Double-Focusing Sector Instruments

The electrostatic sector or electrostatic analyzer (ESA) produces a radial electric field between two oppositely charged plates extending over the ESA angle ( ) (Fig. 4.21). An ion passes the ESA midway on a circular path if 

Note The ESA effects energy dipersion. Thus, the kinetic energy distribution of an ion beam can be reduced. The ESA does not allow for mass separation among monoenergetic ions. 

Upon substitution of v with Eq. 4.3 one obtains the simple relationship 

A different EB design to effect an image plane of 140 mm in length with a linear mass scale for detection on a photographic plate has been published by Bain-bridge and Jordan. [74] Their paper is especially recommended, as it also nicely illustrates the use of photoplates in these days. 

Another famous type of EB arrangement has become known as Nier-Johnson geometry (Fig. 4.23). [75] Here, the ion beam first passes through the electrostatic 

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With few exceptions, magnetic sector instruments are comparatively large devices capable of high resolution and accurate mass determination, and suited for a wide variety of ionization methods. Double-focusing sector instruments are the choice of MS laboratories with a large chemical diversity of samples. In recent years, there is a tendency to substitute these machines by TOE or by Fourier transform ion cyclotron resonance (FT-ICR) instruments. 

MS/MS techniques can be performed on double-focusing sector instruments as well as on quadrupole or hybrid instruments. For the purpose of this discussion will concentrate on a condensed survey of double focusing sector instruments. References that provide substantial accounts on quadrupole and hybrid instruments used for MS/MS can be found in the bibliography at the end of this chapter. 

For many years double focusing sector instruments were the instruments of choice for highly accurate and precise (within lppm) measurements of molecular mass, used to determine molecular compositions of unknown compounds or at least limit the possibilities to a small number. This role is now usually fiUed by upper-end TOF analyzers (less expensive, superior ease of use, but lower accuracy and precision) and by FTICR and Orbitrap instruments (more expensive but higher RP possible). Magnetic sector instruments are still used by the petroleum industry in type analysis (see Preface), and also for GC/HRMS quantitation as mentioned above, since TOF, FTICR and Orbitrap instruments do not posses the necessary combination of figures of merit for such trace quantitative analyses. 

As a result of the introduction of alternative MS-MS instruments, which are more cost-effective and easier to operate, the double-focusing sector instruments are hardly used in MS-MS. The same holds for hybrid MS-MS systems comprising sector instruments combined with quadrapole or ion trap building blocks. 

Until a decade ago, double-focusing sector instruments were the choice of MS laboratories with a large chemical diversity of samples. In recent years, however, there has been a strong tendency to substitute sector instruments by TOE, orbitrap, or FT-ICR instruments, and moreover, to even substitute classical ionization methods such as El, Cl, or FAB with APCI or ESI just to redirect the stream of samples to those preferable modem mass analyzers. 

Figure 2 Geometries of double-focusing sector instrument Nier-Johnson (EB) geometry and reversed geometry.

A double-focusing sector instrument allows collision-activated ion dissociation (CID) for tandem mass spectrometry (MS-MS) in the field-free regions, e.g., between the entrance slit and the first sector, or in between the two sectors. Special linked scanning procedures of the instrument, such B/E linked scans, allow the acquisition of product ion mass spectra. 

A double-focusing sector instrument allows high-resolution measurements, enabling (1) determination of accurate m/z of ions (at 1 ppm or better) and (2) improvement of selectivity in the analysis of compounds in complex matrices. 

Double-focusing sector instrument are still widely used in GC-MS for a number of applications, e.g., in the analysis of polychlorodibenzodioxins and related compounds. 

Although in most applications of GC-MS a linear quadrupole or quadrupole ion trap is used for mass analysis, yielding unit-mass resolution only, higher mass resolution can be achieved as well, especially by means of a double-focusing sector instrument. Such instruments have been for many years, and still are, routinely used in various application areas of GC-MS, e.g., analysis of polychlorinated compounds such as polychlorodibenzodioxins and polychlorobiphenyls. Highly sophisticated and dedicated instruments are available for this purpose. 

Tandem mass spectrometry is applicable to all types of biomolecules but typically requires specialized mass spectrometry instrumentation for implementation. The most common tandem mass spectrometer is a triple quadrupole instrument, wherein the first and third quadrupoles are used as mass analysers and the middle quadrupole is used as a collision chamber. MS/MS can be performed with double focusing sector instruments and in some cases with specialized TOF mass analysers. Quadrupole ion traps and FTICR mass spectrometers are ideally suited for MS/MS experiments, and due to the operational characteristics of these mass analysers... 

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