Trapped charge can lead to a source of error when making low level Photon Transfer ("PTC") measurements. Residual signal can corrupt low level dark and light-on measurements leading to incorrect signal measurements when performing frame differencing required for the data reduction in the PTC procedure. To quantify the effect of trapping, the trap characteristics of capacity and decay rate versus temperature were studied for a commercially produced full frame CCD. Photon Transfer noise analysis was used to measure trap capacity, trap leakage rate, dark shot noise and thermal generation of dark signal as a function of cooling.