Effects of electrode geometry on quadrupole ion trap performance

Enhancement in the performance and applicability of quadrupole ion trap mass spectrometers is sought through variation of the physical structure of the device. Improvements in mass measurement accuracy are pursued through the study of chemical mass shifts, i.e. large, compound-dependent inaccuracies known to occur in traps with non-optimized geometry and believed to be related to ion structure. Experiments and simulations are used to elucidate the cause these shifts, and they indicate that the chemical mass shift is the result of two interacting factors: (i) delayed ion ejection from the trap during mass analysis due to field imperfections associated with the end-cap electrode apertures; (ii) the compound-dependent modification of this delay by collisions with the bath gas. The shifts are removed by appropriate manipulation of the ion trap geometry, by careful choice of resonance ejection working point, or by performing the mass analysis scan in the reverse direction. Use of the chemical mass shift for isomer distinction is demonstrated. The performance of cylindrical ion traps (CITs) operated in the mass-selective instability mode is characterized to test the suitability of this simplified geometry for miniature instrument applications. The performance of the device is optimized by experimental variation of the electrode geometry guided by electric field calculations, and performance comparable to standard hyperbolic ion traps is achieved. Nondestructive, image current detection of ions in a CIT is also reported