LC-MS/MS analysis of PG and its metabolites in both dog and human plasma was performed to further understand the potential differences of PG disposition and its metabolism between humans and dogs. A highly sensitive 10-plex method for the quantification of PG and its phase I & II metabolites was developed. Figure shows a chromatogram of all targeted metabolites and an example of PG assay performances including PG intra/inter assay quality controls in human plasma, demonstrating appropriate precision and accuracy. The 3-plexed method of PG and its phase I metabolites (GA and 4OMGA) was fully validated per 2018 FDA guidance [18], with LLOQ of propyl gallate at 20 pg/mL in human plasma and 40 pg/mL in dog plasma. Other major metabolites such as glutathione and glucuronide conjugates were detected at low nanogram levels, which were sufficient for quantifying the study samples. Table summarizes LLOQ of each analyte.