Design, optimization and initial performance of a toroidal rf ion trap mass spectrometer

The design, optimization, and initial performance of a novel ion trap mass analyzer is reported. This analyzer geometry is based on the edge rotation of an ion trap cross-section into the shape of a torus. The advantages of this design are that for a given device analyzer radius (r ₀), significantly higher ion storage capacity than that obtained with a traditional three-dimensional quadrupole ion trap may be possible. Initial performance of this device was poor however, due to the significant contribution of additional nonlinear fields introduced by the rotation of the symmetrical ion trap cross-section. These nonlinear fields contributed to poor mass resolution and sensitivity as well as erratic ion ejection behavior. Using field analysis and ion trajectory simulation computer programs to guide the optimization, the geometry of this toroidal rf ion trap analyzer was modified in an attempt to correct for these nonlinear fields. These programs revealed that the original, symmetric toroid analyzer trapping field had a significant, sublinear component. Increasing the endcap separation and intentionally skewing the cross-sectional symmetry of the device minimized the field faults. Ion trajectory simulations indicated that the mass resolution and sensitivity of this asymmetric analyzer should be dramatically improved. Based on these results, an asymmetric version of the toroidal rf ion trap analyzer was constructed and this device has demonstrated unit mass resolution performance.