QMS signals of the m/z 18 u (associated with H₂O photodesorption, in dark red) and 46 (associated with the photodesorption of HCOOH, in dark blue) monitored during the irradiation of the mixed ices at fixed energy. The top panels display the signals from the ¹³CO:HCOOH (1:1 and 10:1) ices and the bottom panels display the signals from the H₂O:HCOOH (1:1 and 10:1) ices. The photon fluence received by the ice before the measurements was ~1 × 10¹⁶ photons cm⁻². The signals are scaled for more clarity.