Your estimation is possibly right.

You can see absorption specta of the phenol red here (Fig. 7),

https://www.researchgate.net/publication/221925346_Plastic_Optical_Fiber_pH_Sensor_Using_a_Sol-Gel_Sensing_Matrix/figures

and you can see emission specra of several scintillators here (Fig.2, blue lines)

https://www.researchgate.net/publication/224332233_Optical_and_Scintillation_Properties_of_Inorganic_Scintillators_in_High_Energy_Physics/figures

If there is overlap between the emission spectrum of the scintillator and the absorption spectrum of the phenol red, the scintillation photons are absorbed by the pigment.

What is the scintillator used in your system? Most scintillators overlap.

Dear Seba, first of all, why is there phenol red in your scintillation sample? In general when using sintillation counting, one should try to have all samples and standards have about the same chemical composition. Apart from light absorption, such as by phenol red, one could also get chemiluminescence eg when strong acids are present or certain cleaning agents. I had this problem when assaying swab tests for 3H contamination. In short, take care to have all scintillation samples with a comparable chemical background.

I hope this answer is helpful. Kind regards, Harrie Verhoeven

Easy, to test. Just use somethng you want to count and then add phenol portion by portion and recount each time.

Till