The kinetic energy that is released upon bond rupture is often represented as T_1/2. A value that is derived from the FWHM of a fragment peak by the use of two different conversion formulas. The choice of formula depends on whether the peak is recorded by scanning a magnetic sector or an electrostatic analyzer. We have conducted a systematic variation of the possible scan configurations of two different double focusing instruments in two countries. We have found that a double focusing mass spectrometer in normal geometry gives rise to T_1/2 values that are 1.4 times larger when analyzing peaks that are obtained from magnet scans compared to the peaks that are obtained by scanning an electrostatic analyzer. The E scans (MIKE experiments) give rise to the same values for both of the employed mass spectrometers. The results are explained in terms of energy defocusing when the reactions take place too far away from the focal points and show that only the E scans T_1/2 values can be compared from instrument to instrument.