RF-only quadrupole

A time-of-flight (TOF) mass spectrometer, one of the simplest mass-analyzing devices, is currently in high demand. In combination with matrix-assisted laser desorption/ionization (MALDI), it has emerged as a mainstream technique for the analysis of biomolecules. For further reading, several reviews [11-16] and the book cited at the end of the chapter may be consnlted. 

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The triple quadrupole instrument consists of two mass analyzers separated by an rf-only quadrupole. In the rf-only mode, ions of all masses are... 

One of the most common couplings of analyzers for tandem MS is the triple quadrupole. Its notation is QqQ, where the capital Q indicates a quadrupole analyzer and the lowercase q is for a RF-only quadrupole that constitutes the collision cell. 

Lock, C.M. Dyer, E. Simulation of Ion Trajectories Through a High Pressure RF-only Quadrupole Collision Cell by SIMION 6.0. Rapid Commun. Mass Spectrom. 1999., 75,422-431. 

Yost, R.A. Enke, C.G. McGilvery, D.C. Smidi, D. Morrison, J.D. High Ef-lici cy Collision-Induced Dissociation in an Rf-Only Quadrupole. Int. J. Mass Spectrom. Ion Phys. 1979, 50, 127-136. 

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... 

Figure 2.10 shows the general diagram of an instrument with three quadrupoles [8], Quadrupole mass spectrometers are symbolized by upper case Q, and RF-only quadrupoles with a lower case q. A collision gas can be introduced into the central quadrupole at a pressure such that an ion entering the quadrupole undergoes one or several collisions. 

B. A. Mansoori, E.W. Dyer, C.M. Lock, K. Bateman, R.K. Boyd, B.A. Thomson, Analytical performance of a high-pressure RF-only quadrupole collision cell with a axial field applied by using conical rods, J. Am. Soc. Mass Spectrom., 9 (1998) 775. 

Since ions analysed with a quadrupole instrument have low translational kinetic energies, it is possible for them to undergo bimolecular reactions with species inside an RF-only quadrupole. These bimolecular reactions are often useful for the structural characterization of isomeric species. An example of this is the work of Harrison and co-workers [17]. They probed the reactions of CH.iNHTions with isomeric butenes and... 

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.

The quadrupole is scanned with the U/V ratio kept constant, i.e., along the scan line of Figure 1.18, which allows the successive separation of different masses by sequentially bringing each of them into stable trajectories through the rods. If U = 0 (no EXT), all ions above a certain mass limit (determined by V) have stable trajectories through the quadrupole such an rf-only quadrupole is therefore ideally suitable as a collision cell in a quadrupole-based tandem mass spectrometer (vide infra). ... 

Quadrupole-Orthogonal Acceleration TOF Instrument As of today, quadrupole (Q)-orthogonal acceleration (oa) TOF (oa-TOF) instrument is the most popular hybrid instrument [58,59] a simphstic pictorial representation is shown in Figure 4.11. The quadrupole section consists of a normal massresolving quadrupole and an rf-only quadrupole. The latter serves as a collision cell and as an ion-accumulation device. For precursor-ion scan, the ions desired are mass-selected by the main quadrupole, accumulated in the collision cell, and a packet of the CID product ions is pushed into the TOF analyzer. The precursor-ion selection by the quadrupole is at a medium resolution, but the product-ion analysis by the TOF section is at a reasonably high resolution. 

The precursor is mass-selected by adjusting the field of the mass-resolving quadrupole, collisionally activated in the rf-only quadrupole, and the CID products are mass-analyzed with the TOP mass analyzer. For precursor-ion spectrum, read page 140. 

The first mass-resolving quadrupole (Qi) is adjusted to transmit the chosen precursor, which is collisionally activated in the rf-only quadrupole, and the second mass-resolving quadrupole (Q3) is set to monitor the desired product ion. 

Modes of operation In Figure 2, Qo is an rf-only quadrupole in which the ion beam is focused colli-sionally to enhance the transmission of ions from Qo to Qi, an rf/dc quadrupole mass filter. Precursor ions are mass selected in Qi and transmitted to Qc where they are confined and subjected to resonant dissociation the product ions are ejected axially and mass-selectively. In an MS/MS experiment, where... 

A schematic of the Thermo Scientific MALDI source [124] is shown in Figure 2.24b. The MALDI source region includes a movable sample plate ( 1) with a 1pm positioning accuracy and approximately 12 x 8cm free travel to access sample plate sizes of the standardized microtiter plate format, a RF-only quadrupole with auxiliary DC resistive rods (DC aux) mounted parallel to and in the gaps between the RF rods ( 3). The skimmer ( 4) separates the source region held at about 0.1 mbar from the next pressure region. On the front side, a 4 mm aperture ( 2) is placed equidistant from both the RF rods assembly and the sample plate. 

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). 

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