Mass Analyzers for El

Classical organic chemistry provides a wide variety of potential analytes for electron ionization, the only limitation being that the analyte should be able to evaporate or sublime without significant thermal deconposition. These requirements are usually met by saturated and unsaturated aliphatic and aromatic hydrocarbons and their derivatives such as halides, ethers, acids, esters, amines, amides etc. Heterocycles generally yield useful El spectra, and flavones, steroids, terpenes and comparable compounds can successfully be analyzed by El, too. Therefore, El represents the standard method for such kinds of samples. 

GC-EI-MS can be used for the direct analysis of mixtures, e.g., to analyze synthetic by-products an advantage that made GC-EI-MS benchtop instruments become widespread in modem synthetic laboratories. The GC-EI-MS combination is especially successful in monitoring environmental pollutants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofuranes (PCDFs), or other volatile organic compounds (VOCs). 

While low- and medium-polarity analytes are usually well suited for El, highly polar or even ionic compounds, e.g., diols or polyalcohols, amino acids, nucleosides, peptides, sugars, and organic salts should not be subjected to El unless properly derivatized prior to EI-MS [67-72]. 

There is - as with any other ionization method - no strict upper limit for molecular mass, a range of up to 800-1000 u being a realistic estimate. There are exceptions of up to 1300 u if the analyte is extremely nonpolar, e.g., from numerous fluoroalkyl or trialkylsilyl groups which also significantly contribute to molecular mass. 

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Decomposition (fragmentation) of a proportion of the molecular ions (M +) to form fragment ions (A B+, etc.) occurs mostly in the ion source, and the assembly of ions (M +, A+, etc.) is injected into the mass analyzer. For chemical ionization (Cl), the Initial ionization step is the same as in El, but the subsequent steps are different (Figure 1.1). For Cl, the gas pressure in the ion source is typically increased to 10 mbar (and sometimes even up to atmospheric pressure) by injecting a reagent gas (R in Figure 1.1). 

For solids, there is now a very wide range of inlet and ionization opportunities, so most types of solids can be examined, either neat or in solution. However, the inlet/ionization methods are often not simply interchangeable, even if they use the same mass analyzer. Thus a direct-insertion probe will normally be used with El or Cl (and desorption chemical ionization, DCl) methods of ionization. An LC is used with ES or APCI for solutions, and nebulizers can be used with plasma torches for other solutions. MALDI or laser ablation are used for direct analysis of solids. 

The beam of substrate molecules then passes straight into the ion source (electron ionization, El, or chemical ionization. Cl) for ionization before entry into the mass analyzer. 

The compatibility is excellent with continuous ion sources such as ESI, dynamic SIMS, CF-FAB, ICP, El, Cl, etc. Sector instalments are not well-suited for pulsed ionization methods, although there are examples where MALDI sources have been utilized [225-229]. Sector instruments are usually larger and more expensive than other mass analyzers, such as TOFs, quadrupole filters, and traps. 

The dwelltime of ions within the ion source is defined by the extraction voltages applied to accelerate and focus them into an ion beam and by the dimensions of that ion source. In standard El ion sources the freshly formed ions dwell about 1 ps before they are forced to leave the ionization volume by action of the accelerating potential. [41] As the ions then travel at speeds of some 10 m s they pass the mass analyzer in the order of 10-50 ps (Fig. 2.9). [9] Even though this illustration has been adapted for a double focusing magnetic sector mass spectrometer, an ion of m/z 100, and an acceleration voltage of 8 kV, the effective time scales for other types of instruments (quadrupole, time-of-flight) are very similar under their typical conditions of operation (Table 2.4). 

With external ion sources it became feasible to interface any ionization method to the QIT mass analyzer. [171] However, commercial QITs are chiefly offered for two fields of applications i) GC-MS systems with El and Cl, because they are either inexpensive or capable of MS/MS to improve selectivity of the analysis (Chap. 12) and ii) instruments equipped with atmospheric pressure ionization (API) methods (Chap. 11) offering higher mass range, and some 5-fold unit resolution to resolve isotopic patterns of multiply charged ions (Fig. 4.47). [149,162,172,173]... 

Provided El spectra have been measured under some sort of standard conditions (70 eV, ion source at 150-250 °C, pressure in the order of 10 " Pa), they exhibit very good reproducibility. This is not only the case for repeated measurements on the same instrument, but also between mass spectrometers having different types of mass analyzers, and/or coming from different manufacturers. This property soon led to the collection of large El mass spectral libraries, either printed [76-78] or computerized. [79] The best established El mass spectral databases are the NIST/EPA/NIH Mass Spectral Database and the Wiley/NBS Mass Spectral Database, each of them giving access to about 120,000 evaluated spectra. [80-83]... 

Whether an analyte is suitable to be analyzed by Cl depends on what particular Cl technique is to be applied. Obviously, protonating PICI will be beneficial for other compounds than CE-CI or EC. In general, most analytes accessible to El (Chap. 5.6) can be analyzed by protonating PICI, and PICI turns out to be especially useful when molecular ion peaks in El are absent or very weak. CE-CI and EC play a role where selectivity and/or very high sensitivity for a certain compound class is desired (Table 7.4). The typical mass range for Cl reaches from 80 to 1200 u. In DCI, molecules up to 2000 u are standard, but up to 6000 u may become feasible. [92]... 

For the choice of a mass analyzer to be operated with a Cl ion source the same criteria as for El apply (Chap. 5.5). As mentioned before, sufficient pumping speed at the ion source housing is a prerequisite. 

Electron impact (El) ionization is one of the most classic ionization techniques used in mass spectrometry. A glowing filament produces electrons, which are then accelerated to an energy of 70 eV. The sample is vaporized into the vacuum where gas phase molecules are bombarded with electrons. One or more electrons are removed from the molecules to form odd electron ions (M+ ) or multiply charged ions. Solids, liquids and gases can be analyzed by El, if they endure vaporization without decomposition. Therefore the range of compounds which can be analyzed by El is somewhat limited to thermally stable and volatile compounds. The coupling with gas chromatography has been well established for... 

Ferrocenyl-containing zinc complexes 64 and 65 were analyzed by El mass spectrome-try57"58 For 64, the intact M+ has been recorded at 70 eV gj mass spectra for the other heterometallic compounds, such as Zn F. containing mixed-valence Mn Fjy[n( 7... 

Gas Chromatography-Mass Spectrometry. A Finnigan 4500/Incos instrument with a 30-m X 0.32-mm i.d. capillary column coated with SP-B-5 was used. The GC parameters were as follows injector, 270 °C column oven temperature programmed, 50 °C (0.1 min, hold) 15 °C/min to 100 °C, 5 °C/min to 270 °C internal standard, anthracene-djo helium flow, 3.0 mL/min sample size, 3.0 /xL. MS conditions were as follows El, 70 eV scan (m/z), 35-650 daltons source temperature, 250 °C filament current, 0.5 A sensitivity, 10-8 A/V. (NOTE When the name of a compound is followed by (confirmed) , it means that the standard material was analyzed for confirmation under conditions identical to those of the sample when the name is followed by (tentative) , it means that the mass fragmentography showed the best fit (>80 ) based on the National Bureau of Standards [NBS] library computer search.)... 

The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9) ultrahigh mass resolution (>1,000,000 at m/z. < 200) with accurate mass measurement even 1n gas chromatography/mass spectrometry experiments sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers versatile Ion sources (electron impact (El), self-chemical ionization (self-Cl), laser desorption (LD), secondary ionization (e.g., Cs+-bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission) trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. 

Example 1.1 One of the applications of using Stokes s law to determine the particle size is the Sedigraph particle analyzer. Table El.l shows the relationship between the cumulative weight percentage of particles and the corresponding particle terminal velocities for a powder sample. The densities of the particle and the dispersing liquid are 2,200 and 745 kg/m3, respectively. The liquid viscosity is 1.156 x 10-3 kg/m s. Find out the relationship of the mass fraction distribution to the equivalent dynamic diameter. 

There are several types of ionization sources [MALDI, ESI, FAB (fast atom bombardment), PD (Cf-252 plasma desorption), El (electron ionization), Cl (chemical ionization) etc.], different types of mass analyzers [combinations of magnetic and electric sectors, quadrupolar filters (Q) and ion traps (IT), time-of-flight (TOF) and FT-ICR] and different detectors, each with its own advantages and drawbacks. We describe herein only the systems that presently have widespread use for the study of biomolecules ESI coupled to a quadrupole (or triple quadrupole, QqQ) mass analyzer or an ion trap, the MALDI source with the linear or reflectron TOF analyzer, and the FT-ICR system which can be equipped with both ESI and MALDI sources. 

In electron impact (El) mass spectra PCDTs show a strong molecular M1 ion and the expected clustering due to chlorine and sulfur isotopes. The major difference in the El mass spectra of the PCDDs and PCDTs is the formation of a strong M+-COCl in the former and a strong M+-2C1 in the latter compounds. PCDDs and PCDFs could be detected with good sensitivity via the reaction M+— (M-COCl)+ by QUAD MS/MS. Mass-analyzed ion kinetic energy (MIKE) MS/MS was found suitable for detecting the PCDTs via the reaction M+ —> (M-2Cl)+, whereas PCDDs and PCDFs were monitored by the reaction M+ —> (M-COCl)+ [31]. 

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