The analysis of collision induced degradation reactions of ions in the gas phase (CA-mass spectrometry [']) has acquired increasing importance recently as a means for investigating ion structures. Ion cyclotron resonance (ICR) is preferably employed for such measurements, especially if ions of long lifetimes are the subject of interest. Otherwise double focusing mass spectrometers of Nier Johnson geometry are used in the so called DADI mode of operation ['], whereby the collision gas is admitted to the field free region between magnetic and electric sector. This mode of operation requires reversal of the instrument's conventional configuration, as the magnetic sector must precede the electric sector and therefore is not easily applicable to existing instrumentation. In addition under these conditions the principle of double focusing (mutual compensation of energy dispersion of electric and magnetic field) is not operative for ions generated in this region. The peaks appear broadened by collisional scattering and tend to overlap, often in a quite disturbing manner.We would like to point out that collision induced degradations can be investigated with double focusing mass spectrometers in their usual configuration (the electric sector preceding the magnetic sector) while simultaneously avoiding the above mentioned drawbacks of the DADI method. If the collision gas is admitted to or just behind the ion source, the products formed by collisional processes in the first field free region can be analyzed by the newly established "linked scan t e~h n i q u e "~~] .The operating principle of this method consists essentially of giving daughter ions m i , m i ... arising from a freely selectable parent ion m: so much additional kinetic energy (velocity) that they acquire the same momentum as their precursor m:. Having the same momentum they are recorded at the same place as m: at the collector site without altering the magnetic field. The required additional kinetic energy is imparted by increasing the accelerating voltage I.: Thereby the ion trajectory in the electric sector is stretched and in order to focus the beam on to the intermediate slit the electric sector voltage E mus* changed simultaneously ("linked) in such a way that VEIVremains constant. A continuous linked scan of this sort produces a daughter ion spectrum of m: much like a DADI scan, save that now the principle of double focusing is operative. The signals are sharp and d o not overlap. The measurement takes place earlier with respect to formation of the investigated ions than in the DADI configuration, reducing the probability of their isomerization before collision.We tested the method by examining the ion C12HloO+ (m/e 170) formed from diphenyl carbonate by decarboxylation under electron impact. the DADI-CA spectrum of this product and compared it with the spectra of diphenyl ether and the isomeric phenylphenols which were determined under identical conditions.

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