Measuring cell for ion cyclotron resonance spectrometer

A method of trapping and exciting ions in a measurement cell of an FTMS spectrometer, the method comprising:

• (a) applying a magnetic field to the measurement cell so as to produce a region of homogeneous magnetic field, having a magnetic field direction, within the cell;
• (b) applying a d.c. trapping potential to a trapping electrode arrangement positioned about a longitudinal axis which extends in a direction generally parallel to that magnetic field direction, so as to trap ions in the cell, in that axial direction within a trapping region defined by the trapping electrode arrangement, the trapping electrode arrangement including first and second trapping electrodes;
• and
• (c) applying an r.f. excitation potential to an excitation electrode arrangement positioned about that longitudinal axis, so as to resonantly excite the ions in the cell, at least a part of the excitation electrode arrangement extending axially outwardly of the trapping region defined by the trapping electrode arrangement, the excitation electrode arrangement including a central excitation electrode part and first and second outer excitation electrode parts axially spaced from the central excitation electrode part, each of the trapping electrodes being interposed between the central excitation electrode part and a corresponding outer excitation electrode part;
• wherein the ions are trapped within the region of homogeneous magnetic field and wherein the ions are further trapped within a homogeneous region of an excitation electric field generated by the application of the r.f. excitation potential to the said excitation electrodes.

A Fourier Transform mass spectrometer, comprising:

• an ion source for generating ions;
• and
• at least one ion guide for transporting the ions to a measurement cell, the measurement cell including:
• an excitation electrode arrangement positioned about a longitudinal axis which extends in a direction generally parallel to the field direction of an applied homogeneous magnetic field, the excitation electrode arrangement including a central excitation electrode part and first and second outer excitation electrode parts axially spaced from the central excitation electrode part;
• a trapping electrode arrangement, also positioned about the longitudinal axis, for trapping ions longitudinally in the cell within a trapping region defined by the trapping electrode arrangement, the trapping electrode arrangement including first and second trapping electrodes located axially between the central excitation electrode part and the first and second outer excitation electrode parts respectively;
• and
• a detection electrode arrangement for detecting ions trapped within the trapping region;
• wherein at least a part of the excitation electrode arrangement extends axially outwardly of the trapping region defined by the trapping electrode arrangement.