Fig. 1.23 Standalone linear ion trap. Because the ions are ejected radially two detectors are required for best sensitivity. Adapted with permission from reference .
Very recently linear ion traps (LIT) or two-dimensional ion traps (2D IT) have gained interest for various applications, either as standalone mass analyzers or coupled with Fourier transform ion cyclotron, three-dimensional ion trap (3D IT), TOF or orbitrap mass analyzers . Physically, a linear ion trap is like a quadrupole formed by four hyperbolic or circular rods placed symmetrically. In a linear ion trap the ions are confined radially by a two-dimensional radio frequency field. To prevent ions from escaping axially, a DC potential is applied to the end electrodes. The same type of experiments which can be performed on 2D or 3D ion traps are basically the same but there are several advantages to trap ions in a 2D trap compared to 3D traps: (i) no quadrupole field along the z-axis, (ii) enhanced trapping efficiencies, (iii) more ions can be stored before observing space charging effects and (iv) strong focusing along the center line instead of focusing ions to a point.
Schwartz et al.  described a standalone linear ion trap where mass analysis is performed by ejecting the ions radially through slits of the rods using the mass instability mode. To maximize sensitivity the detection is performed by two detectors placed axially on either side of the rods (see Fig. 1.23).
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