Quadmpole Mass Filter

The quadmpole mass filter is a relatively small device that can be set to let through ions within a very limited m/z range only. The trajectories of the ions with either higher or lower m/z will bend and the ions will never escape the filter. A mass spectrum is acquired by scanning through the whole m/z range of interest and detecting how many ions pass the filter at each m/z. 

Quadmpole analyzers have generally been considered to give poor mass accuracy. Recently, however, with better machined parts and better electronics, commercially available instruments can perform quite well. A mass accuracy 5 ppm can be obtained with internal calibrants. 

Like sector analyzers, quadrupole analyzers are well suited for continuous ion sources such as ESI, but are not well-suited for pulsed ionization methods. Quadmpole mass spectrometers are generally substantially cheaper and smaller than sector instruments and Qq-TOFs. They are very often used in combination with GC and LC, and single or triple quadmpole mass filters are very common benchtop instruments for routine measurements. 

---

There are otlier teclmiques for mass separation such as tire quadmpole mass filter and Wien filter. Anotlier mass spectrometry teclmique is based on ion chromatography, which is also capable of measuring tire shapes of clusters [30, 31]. In tills metliod, cluster ions of a given mass are injected into a drift tube witli well-defined entrance and exit slits and filled witli an inert gas. The clusters drift tlirough tills tube under a weak electric potential. Since the... 

Quadrupoles. The quadmpole mass filter (16,17), which became available commercially in 1967, consists of four cylindrical rods having circular or, more recentiy, hyperboHc cross sections, arranged with their centers on the perimeter of a circle of radius Tq. Figure 2 is a schematic diagram of a... 

FIG. 35. Vertical cross section of the reaction chamber equipped with the mass spectrometer system. Indicated are QMF. the quadmpole mass filter ESA. the electrostatic analyzer CD, the channeltron detector DE, the detector electronics DT, the drift tube lO, the ion optics TMP, the turbomolecular pump PR, the plasma reactor and MN. the matching network. 

Quadmpole ion traps were originally coupled to continuous ion sources but also work well with pulsed ion sources. The cylindrical QIT is a compact device with a diameter and length of 5 cm and the linear QIT is the size of a quadmpole mass filter, that is, 20 cm long. As stand-alone instruments they are nowadays of benchtop size and, together with quadmpole mass filters, are considered to be standard low-cost devices, which are commonly coupled to LC systems. 

Overall, the quadmpole analyzer rather acts as a mass filter than as a momentum (B sector) or energy (ESA) spectrometer hence the widespread use of the term quadmpole mass filter. 

Chapter 2 systematically defines some of the important PK parameters and guides the reader through the types of quantitative LC-MS experiments performed to elucidate the PK parameters necessary to move a drag through discovery, preclinical development, and clinical stages. Chapters 3, 4, and 5 respectively introduce the readers to quadmpole mass filters and liner ion traps, time-of-flight mass... 

A sehematie diagram of a quadmpole mass filter is shown in figure Bl.7.8. In an ideal, three-dimensional, quadmpole field, the potential ( ) at any point (x, y, z) within the field is deseribed by equation (B 1.7.5) ... 

Ideally, the rods in a quadmpole mass filter should have a h q)erbolic geometry, but more common is a set of four cylindrical rods separated by a distance 2r, where r 1.16r[t(figure B1.7.8). This arrangement provides... 

A schematic diagram of a SIFT apparatus is shown in figure B 1.7.12. The instrument consists of five basic regions, the ion source, initial quadmpole mass filter, flow tube, second mass filter and finally the detector. The heart of the instmment is the flow tube, which is a steel tube approximately 1 m long and 10 cm in diameter. The pressure in the flow tube is kept of the order of 0.5 Torr, resulting in carrier gas flow rates of... 

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. 

Performance has been greatly improved by the introduction of hybrid instruments that combine arrays of one or more quadrupole elements with TOF analyzers. In these types of configuration, MALDI-ions are initially desorbed into a source region at an intermediate pressure of about one to a few millibars, where they experience coUisional cooUng to Umit fragmentation. From the source region they are extracted into a quadmpole mass filter and subsequently are transmitted... 

Date, A.R. Gray, A.L. Plasma source-mass spectrometry using an inductively coupled plasma and a high-resolution quadmpole mass filter. Analyst 1981,106, 1255 1267. 

The theory of operation of a Paul trap is very similar to that for the linear quadmpole mass filter, discussed above, and the following treatment will buUd on that as well as on a more detailed introduction (March 1997). The mechanism of the device as an ion trap (as distinct from a mass spectrometer) can be understood in qualitative terms through a mechanical analogy (Paul 1990) illustrated in Figure 6.17. (With appropriate modifications this analogy also applies to ion motion in the... 

In Eq. (8.50) Ip, I, are the signal amplitudes of product ions and reagent ions (H3O+, H30. H20, etc.) corrected for differential diffusion in the flow tube and mass discrimination in the downstream quadmpole mass filter. Additional parameters fj., f 2 may also be included in the reactions of 1130 reagent ions to account for analytes where not all water cluster ion reactions with the analyte are reactive [98]. 

The common mass spectrometers used in ICP-MS today are scanning-based analysers, such as the quadmpole mass filter. Unfortunately, they suffer from important performance limitations when used as detectors of short transient signals (e.g. those generated in spedation analysis), derived from their inability to perform true simultaneous multielemental analysis. With scanning-based instruments, individual mass-to-charge ratios are measured in a sequential mode, from one isotope to the next. As a result, some difficulties (in terms of precision, sensitivity or accuracy of isotopic and isotope ratio measurements) are expected when fast transient or time-dependent signals (such as those... 

Theoretically, the electrodes of a quadmpole mass filter should have a hyperbolic cross section for optimized geometry of the resulting quadmpole field, and thus for optimized performance [80,81]. However, for ease of manufacture, cylindrical rods are often employed instead. By adjusting the radius of the rods carefully (r= 1.1468 X ro), a hyperbolic field may be approximated [91]. However, even slight distortions of the ideal quadmpole field either from interference with external fields or due to low mechanical precision or inadequate shape of the device cause severe losses of transmission and resolution [92]. The expected advan-... 

The quadmpole mass filter has seen continued performance improvements, but it is still operated fundamentally as it was when it was first introduced. The QMF is in demand today in the form of single-quadmpole, triple-quadmpole, or hybrid mas s spectrometers. The birth of the QIT as a scanning mass spectrometer in 1983 by Finnigan Corporation occurred later because of the need for the often overlooked computer control which was essential for implementation of the scan function and the mass selective instability scan. ... 

Two key performance criteria for the quadmpole mass filter are the mass range and the mass resolution. Both can be calculated, assuming a perfect quadmpolar field. The maximum mass that can be transmitted is given by [47]... 

---