviations for standard solution (7-point calibration curves over the range of zero to 250 ng/mL) were controlled within 10%. Recoveries at the level of 100 ng/mL varied from 79 to 110%. The LOD for PAs in the infusion of (herbal) teas was determined as the concentrations of analyses in a sample that showed a peak divided by the signal-to-noise ratio (S/N) of 3. The LOD was estimated at the range of 10 to 20 ng/L and the limit of quantification (LOQ) was obtained at 50 ng/L. For those PAs with no available standard compounds, the corresponding structurally related PA standards were employed for the semi-quantification of those PA levels.
