The effect of methanol used as sacrificial agent and its by-products, namely formaldehyde and formic acid, was evaluated by constructing calibration curves. Accordingly, standard solutions were prepared in presence of MeOH in the range 1 ÷ 20 % v/v, HCHO in the range 0.01 mg L - 1 ÷ 10 and formic acid 0.001 ÷ 0.1 % v/v, respectively. The slope of each curve was compared with the one obtained in ultrapure water, achieving not significantly different values (p < 0.05).