INTENSE ARTIFACT PEAKS IN MIKE SPECTRA appropriate spectrometer region (e.g. acceleration region, FFR1, etc.) relative to those of the chosen MIKE precursor in FFR2. Such relative rates are functions not only of relative intensities (abundances) of the respective precursors, but also of the relative rate constants for fragmentation. This conclusion is nicely confirmed by a comparison of Fig. 1 with a MIKE spectrum obtained using a triple-analyzer instrument of EBE configuration (Kratos MS5O-TA). The double-focusing E B combination was used to select the MIKE precursor, and the spectrum obtained by scanning the second electric sector. The intensities of the artifact peaks are now very low relative to that due to formation of the monomer [(Lys-Lys)H]+. This reflects the much longer time required for the precursor ions [(LysL~s )~( H C I )~H ] + to reach the field-free region immediately preceding the magnet, where the artifact signals originate. In the case of the EBE instrument, most of these ions thus decompose before they reach this region, and the first electric sector filters out all the product ions thus formed in upstream regions (acceleration, FFR1).The present example thus confirms and extends the previous findings4-'' concerning pitfalls in MIKE spectrometry. Artifact peaks must always exist whenever ions with values of mlz higher than those of the chosen MIKE precursor are present in the ion beam extracted from the ion source. The question of their ibtensity relative to those of the genuine MIKE peaks is a complex one. The present example, however, illustrates that the problem is not trivial. Clearly, it is useful to have a third analyzer (e.g. an electric sector,'" or a quadrupole mass filter as used here) following the B E combination appropriate to MIKES.

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Aba t r a c t . ~~~~~~~~~Flight miiss spcctromctcrs;irc desc*ribd tliiit iwlricw, i n a s resolving powers of trz/:lm 3 lo3 to ;2 lo' by using electron-impact ionisiition. The most important parts in such systems arc an ion source in which ions ;ire prodi1c.d iind stored continilnitsly betnwn extraction piilses ;is well a s R grid-free ion mirror ivhicli enstires good I;iterul focusing iincl ii totcil flight tinit: t.lint is in&pendent of differencw in ion energies.

A new tandem mass spectrometer has been developed for the study of molecular fragmentation reactions. The first stage of this tandem mass spectrometer is a double-focusing sector field instrument in the BE configuration. The second stage is a double-focusing combination of a Wien filter and a sector magnet that allows simultaneous detection of a wide range of masses owing to the approximately equal velocities of the fragment ions. By using a microchannel plate as a collision target, high fragmentation effi.ciency and high ion transmission were achieved.