The Magnetic Sector Mass Spectrometer

FIGURE 10.15 Ail illustration of a magnetic sector mass spectrometer. 

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The mean free path is the average distance a molecule travels before colliding with another molecule. The mean free path, X, is given by X = kT/ jr2 itP). where k is Boltzmann s constant, Tis the temperature (K), P is the pressure (Pa), and cr is the collision cross section. For a molecule with a diameter d, the collision cross section is ltd2. The collision cross section is the area swept out by the molecule within which it will strike any other molecule it encounters. The magnetic sector mass spectrometer is maintained at a pressure of 10-5 Pa so that ions do not collide with land deflect) each other as they travel through the mass analyzer. What is the mean free path of a molecule with a diameter of 1 nm at 300 K in the mass analyzer ... 

From the aforementioned equation, one can see that there are two modes of operation for the magnetic sector mass spectrometer. One method holds the magnetic field constant and scans the mass by varying the potential applied to the ion optics the other method varies the magnetic field to control the selected mass while keeping the kinetic energy of the ions constant. 

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. 

The ion optics of a magnetic-sector mass spectrometer cause the ion beam leaving the ion source to arrive at a collector after being separated into individual m/z values and focused. 

Tandem quadrupole and magnetic-sector mass spectrometers as well as FT-ICR and ion trap instruments have been employed in MS/MS experiments involving precursor/product/neutral relationships. Fragmentation can be the result of a metastable decomposition or collision-induced dissociation (CID). The purpose of this type of instrumentation is to identify, qualitatively or quantitatively, specific compounds contained in complex mixtures. This method provides high sensitivity and high specificity. The instrumentation commonly applied in GC/MS is discussed under the MS/MS Instrumentation heading, which appears earlier in this chapter. 

Reactions of D with D20 and of 0 with 02, N20, and N02 have been studied with a magnetic sector mass spectrometer. Competition between electron transfer and ion-atom interchange has been observed in the production of 02 by reaction of 0 with 02, an endothermic reaction. The negative ion of the reacting neutral molecule is formed in 02, N2Of and N02 but not in D20. Rate constants have been estimated as a function of repeller potential. 

Thermospray ionisation sources are usually outfitted with a quadrupole or magnetic sector mass spectrometer (including hybrids or tandem forms). Thermospray operation allows a reversed-phase solvent system, e.g. a 50 50 (v/v) water-methanol or acetonitrile mix containing 0.1 M ammonium acetate. This ensures compatibility with the universal HPLC procedures available in many industrial research laboratories. 

Magnetic sector mass spectrometers accelerate ions to more than 100 times the kinetic energy of ions analysed in quadrupole and ion trap mass spectrometers. The higher accelerating voltage contributes to the fact that ion source contamination is less likely to result in degraded sensitivity. This is particularly important for analysis that requires stable quantitative accuracy. 

The main characteristics of sector mass spectrometers are shown in Table 6.29. Magnetic sector mass spectrometers are often considered more difficult to operate than QMS and ToF-MS the high-voltage source is more demanding to chromatographic interfacing. For figures of merit, see Table 6.27. 

Applications Sector instruments are applied for niche applications such as high-resolution measurements and fundamental ion chemistry studies. Magnetic sector mass spectrometers remain the instrument of choice in areas of target compound trace analysis, accurate mass measurement and isotope ratio measurement. 

Imaging SIMS. Steeds et al. (1999) included this technique in their study of the distribution of boron introduced into diamond, where it is a well-established dopant that controls the electrical conductivity. SIMS was performed with a field-emission liquid gallium ion source interfaced to a magnetic sector mass spectrometer capable of about 0.1 pm spatial resolution. 

How can metastable ions be registered with a classic magnetic sector mass spectrometer (See Chapter 2, Section 2.2.2) Let ion mj+ leave the ion source and after acceleration with accelerating voltage V fragment, with formation of ion m2+ and a neutral particle m3° between the source and magnetic analyzer (first field-free region, 1 FFR). 

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