Micro-titer plate assay for measurement of total phenolic and total flavonoid contents in medicinal plant extracts

Total phenolic content (TPC) measured by using the Folin-Ciocalteu method and Total flavonoid content (TFC) based on a complex formation with AlCl3 assay are widely applied as part of herbal extract quality control. Conducting the assay measurements in microtiter plates may save resources and time. The objective of this study was to evaluate the validity of the micro-titer plates assay to determine the TFC and TPC in several medicinal plants’ extracts. To compare the developed TPC and TFC methods to the conventional assays, the confidence intervals of linear regression were used with a significance set as p < 0.05. Both of the standards showed good linearity in the range of 10–60 mg L−1 of gallic acid and 40–200 mg L−1 of quercetin, (R2 > 0.999). Based on the micro-titer plate assay, the limits of detection (LODs), and quantification (LOQs) for TPC ranged between 1.19 mg L−1 and 3.98 mg L−1 whereas for TFC, they were 1.47 mg L−1 and 4.90 mg L−1. The percent coefficient of variation (CV) of the intra-day and inter-day assays were lower than 1.46 and 1.91, respectively, showing adequate precision. The recovery ranged between 101.25 and 105.93 for the TPC assay and between 96.78 and 101.28 for the TFC assay. Twelve samples of medicinal extracts were analyzed by validated microplate assay for TPC and TFC and then compared with the conventional method. Notably, there were no significant differences in the TPC and TFC content detected between the microplate and conventional methods for all samples (p > 0.05). In conclusion, the microplate assay can be potentially used for TPC and TFC determinations.