Flow injection analysis of DPPH radical based on electron spin resonance

In order to construct a rapid and selective determination system of free radicals, we developed an FIA system using an electron spin resonance (ESR) spectrophotometer (flow injection spin analysis) equipped with a flow-through flat cell. In the present investigation, 1,1-diphenyl-2-picrylhydrazyl (DPPH) was used as a model free radical. Using a single line flow system, 0.5 mM DPPH was repetitively injected. When the magnetic field was fixed at 335.3 mT, the largest change in the ESR signal was observed and obtained peak height was proportional to the concentration of DPPH radical. A double line flow system was constructed in which a carrier stream containing 0.15 mM DPPH was fed into the flat cell after confluence with a sample stream. When ascorbic acid was injected as a typical DPPH radical scavenger, a negative peak appeared in proportional to the concentration. Lower detection limit of ascorbic acid was 0.01 mM (S/N = 4), sampling frequency was 13 samples per h, and a satisfactory reproducibility (CV = 3.2%, 0.1 mM, n = 5) was obtained. The present system was also applied to estimate the DPPH radical-scavenging activity of other substances and food samples. (C) 2002 Elsevier Science B.V. All rights reserved.