Magnetic sector spectrometers

Mass spectrometer can be used as a detector in HPLC analysis. A special interface between the column and the ion source removes the mobile phase solvent and vaporizes the nonvolatile analytes without destroying or rearranging them. The molecules are ionized, and the ions are directed into a quadrupole mass analyzer or into a magnetic sector spectrometer. 

A sample contains a mixture of three compounds, the molecular ions of which can be separated by a mass spectrometer at a resolution of 9500. Your laboratory is equipped with the following instruments (a) a double-focusing magnetic-sector spectrometer, (b) a single-quadmpole spectrometer, (c) a quadrupole ion-trap spectrometer, (d) a linear time-of-flight (TOP) spectrometer, (e) a refiectron-TOF spectrometer, and (f) a Fourier transform ion cyclotron mass spectrometer. Suggest aU possible choices among these instruments that you can employ for the analysis of this mixture. 

Sector instruments and hybrid quadrupole-seclor instruments have also been used in a tandem manner. The first tandem mass spectrometers were sector instruments that combined an electric sector spectrometer with a magnetic sector spectrometer, either in forward geometry (electric sector followed by magnetic... 

Before ehding this presentation on mass spectrometry, we should cite the existence of spectrometers for which the method of sorting ions coming from the source is different from the magnetic sector. These are mainly quadripolar analyzers and, to a lesser degree, analyzers measuring the ion s time of flight. 

The quadripolar spectrometers whose resolution is limited to about 2000 are of simpler design than the magnetic sectors and are less costly. They are often used in conjunction with gas chromatography (see section 3.3) for purposes of identification. 

The chapter is divided into sections, one for each general class of mass spectrometer magnetic sector, quadnipole, time-of-flight and ion cyclotron resonance. The experiments perfonned by each are quite often unique and so have been discussed separately under each heading. 

Figure Bl.7.4. Schematic diagram of a reverse geometry (BE) magnetic sector mass spectrometer ion source (1) focusing lens (2) magnetic sector (3) field-free region (4) beam resolving slits (5) electrostatic sector (6) electron multiplier detector (7). Second field-free region components collision cells (8) and beam deflection electrodes (9).

Magnetic sectors can be used on their own, or in conjunction with energy analysers to fomi a tandem mass spectrometer. The unique features of the reverse geometry instrument are presented from this point. 

A single magnetic sector can be used as a mass filter for other apparatus. However, much more infonnation of the simple mass spectrum of a species can be obtained using the tandem mass spectrometer. 

Another approach to mass analysis is based on stable ion trajectories in quadnipole fields. The two most prominent members of this family of mass spectrometers are the quadnipole mass filter and the quadnipole ion trap. Quadnipole mass filters are one of the most connnon mass spectrometers, being extensively used as detectors in analytical instnunents, especially gas clnomatographs. The quadnipole ion trap (which also goes by the name quadnipole ion store, QUISTOR , Paul trap, or just ion trap) is fairly new to the physical chemistry laboratory. Its early development was due to its use as an inexpensive alternative to tandem magnetic sector and quadnipole filter instnunents for analytical analysis. It has, however, staned to be used more in die chemical physics and physical chemistry domains, and so it will be described in some detail in this section. 

After ions have been formed by El, they are examined for mass and abundance by the analyzer part of the mass spectrometer, which can incorporate magnetic sectors, electric sectors, qua-drupoles, time-of-flight tubes, and so on. The region in which the ions are first formed is called... 

An AutoSpec-TOF mass spectrometer has a magnetic sector and an electron multiplier ion detector for carrying out one type of mass spectrometry plus a TOF analyzer with a microchannel plate multipoint ion collector for another type of mass spectrometry. Either analyzer can be used separately, or the two can be run in tandem (Figure 20.4). 

Some Factors Important in Choosing between Quadrupole and Magnetic-Sector Mass Spectrometers... 

Modem mass spectrometers are used in a very wide variety of situations, so it is almost impossible to have a simple set of criteria that would determine whether a quadrupole or magnetic sector instmment would be best for any particular application. Nevertheless, some attempt is made here to address major considerations, mostly relating to cost. 

Peak matching can be done on quadrupole and magnetic-sector mass spectrometers, but only the latter, particularly as double-focusing instruments, have sufficiently high resolution for the technique to be useful at high mass. 

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