Quadrupole rods

This is probably the most widely used MS-MS instrument. The hardware, as the name snggests, consists of three sets of quadrupole rods in series (Figure 3.8). The second set of rods is not used as a mass separation device but as a collision cell, where fragmentation of ions transmitted by the first set of quadrupole rods is carried out, and as a device for focussing any product ions into the third set of quadrupole rods. Both sets of rods may be controlled to allow the transmission of ions of a single mjz ratio or a range of mjz values to give the desired analytical information. 

Triple quadrupole A mass spectrometer consisting of three sets of quadrupole rods in series, which is used extensively for studies involving MS-MS. 

The quadrupole mass analyzer utilized by Q-ICPMS is comprised of quadrupole rods with combined DC and RF potentials that can be set to allow analyte ions with a specific... 

Typical corrective measures for poor performance are resilylation, replacing injector glass insert, breaking off a few centimeters at the front end of the column, and replacing column. Serious loss of sensitivity may require cleaning the source, replacing the filaments, or cleaning the quadrupole rods. 

One might think that quadrupole mass spectrometers would be less susceptible to this limitation because of their much larger entrance apertures. Unfortunately, this is only marginally true, and only for ions of low mass. This is because the electrical entrance aperture of the quadrupole Is usually considerably smaller than the physical entrance aperture. By "electrical entrance aperture" we mean that area of the physical entrance aperature through which ions must travel if they are to be transmitted through the quadrupole rod structure. The radius of this aperture, r, is small and mass-dependent as seen by the following equation... 

The ions accelerated along the z axis enter the space between the quadrupole rods and maintain their velocity along this axis. However, they are submitted to accelerations along x and y that result from the forces induced by the electric fields (Figure 2.5) ... 

The operation of the QIT is based on the same physical principle as the quadrupole mass spectrometer described above. Both devices make use of the ability of RF fields to confine ions. However, the RF field of an ion trap is designed to trap ions in three dimensions rather than to allow the ions to pass through as in a QMF, which confines ions in only two dimensions. This difference has a significant unpact on the operation and limitations of the QIT, The physical arrangement of a QIT is different from that of a QMF. If an imaginary axis is drawn through the y-axis of the quadrupole rods, and the rods are rotated around the axis, a solid ring with a hyperbolic inner surface results from the x-axis pair of rods. 

The second configuration of a linear IT commercially available is schematized in Fig. 2.22. It operates, with respect to ion injection and storing, in the same way as described above, playing on the voltages applied to Gates 1 and 2, but in this case the ions are no longer ejected axially, but radially, thought two narrow slits present on two opposite quadrupole rods (Schwartz et al., 2002). 

Fig. 18. Diagram of reactive scattering apparatus for the study of non-metal reactions A, scattering chamber B, source chambers C, liquid nitrogen cooled cold shield D, detector E, source bulkheads G, liquid nitrogen trap H, oil diffusion pumps N, free radical source P, nozzle source Q, skimmer E, ion source H, liquid He trap I, ion lenses P, photomultiplier Q, quadrupole rods R, light baffle S, slide valve T, radial electric field pumps (from C. F. Carter et al. 02 by permission of the Chemical...

The quadrupole mass analyser is a mass filter . Combined DC and RF potentials on the quadrupole rods can be set to pass only a selected mass-to-charge ratio [36]. All other ions do not have a stable trajectory through the quadrupole mass analyser and will collide with the quadrupole rods, never reaching the detector. The operation of a quadrupole mass analyser is usually treated in terms of a stability diagram that relates the applied DC potential and the applied RF potential, and the RF frequency to a stable vs unstable ion trajectory through the quadmpole rods. A schematic diagram of a quadmpole mass filter is shown in Fig. 10.5. 

Increasing the resolution decreases the number of ions that reach the detector. Good resolution also depends on the quality of the machining for the quadrupole rods. Quadrupole rods can have other functions besides their use as a mass filter. An RF-only quadmpole will act as an ion guide for ions within a broad mass range. For example, the collision region of a triple quadrupole mass spectrometer uses an RF ion guide. A DC-only quadrupole is used as a lens element in some ion optical systems. 

---