Instrumentation sector

The detector converts ions of a given m/z value into a measurable electrical signal whose intensity is proportional to the corresponding ion current. With beam instruments (sectors, quadrupoles, or TOF analyzers) and the quadru-pole ion trap, the ions are first separated according to their m/z value before detection, usually by an electron multiplier or a photon multiplier. The operation of these most common detectors is briefly outlined below. 

Some ionization techniques (El, FAB, and SIMS) are compatible with all mass analyzers. PD, LD, and MALDI are most suited to TOF analyses. Atmospheric pressure ionization methods (TSP, ESI, APCI) are best coupled with quadrupole and ion trap instruments. Sector and FTICR instruments can also operate with chromatographic interfaces however, a significant reduction of pressure in the system is required. Consequently, in FTICR, the ion source and the ICR cells must be separated by a distance of about 1 m. Powerful ion optics is required for the transmission of ions for these long distances. This inconvenience, however, is offset by the advantages of FTICR, such as extremely high resolution and the ability to store the ions of interest for long periods. 

The Italian National Federation of Electrotechnical and Electronic Industries, (AssoAutomazione-GISI) carried out a 2005 analysis of the market situation in the automation, industrial, civil and laboratory instrumentation sectors as well as the public utility and traffic networks. They estimated that overall sales growth in 2005 would average 2.5%. They were cautiously optimistic as far as 2006 was concerned. 

Most instruments are configured with a fixed value for the radius of curvature, r, so changing the value of B selectively passes ions of particular values of momentum, mv, tlirough tlie magnetic sector. Thus, it is really the momentum that is selected by a magnetic sector, not mass. We can convert this expression to one involving the accelerating potential. 

Magnetic sector instruments typically operate with ion sources held at a potential of between 6 and 10 kV. This results in ions with keV translational kinetic energies. The ion kinetic energy can be written as zt V = Ifur and thus the ion velocity is given by the relationship... 

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. 

Ions accelerated out of the ion source with keV translational kinetic energies (and m/z selected with the magnetic sector) will arrive in the FFR of the instrument in several microseconds. Ions dissociating on this... 

Figure Bl.7.7. Summary of the other collision based experiments possible with magnetic sector instruments (a) collision-mduced dissociation ionization (CIDI) records the CID mass spectrum of the neutral fragments accompanying imimolecular dissociation (b) charge stripping (CS) of the incident ion beam can be observed (c) charge reversal (CR) requires the ESA polarity to be opposite that of the magnet (d) neutiiralization-reionization (NR) probes the stability of transient neutrals fonned when ions are neutralized by collisions in the first collision cell. Neutrals surviving to be collisionally reionized in the second cell are recorded as recovery ions in the NR mass spectrum.

The beam entering the ion chamber is suitable for both electron (El) and chemical (Cl) ionization, and either mode can be used (Figure 12.3). Mass analysis follows in the usual way, typically using quadruple or magnetic-sector instruments. 

This chapter provides brief descriptions of analyzer layouts for three hybrid instruments. More extensive treatments of sector/TOF (AutoSpec-TOF), liquid chromatography/TOF (LCT or LC/TOF with Z-spray), and quadrupole/TOF (Q/TOF), are provided in Chapters 23, 22, and 21, respectively. 

In the magnetic-sector/TOP hybrid, ions produced in an ion source pass through the magnetic sector first and then might enter the TOF section, depending on how the hybrid is operated. The hybrid can be used as two separate instruments or as two instruments in conjunction with each other. 

Also in general terms, the TOF part of the hybrid is used mostly for MS/MS studies in which ions produced in the magnetic sector are collided with neutral gas molecules to induce decomposition (see Chapter 23). In this mode the instrument produces more highly resolved product ion spectra than can be attained in simple magnetic-sector instruments. 

A further important property of the two instruments concerns the nature of any ion sources used with them. Magnetic-sector instruments work best with a continuous ion beam produced with an electron ionization or chemical ionization source. Sources that produce pulses of ions, such as with laser desorption or radioactive (Californium) sources, are not compatible with the need for a continuous beam. However, these pulsed sources are ideal for the TOF analyzer because, in such a system, ions of all m/z values must begin their flight to the ion detector at the same instant in... 

Hybrid Magnetic-Sector Time-of-Flight (Sector/TOF) Instruments... 

Like the magnetic sector, the TOP section by itself is not capable of MS/MS operation, but allied with the sector, the two make an excellent MS/MS instrument. 

Operation of the Combined Magnetic and TOF Sectors as a Hybrid Instrument... 

The AutoSpec -TOF hybrid mass spectrometer combines the advantages of a magnetic/electric-sector instrument with those of time-of-flight to give a versatile instrument capable of MS or MS/MS at high or low resolution. 

There are other characteristics of quadrupoles that make them cheaper for attainment of certain objectives. For example, quadrupoles can easily scan a mass spectrum extremely quickly and are useful for following fast reactions. Moreover, the quadrupole does not operate at the high voltages used for magnetic sector instruments, so coupling to atmospheric-pressure inlet systems becomes that much easier because electrical arcing is much less of a problem. 

Three main types point ion collectors are in use for quadrupole, magnetic-sector, and TOF instruments, and they are discussed here. The multichannel plate collector (or time-to-digital converter)... 

Ion trajectory through a conventional (EB) sector instrument, showing three field-free regions in relation to the sectors, the source, and the ion detector. 

In a sector instrument, which acts as a combined mass/velocity filter, this difference in forward velocity is used to effect a separation of normal and metastable mj" ions (see Chapter 24, Ion Optics of Magnetic/Electric-Sector Mass Spectrometers ). However, as discussed above, the velocity difference is of no consequence to the quadmpole instrument, which acts only as a mass filter, so the normal and metastable mj ions formed in the first field-free region (Figure 33.1) are not differentiated. 

In an EW- of a B/E-linked scan using an electric/magnetic-sector instrument, a precursor ion is selected. In this case it is m, which might be a molecular ion but equally could be any fragment ion. All product ions (mj, m3, m4) from decomposition of m, in the first field-free region between the ion source and the ion collector are found, thereby giving connections mpm, m -m3, m -m4. 

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