Quadrupole mass, generally filters

Quadrupole mass filters can easily be combined with chromatographic techniques so are often the analysers used in GC-MS and LC-MS instruments (see Section 5.6). In general, quadrupole mass filters are often used to provide low-resolution spectra and are much cheaper and require less space than magnetic sector instruments. 

Ions exiting the drift tube are mass analyzed in mass spectrometer MS2, an important feature if reactions are occurring in the drift cell. Ions are generally detected after MS2 by ion counting techniques. The mass spectrometers MSI and MS2 are typically quadrupole mass filters, and either one or the other can be run in RF-only mode for better signal but without mass selection, if desired. 

An exhaustive description of the physical aspects of the behavior of ions in a quadrupole field is beyond the aim of this book. For those interested in this aspect we suggest the March and Todd books, which describe in detail all the theoretical and practical aspects of quadrupole mass filters and ion traps very well. We will try to give a general idea of how the quadrupole mass filter operates, with the intent of making the reader conscious of the pros and cons of this instrumental approach. 

Quadrupole ICP-MS instruments are considerably cheaper and easier to operate, but the detection limits are generally inferior compared to SF-ICP-MS. Furthermore, as the quadrupole mass filter provides a resolution of only 300, polyatomic interferences are likely, especially in the mass range 40-80 amu. A well-known interference appears on As (ICP-MS Interferenz Tabelle 1995). The As isotope is a mono-isotope, which... 

Time-of-flight mass spectrometers, quadrupole mass filters, and several types of magnetic-sector mass spectrometer have all been used in photoionization studies. Specific experiments may favor one or another of these types. Most experiments favor a mass spectrometer with high transmission and only modest mass resolution (e.g., 1/100). The time-of-flight mass spectrometer is not as generally useful as the other two types... 

Finally in this section, it is noted that single quadrupole mass filters (as opposed to triple quadrupole instruments) are generally used in trace quantitative analyses only for GC-EI(CI)-MS methods for thermally stable analytes (e.g. pesticides in various matrices) in SIM mode. (It is interesting that GC/MS instruments still appear to account for a large fraction of mass spectrometer units sold worldwide.) The same linear quadrupole devices can also be operated as 2D ion trap mass spectrometers, discussed in Sections 6.4.5 and 6.4.6. 

In 1989 the Nobel Prize in physics was shared by Wolfgang Paul (for development of the three-dimensional quadrupole ion trap as an extension of the linear quadrupole mass filter) and Hans Dehmelt (for spectroscopic studies of ions suspended in ion traps of various kinds, including the Paul trap) the Nobel award lectures (Paul 1990 Dehmelt 1990) incidentally also provide accounts of their work that are interesting historically and also lucid and accessible to nonexperts. Other early work on development of the same general principles for ion trapping (Good 1953 Wuerker 1959) should also be... 

ThG SGparation Process. Once the anal3d is in the gas phase as an ion, analytes must be separated on the basis of mass to charge (m/z). A variety of separation techniques are available. The most common form of ms separation is the quadrupole mass filter (3,4). Because this method is limited to m/z less than 3000 or so, it is not generally used with MALDI. However, a quadrupole is commonly used with esi where multiply charged analytes allow one to examine high masses for m/z less than 3000. 

In Eq. (8.49), k is the rate coefficient for reaction, the quantities in square brackets are the number densities of the ions and analyte A, t is the reaction time and De is a differential diffusion enhancement coefBcient that accommodates the difference in diffusion rate to the walls between reagent ions and product ions. Ions with larger m/z generally have slower rates of diffusion than smaller ions. Another important factor when using quadrupole mass filters is to account for the reduced transmission of heavier ions. The two opposing effects of diffusion and transmission oppose each other. A proper analysis will account for both effects. 

Of several methods available for the separation of charged ions only two are in general use in organic mass spectrometry, namely magnetic field deflection and quadrupole mass filtering. 

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. 

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