Experimental calibration of the SORI-CID internal energy scale: energy uptake and loss

This paper describes a novel method to experimentally determine the net amount of internal energy deposited into biomolecules during sustained off-resonance irradiation collision-induced dissociation (SORI-CID). The method of calibration is based on a controlled manipulation of the initial internal energy of a trapped ion population prior to dissociation, A decrease in the initial internal energy will lead to an increase of the amount of internal energy needed to reach the same degree of dissociation. The number of SORI cycles needed to reach 50% dissociation for different initial internal energies has been determined for leucine enkephalin. The number of SORI cycles is proportional to the amount of the internal energy accumulated. The ratio between the change in internal energy and the change in the number of SORI cycles (needed to reach 50% dissociation) hence yields the net amount of internal energy deposited per SORI cycle. This methodology was applied to ion populations at room temperature and at temperatures down to 143 K. The latter temperatures were reached in a novel liquid nitrogen cooled ICR cell. The calibration of the SORI internal energy scale also revealed that at low environmental temperatures the amount of internal energy loss from an activated ion population is strongly increased. With this novel methodology the net internal energy loss during SORI was quantified, and it is argued that the main loss mechanism is the emission of IR photons. (C) 2002 Elsevier Science B.V. All rights reserved.