ANALYSIS OF ISOMERIC MIXTURES USING BLACKBODY INFRARED RADIATIVE DISSOCIATION - DETERMINING ISOMERIC PURITY AND OBTAINING INDIVIDUAL TANDEMMASS-SPECTRA SIMULTANEOUSLY

A new method that makes possible, for the first time, simultaneous acq uisition of individual dissociation mass spectra of isomeric ions in m ixtures is presented. This method exploits the exquisite sensitivity o f blackbody infrared radiative dissociation kinetics to minor differen ces in ion structure. Instead of separating precursor ions based on ma ss (isomers have identical mass), fragment ions are related to their o riginal precursor ions on the basis of rate constants for dissociation . Mixtures of the peptide isomers des-R-1 and des-R-9 bradykinin are d issociated simultaneously at several temperatures. By fitting the kine tic data to double-exponential functions, the dissociation rate consta nt and abundance of each isomer in the mixture are obtained. To overco me the difficulty of fitting double-exponential functions, a novel glo bal analysis method is used in which several dissociation data sets co llected at different temperatures are simultaneously fit. The kinetic data measured at multiple temperatures are modeled with the preexponen tials (corresponding to the abundance of each isomer) as ''global'' pa rameters which are constant for all data sets and the exponentials (ra te constants) as ''local'' variables which differ for each data set. T he use of global parameters significantly improves the accuracy with w hich abundances and dissociation rate constants of each individual com pound can be obtained from the mixture data. Fragment ions produced fr om a mixture of these two isomers are related back to their respective precursor ions from the kinetic data. Thus, not only can the composit ion of the isomeric mixture be determined but an individual tandem mas s spectrum of each component in the mixture can be obtained.