Quadrupole mass spectrometers principle

In recent years several new instruments have been developed based on different mass-spectrometer principles. Two different categories of ICP-MS instruments are currently commercially available low-resolution instruments (using either QMS, ITMS or ToF-MS) and focusing high-resolution instruments (DFS, FTMS). Selected specifications for these two categories are shown in Table 8.63. Both the quadrupole-based and the double-focusing instruments allow a sequential multielement measurement, whereas ICP-ToFMS allows... 

A diagram showing the basic operating principle of a triple quadrupole mass spectrometer is shown in figure 4. The sample enters the ion source from the column, via a suitable interface if necessary, and, in GC, is usually fragmented by either an electron impact or... 

Figure 2 Schematic representation of a triple quadrupole mass spectrometer. The principle of frequently used scan modi is indicated. Please note that MRM is based on monitoring multiple specific precursor/product ion pairs.

The physical principles underlying the operation of a quadrupole mass spectrometer require the solving of a complicated differential equation, the Mathieu equation. In operation when an ion is subjected to a quadrupoiar RF field, its trajectory can be described qualitatively as a combination of fast and slow oscillatory motions. For descriptive purposes, the fast component will be ignored here and the slow component emphasized, which oscillates about the quadrupoiar axis and resembles the motion of a particle in a fictitious harmonic pseudopotential. The frequency of this oscillation is sometimes called the secular frequency. 

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. 

Probably the first pyrolysis-mass spectrometer was described by Meuzelaar and Kistemaker. This instrument was based on a Riber quadrupole mass spectrometer and used the Curie-point pyrolysis method first described by Giacobbo and Simon. This development eventually produced two commercial instruments — the Extra-nuclear 5000 (Extranuclear Laboratories, Pittsburgh, PA), effectively a copy of the FOM machine, and the Pyromass 8-80 (VG Gas Analysis, Middlewich, England), based on the same principles but using a small magnetic mass spectrometer. 

Another LDI instrument that was similar in principle to LAMMA was developed by Perchalski (1985) that featured the additional selectivity of two stages of mass analysis provided by a triple quadrupole mass spectrometer (QqQ). The LDI QqQ was shown to have potential for use as a probe-type analyzer for molecular analysis of mixtures, as demonstrated by the detection of a mixture of nine antiepileptic drugs by monitoring the precursor ion/product ion pair for each drug (Perchalski et al., 1983). The LDI—QqQ, however, was determined to be too slow to adequately characterize molecules ionized by cationization or anionization after desorption by a single-shot laser. Also, the vaporization/ionization process on the LDI—QqQ was unable to ionize polar, nonvolatile, and/or thermally unstable molecules (Perchalski, 1985). 

The relative ion composition of a plasma is monitored by mass spectrometry today almost exclusive of the other techniques. Excellent description and discussion of the principles and operation of many mass spectrometers are given in several texts. The characteristics of a mass spectrometer for sampling plasmas have been discussed previously. It will only be mentioned here that the quadrupole mass spectrometer is well suited for plasma sampling. 

Assays in SIM-mode and El-technique exist for many prostanoids and in principle can be set up for all prostanoids and their metabolites. The well equipped mass spectrometers - in most cases quadrupole mass spectrometers - are easy to operate. For quantification of prostanoids the selected-ion-monitoring in electron inpact mode reveals excellent specifity and sensitivity (27). The responses of selected pairs of ions of high relative abundance are compared. As the amount of added deuterated internal standard is known the ratios of the endogenous prostanoid signal and the one of the internal standard allow quantification (27). 

In principle, a helium leak detector detects the partial pressure of a tracer gas in vacuum with a suitable sensor. Small mass spectrometers have become common as a sensor, especially small magnetic sector-field mass spectrometers with permanent magnets. Also the quadrupole mass spectrometer well known from residual gas analysis can be found in some leak detectors. 

Principles and Characteristics Simultaneous thermal analysis techniques, such as TG-DSC/DTA offer vital information on polymer structure based on heat flow behaviour and mass change [290], but little direct information on the composition of evolved gas products. A more complete thermal profile is provided when a thermal analyser is coupled to an identification tool. Henderson et al. [433] have recently described TG-DSC/DTA with evolved gas analysers (MS and FTIR). The skimmer coupling is the most advanced commercial way of combining a thermobalance or simultaneous TG-DSC/DTA instrument with a quadrupole mass spectrometer [338]. For descriptions of interface techniques in this coupled instrumentation, cfr. ref. [411]. Simultaneous TG-DSC-MS is capable of operation up to 2000°C [434]. 

In principle, any type of magnetic or quadrupole mass spectrometer can be utilized for the analytical pyrolysis of organic materials, if a direct introduction system capable of producing a desired tempera-ture/time profile is available. For example, direct insertion probes (DIPs) and direct exposure probes (DEPs) are Avidely used for sample introduction and such probes are supplied with control units that allow heating and temperature programming of the sample up to 500-800°C. Therefore, such modules should be considered as the most readily available probes for Py-MS studies. 

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