Method and apparatus for detecting a photochemically active chemical species in a sample

A method for detecting a photochemically active chemical species in a sample, comprising the steps of:

• a) illuminating the sample(S), according to an illumination sequence, with light (LB1) of at least a first wavelength (λ1) suitable to be absorbed by the chemical species triggering a first reaction (R1) affecting at least one optical property of the chemical species and comprising at least one photochemically-activated step (PAS1) and one thermally-activated step (TAS1), wherein the illumination sequence is such that:
• in at least a first time window (TW1, TW1′) of the illumination sequence, a rate of the first reaction is limited by the photochemically-activated step;
• and in at least a second time window (TW3, TW2′) of the illumination sequence, the rate of the first reaction is limited by the thermally-activated step;
• b) measuring the evolution of the at least one optical property of the chemical species during the first and the second time windows;
• c) determining, from said measuring, at least a first and a second time constants representing rate constants of the first reaction in the first and the second time windows, respectively;
• and
• d) using the determined at least the first and second time constants for detecting the chemical species.