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Investigations on ion injection into a laboratory‐built quadrupole/ion trap tandem instrument (Nermag R10‐10/ITMS Finnigan) with a 90° geometry are described. The quadrupole dual electron impact/chemical ionization (EI/CI) source was used as an external source to prepare ions under EI (or CI) conditions. They were selected by the quadrupole analyzer and injected into the ion trap by using an electrostatic quadrupolar deflector. The effects of major trapping parameters (i.e. ion injection r.f. level and helium pressure) were considered. The injection efficiency of the stable CF3+ (m/z 69) fragment ion from FC43 and also those of the m /z 40 and 84 atomic cations of argon and krypton depend upon the injection r.f. level conditions. As expected, effects of non‐linear resonances were observed during these experiments. Under certain injection and trapping conditions, ion motion can be strongly affected, yielding ion ejection, because non‐linear fields (resulting from device imperfections) are present in addition to the quadrupolar potential required for ion trapping. The influences of non‐linear effects according to their strength were investigated to show the best and worst trapping conditions. The non‐linear resonances are characterized by the expected β z values. In order to determine the average internal energy deposited on the selected polyatomic ions during injection, butylbenzene molecular ions were chosen as a model for internal energy measurement. The effects of the r.f. injection level and helium pressure on the injected ion fragmentation were studied. The M +• ions were only observed through a narrow injection qz window. At low qz values, only low critical energy m/z 92 ions were detected, while the higher critical energy m/z 91 ions were observed at higher qz values. Copyright © 1999 John Wiley & Sons, Ltd.

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