RF-only hexapoles

An RF-only hexapole is used in systems from Waters (Figure 5.7). In more recent Waters systems, it is replaced by an ion-tunnel device (see below). 

Waters introdnced a stacked-ring RF ion-transmission device (the MassTransit ion tutmel), replacing the RF-only hexapoles [43]. The device consists of a series of constant-apertnre and eqttally-spaced ring electrodes. An RF voltage is applied with 180° phase shift to adjacent plates, generating a field that constrains the ions to the centre region of the device. 60-80% improved ion transmission was observed. 

It should also be noted that the RF-only quadrupoles (as well as the related RF-only hexapoles and octapoles) can be operated as ion lenses, often referred to as RF-only ion guides or simply ion guides. RF-only quadrupoles (denoted as lower case q , nonitalicized, to distinguish them from the closely related quadrupole mass filters Q) also act as efficient collision cells for collision induced dissociation in triple quadrupole analyzers (QqQ, Section 6.4.3) and hybrid tandem quadrupole-time of flight instruments (QqTOF, Section 6.4.7). Discussion of these devices must be postponed imtil the principles of all hnear quadrupoles have been considered (Section 6.4.2). 

For NSD it is not required to have a true nozzle-skimmer arrangement. Instead, a potential drop of 20-100 V across a suitable pressure region of any ESI interface design suffices to induce this sort of CID. The Bruker Apollo II ESI source, for example, features a dual ion funnel setup with the first ion funnel in the rough vacuum of the supersonic expansion plume ( 3-4 mbar) and the second ion funnel prior to an RF-only hexapole used to alternatively guide or accumulate ions. 

Common tandem-in-space instruments employ a quadrupole as the first mass analyzer, a multipole collision cell (usually hexapole) operated in RF-only mode, and then either a second quadrupole or a TOF tube as the second mass analyzer. These instruments are termed triple or tandem quadrupole and quadrupole-time-of-flight mass spectrometers. 

Turner et al. [114] described an ICP-MS with a hexapole transfer lens (Fig. 3.12a) in a tube that allows the pressure to be maintained. The cell was initially reported to contain He, although it is likely that H2 or H20 vapor was also in the cell and is now purposely added to the cell [115]. Ar2+, ArCl+, ArO+, and Ar+ signals were reduced relative to Se+, As+, Fe+, and Ca+, respectively. Detection limits for Fe, Ca, K, Se, and As near 10 parts per trillion (ppt) have been reported [115]. Recently, Beaty and Liezers [116] also described a collision-reaction cell at a pressure of 30 mtorr that reduced the ion kinetic energy spread as well as continuum ICP-MS background to less than 1 count/sec. Previously, Douglas and French had described the use of an rf-only quadrupole for collisional focusing and reduction of the ion kinetic energy spread [117]. 

Collision-Reaction Cells. Perhaps the most exciting development regarding molecular ion removal in ICP-MS is the use of collision or reaction cells, introduced in Sec. 3.1.5. These rf-only quadrupole, hexapole, or octapole cells, typically operated at pressures around 10 mtorr, can provide two benefits The collision cell can be used to reduce the ion kinetic energy and to focus ions toward... 

Another important application of a linear quadrapole is operation in the RF-only mode, i.e., where the DC voltage is zero. In this mode, all ions with m/z between a low-mass and high-mass cut-off are transmitted. RF-only quadrapoles (or hexapole or octapole) are used as an ion guide between API sources and mass analysers (Ch. 5.4.5) and as collision cells in various MS-MS instruments. 

Figure 11 Schematic representation of the home-built ESI-qQq-FTMS with a 7-Tesla actively shielded magnet, used to generate some of the data shown in Fig. 17 (27). QO, rf only quadrupole Ql, resolving quadrupole Q2, colhsion cell HEXl and HEX2, //-only transfer hexapoles ICR, ion cyclotron resonance ceU.

Fig. 2.4 Triple quadrupole MS, QQQ, where Q1 and Q3 indicate scanning quadrupoles and Q2 indicates the collision cell consisting of either an RF-only quadrupole, hexapole, or octapole. In some hybrid instruments Q3 is replaced by either a TOF or a linear quadrupole ion trap. (See text for more details and references for further reading)...

RF-only quadrupole, hexapole, or octopole collision cells [96,104,105] are part of so-called triple quadrupole mass spectrometers, which essentially represent QqQ, QhQ, or QoQ instruments, respectively, depending on the type of RF-only collision cell actually in place. They are efficient tools for tandem mass spectrometry (Chap. 9). 

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