Influence of measurement wavelength on absorbance values. A. The OPD-solution at pH 5 shows the highest maximum absorbance at 450 nm (purple curve). The peroxidase reaction is stopped for end-point measurements by acidification (addition of stopping solution), which leads to a shift in the absorbance spectrum with an absorbance maximum at 490 nm (green curve). B. + C. Dilution series of OPD-reaction product in CP-buffer is displayed along the x-axis [log scale]. Concentration dependent differences in the absorbance values measured at 450 nm and 490 nm before (purple lines) and after the addition of stopping solution (green lines). B. Range of linear relationship between the measurements at the two wavelengths is highlighted by coloured backgrounds. C. The relative difference between the measurements of the stopped solution [A450/A490] is ~40%, while it is ~46% for the non-stopped reaction [A490/A450].