2D traps

LIT The linear ion trap (LIT) (also referred to as a two-dimensional, or 2D, trap) is a variation on the transmission quadrupole mass analyzer. In the LIT, the quadmpole is constructed such that either ions can be analyzed immediately or, ions can be trapped and held in the quadrupole region and then analyzed (Hager, 2002 Schwartz et al., 2002). Various types of MS/MS can be performed, as described in Chapter 3. 

The commercially available stand-alone LITs, marketed under the name LTQ, are made of four hyperbolic cross-sectional rods (Fig. 1.25). Since ions are trapped in an axial mode as opposed to central trapping on 3D ion traps, LTQs have been successfully coupled with Orbitrap and FTICR for achieving high-resolution capabilities (Peterman et al., 2005 Sanders et al., 2006) (Chapter 5). Functional improvements in 2D traps over 3D traps include 15 times increase in ion storage capacity, 3 times faster scanning, and over 50% improvement in detection efficiency and trapping efficiency. 

When used as a linear ion trap (2D trap) many of the same techniqnes nsed with Paul traps can be used with only minor modifications. Trapping of ions with selected mJz values can be achieved, as in 3D traps, by resonant excitation methods or hy methods that exploit the boundaries of the stahihty diagram. Selective ion trapping is necessary if the hap is to be used also to create fragment ions by in-hap collision induced dissociation for subsequent analysis by a Panl hap or by FTICR or time of flight analyzers. Also, particnlarly in the case of downsheam trapping analyzers, excess unwanted ions that cause space charge problems can be selectively ejected. Resonant excitation at the ions secnlar frequencies (Eqnation [6.33])... 

Figure 6.30 Sketch of a triple quadrupole instrument adapted to demonstrate use of a linear quadrupole as a 2D trap mass spectrometer with axial ejection. Qi was operated as a conventional linear quadrupole miz filter. Either the pressurized (> 0.1 mtorr) RF-only quadrupole q2/x or the evacuated ( 10 torr) Q3/X could be used in trapping mode. The RF-only quadrupole q2/x could be operated as a conventional collision cell, and Q3/X could also be used as a conventional RF-DC quadrupole m z analyzer. The ion guide quadrupole qo is 12 cm long and is maintained at 7 mtorr. All of Qj, q2/x and Q3/X are 127 mm long with ro = 4.17 mm, and the RF drive frequency was 1 MHz. IQl - IQ3 are differential pumping apertures that can also act as ion lenses. Adapted from Hager, Rapid Commum. Mass Spectrom. 16, 512 (2002), with permission of John Wiley Sons, Ltd.

However, a truly surprising result was a remarkable insensitivity to space charge for mass spectra obtained by axial injection from Q3/2. in 2D-trap mass spectrometer mode. For example, a spectrum of the [M-i-H]+ ions of reserpine (miz 609) obtained using us a collision cell and in 2D-trap MS mode at an ion... 

The current pace of instrument development is such that these snmmarized comments will prohahly have to be rewritten sooner rather than later. The potential role of the 2D traps in quantitative analyses was already mentioned and very recently a modification to a QqTOF instrument that permits SIM/MRM operation with high duty cycle has been described the higher product ion resolution could be useful in bioanalytical and similar assays where isobaric interferences from unknown metabolites could be a factor. However, it seems likely that, for the apphcations of interest in this book, the triple quadrupole instrument will remain the workhorse instrument of choice for the foreseeable future. The recent introduction of a triple quadrupole design in which Qj is capable of enhanced RP (peak widths as low as 0.1-0.2Th) permits increased selectivity via precursor ion selection. 

Qn/r- notation conventionally used to denote a linear quadrupole device that can be used either as a conventional m/z filter or as a 2D trap mass spectrometer. 

The ion detectors used in practice are diseussed in more detail later in this chapter but are introduced in general terms here. All analyzers based on quadmpolar electric fields (including 3D and 2D traps) use electron multiplier detectors, usually channel electron multiphers (CEMs) but in some cases an external photomultipher connected optically to a phosphor target. Magnetic sector instruments also use electron multipliers, including CEMs, photomultipliers and the discrete dynode type some... 

Quadnipole fundamentals that can be applied to 3D- and 2D-quadmpole field ion traps can be found in the book by Dawson. We will use the acronym XQIT in this text, where X is a variable that refers to all quadrupole field ion traps whether they have 2D- or 3D-quadrupole fields. For the 2D traps, X will be replaced by L for linear, RL for rectilinear, T for toroidal, and C for curved, allowing for a nomenclature system for future geometries. 

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