SGMS Meeting 2000

Varying the RF frequency: a new scanning mode for quadrupole ion traps

Christian Rolando, Gérard Simonneau, Sébastien Valette
Université des sciences et technologies de Lille (Lille 1)
UFR de Chimie, Bâtiment C4
UPRESA 8009 du CNRS, Chimie Organique et Macromoléculaire
59655 Villeneuve d'Ascq Cedex, France

We showed previously that quadrupole analyzers can be scanned by varying the RF frequency at constant amplitude. A new mass-selective instability scanning mode for quadrupole ion traps has been developed using the same method. In agreement with the theoretical equations of ion motion in a tridimensionnal quadrupolar field all classical scanning method obtained by varying the voltage on electrodes at constant RF frequency can be transposed by varying the RF frequency keeping constant the amplitude. Preliminary results using a RF frequency scanning range from 0.2 MHz to 0.6 MHz and an ac amplitude up to approximately 1000 V peak-to-peak will be reported for the mass-selective instability mode. All the experiments have been performed on a modified ITD 700 (Finnigan, San Jose, USA). The original software and electronics has been used for instrument control and data acquisition. The RF coil and HF amplifier have been removed and replaced by a home made HF power supply and a large band transformer which gave a flat voltage in the scanning range described previously. The annular and cap electrodes have been isolated in order to add an axial modulation or a DC component. The typical ion trap cycle is performed according to the following steps: during the ionization period the frequency is at a high value, then it is decreased to the value corresponding to the low mass of the scan range and then scanned down to a lower value corresponding to the high mass of the scan. Studies on dibromomethane showed that as expected the obtained resolution is proportional to the value of the RF voltage. However by adding an axial modulation a unit resolution can be reached at a few hundred volts which gave access to an extended mass range, allowing to record the full spectrum of perfluoroheptyltriazine. The best ratio axial modulation to main radio frequency appeared correlated with the RF frequency. The classical 0.5 value is obtained at high frequency (ca 0.5 MHz) whereas a smaller value is obtained at lower frequency. The influence of other classical parameters (added DC voltage, scanning time) will be also presented.

Reference

Landais B., Beaugrand C., Capron-Dukan L., Rolando C., Simonneau G. Rapid. Comm. Mass Spectrom., 12, 302-306 (1998).