Abstract: The main objective of the current study was to develop a universal method for a protein binding assay of complicated herbal components,and to investigate the possible relationship between compound polarity and protein binding using Schisadra lignans as an example.Firstly,the rat,dog and human plasma were spiked with three different concentrations of Schisandra chinensis extract(SLE),and ultramicrofiltration was used to obtain the unbound ingredients.Secondly,thirty-one Schisandra lignans in total plasma and ultrafiltered fluid were measured by LC-IT-TOFMS.Lastly,a relative exposure approach,which entailed calculating the relative concentrations of each Schisandra lignan from the corresponding calibration equation created from the calibration samples spiked with the stock solution of SLE,was applied in order to overcome the absence of authentic standards.The results showed that Schisandra lignans exhibited a high capability to bind with plasma protein,furthermore,the protein binding ratio of the lignan components increased proportionally with their individual chromatographic retention time,which indicated that the ratio of protein binding of lignans might increase accordingly with decreasing polarity.This study suggested that the compound polarity might be an important factor affecting the plasma protein binding of herbal components.