Secondary Fragmentation of Benzylpyridinium Thermometer Ions Bearing a 4-Trifluoromethyl Group Extends the Internal Energy Calibration Range of Gas-Phase Ions

Secondary fragmentation of substituted benzylpyridinium “thermometer ions” has been observed previously, but they have not been used as quantitative probes for internal energy calibration. Here, we demonstrate that the secondary fragmentation of 4-trifluoromethyl-substituted benzylpyridiniums provides a high-energy survival-yield probe that extends the measurable internal energy range in collision-induced dissociation (CID). CID of 4-trifluoromethyl-substituted benzylpyridiniums proceeds by N–C bond cleavage to form the corresponding benzyl cation, followed by secondary CF2 loss. This secondary fragmentation can be incorporated into internal energy calibration, provided that the effective dissociation energy for the secondary fragmentation is assigned by considering how excess energy is partitioned during the primary N–C bond cleavage. Using the secondary-fragmentation survival yield extended the CID calibration range from 30 to 60 eV, revealing that internal energy distributions shift toward higher energies and broaden with increasing collision energy. This approach also provide a quantitative probe of high-energy regions inaccessible through primary fragmentation alone.