Charge capacity limitations of radio frequency ion guides in their use for improved ion accumulation and trapping in mass spectrometry.

The use of radio frequency (rf) ion guides as "linear" two-dimensional ion traps and ion guides for ion storage and accumulation, respectively, is becoming increasingly important for realizing improved sensitivity in mass spectrometry. Analytical relationships describing the ion accumulation operation mode of rf ion guides are reported. Comparisons are made between the rf quadrupole ion guide, higher-order rf multipoles and rf stacked ring ion guides, in terms of the charge capacity limitations due to the instability of ions, rf focusing efficiency limits, and effects due to rf ion heating (i.e., collisional activation due to rf oscillations of ions). Analytical relations for the stored charge quantity are derived in the low ion energy approximation, which is shown to be reasonable for the systems considered. The ion density spatial distribution is derived, an exponential form of which proved to provide a good approximation for high-order rf multipoles and stacked ring rf ion guides. The limit on the stored charge dependence upon rf is shown to be directly related to the thermal dissociation thresholds for the ions being studied; the limitation is weaker for higher-order multipoles and stacked ring ion guides. These results suggest that rf quadrupoles provide an optimum configuration when accumulation of a moderate ion density is sufficient (below 10(9) elementary charges/m). Alternatively, accumulation of an appreciable density for more fragile species, such as noncovalent complexes, may be realized using higher-order multipoles and stacked ring ion guides.