Charge capacity limitations of radio frequency ion guides in their use forimproved 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 bec oming increasingly important for realizing improved sensitivity in mass spe ctrometry, Analytical relationships describing the ion accumulation operati on 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 gu ides, 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 rela tions for the stored charge quantity are derived in the low ion energy appr oximation, which is shown to be reasonable for the systems considered, The ion density spatial distribution is derived, an exponential form of which p roved to provide a good approximation for high-order rf multipoles and stac ked ring rf ion guides. The limit on the stored charge dependence upon rf i s shown to be directly related to the thermal dissociation thresholds for t he ions being studied; the limitation is weaker for higher-order multipoles and stacked ring ion guides. These results suggest that rf quadrupoles pro vide an optimum configuration when accumulation of a moderate ion density i s sufficient (below 10(9) elementary charges/m), Alternatively, accumulatio n of an appreciable density for more fragile species, such as noncovalent c omplexes, may be realized using higher-order multipoles and stacked ring io n guides.