Mass-to-charge limit

A mass spectrometer consists of four basic parts a sample inlet system, an ion source, a means of separating ions according to the mass-to-charge ratios, ie, a mass analyzer, and an ion detection system. AdditionaUy, modem instmments are usuaUy suppUed with a data system for instmment control, data acquisition, and data processing. Only a limited number of combinations of these four parts are compatible and thus available commercially (Table 1). 

Ion traps are favored for proteomics studies because of their ability to perform multistage mass analysis (MSn), thereby increasing the structural information obtained from molecules. Ion traps, however, do not provide information for ions that have lower mass-to-charge values (the one-third rule). Additionally, the sensitivity of ion traps can also be limiting because only about 50% of the ions within a trap are ejected to the detector. Ion traps are also subject to a space charging phenomenon that may occur when the concentration of ions in the trap is high and produces ion repulsion within the trap. Nevertheless, the versatility and robustness of ion trap MS underlies its popularity for several proteomics-related applications. 

MS involves the separation of ions based on their mass-to-charge ratio (m/z). The concept was invented a century ago1 with a dramatic impact on analytical chemistry.2-3 The fundamental principle of MS requires vaporization of the molecules in the gas phase and in ionization. Early ionization methods such as electron impact (El) and chemical ionization (Cl)4-5 were limited to small organic molecules that were volatile and stable to heat and amenable to transfer into high vacuum. Introduction of the fast-atom-bombardment (FAB) method of ionization6... 

A/Q = 3 (tritons), those species with mass-to-charge ratios of 2 or less were reduced by more than 2 orders of magnitude. (Scattered background events limited the measurement of this reduction factor.) This mass-to-charge filtering is particularly important in our future experiments because the protons, deuterons, and alpha particles are produced with yields that are, respectively, 105, 104, and 103 times larger than those of the ions for which measurements are planned. 

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