Optical evaluation of antioxidant activity: A comparison between a spectrophotometer and a homebuilt photometer

¹ Mahidol University International Demonstration School, Salaya, Phuttamonthon, 73170 Nakhon Pathom, Thailand
² Department of Physics, Kasetsart University, Jatujak, 10900 Bangkok, Thailand
³ Biomedical Engineering program, Department of Industrial Physics and Medical Instrumentation, Lasers and Optics Research Center, King Mongkut’s University of Technology North Bangkok, Wongsawang, Bangsue, 10800 Bangkok, Thailand

^* Corresponding author: thidarat.w@sci.kmutnb.ac.th

Abstract

The antioxidant activity of an orange peel extract is determined using a DPPH (2,2-diphenyl-1-picrylhydrazyl) solution. Sample absorption is measured with both a commercial spectrophotometer and a homebuilt photometer equipped with a 525-nm LED. Small, dried pieces of orange peel are extracted using 95% ethanol. Seven DPPH solutions with concentrations ranging from 0.1 to 0.4 mM are measured to establish calibration equations. For the spectrophotometer, absorbance exhibits a linear increase with DPPH concentration at both 517 nm (absorbance peak) and 525 nm (matching the LED in the photometer). In the homebuilt photometer, three piecewise linear fittings for concentration 0.1– 0.4 mM provide a better representation of absorption data compared to a single overall fitting across 0.1–0.4 mM. Different volumes of orange peel extract are added into the DPPH solution (0.4 mM DPPH after mixing) and stored in darkness for 30 minutes before the measurement. In both spectrophotometer and photometer analyses, antioxidant activity increases with the extract volume. Using spectrophotometer measurements as reference, the photometer with piecewise fittings produces significantly lower errors compared to those obtained with overall fitting.