The ferric reducing antioxidant power (FRAP) assay was recently adapted to a microplate format. How- ever, microplate-based FRAP (mFRAP) assays are affected by sample volume and composition. This work describes a calibration process for mFRAP assays which yields data free of volume effects. From the results, the molar absorptivity (e) for the mFRAP assay was 141,698M 1cm 1 for gallic acid, 49,328 M 1 cm 1 for ascorbic acid, and 21,606 M 1 cm 1 for ammonium ferrous sulphate. The signifi- cance of e (M 1 cm 1) is discussed in relation to mFRAP assay sensitivity, minimum detectable concen- tration, and the dimensionless FRAP-value. Gallic acid showed 6.6 mol of Fe2+ equivalents compared to 2.3 mol of Fe+2 equivalents for ascorbic acid. Application of the mFRAP assay to Manuka honey samples (rated 5+, 10+, 15+, and 18+ Unique Manuka Factor; UMF) showed that FRAP values (0.54–0.76 mmol Fe2+ per 100 g honey) were strongly correlated with UMF ratings (R2 = 0.977) and total phenols content (R2 = 0.982)whilst the UMF rating was correlated with the total phenols (R2 = 0.999). In conclusion, mFRAP assay results were successfully standardised to yield data corresponding to 1-cm spectrophotom-eter which is useful for quality assurance purposes. The antioxidant capacity of Manuka honey was found to be directly related to the UMF rating.
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