Ionization methods, mass cationic species

A little recognized systematic error in the calculation of accurate masses of, for example, small radical cation molecular ions (as in electron ionization (El)) or protonated molecular ions (as seen in the soft ionization methods) is the fact that the electron has a small, but finite mass. The accurate masses of radical cations, in which a valence electron has been removed, of anions that have been created by capture of an electron, and of protonated species produced by soft ionization processes, should take into consideration this small mass difference [19]. For example, there is a small difference between the relative atomic mass of a neutral hydrogen atom and a proton. The accepted accurate mass of 1H° is 1.007825 Da. The accurate mass of 1H+ is 1.007276 Da. To be completely correct, expected accurate masses of protonated molecular ions, [M+H]+, produced by electrospray should be calculated using the mass of one H+, rather than all of neutral hydrogen atoms. Mamer and Lesimple do acknowledge, however, that, for large molecules, the error is of little consequence. 

Whatever the ionization method used, the mass spectra of oligosaccharides analysed by ESI, MALDI or FAB display intense ions of the molecular species resulting from protonation (M -(- H)+ or cationization by an alkali metal ion (M + alkali metal)"1" in the positive ion mode or from deprotonation (M — II) in the negative ion mode. In ESI, multiply charged ions also are produced. 

Ziegler-Natta polymerization [241,242] is an important method of vinyl polymerization because it allows synthesis of polymers of specific tacticity. As reported by Santos and Metzger, Ziegler-Natta polymerization can be carried out in a flow microreactor system coupled directly to the electrospray ionization (ESI) source of a quadrapole time-of-flight (Q-TOF) mass spectrometer (Fig. 37) [243]. In the first micromixer (Ml), a catalyst (CP2Z1O2/MAO) and a mmiomer solution are mixed continuously to iiutiate the polymerization. The polymerization occurs in a microtube reactor. The solution thus obtained is introduced to the second micromixer (M2), where the polymerization is quenched by acetonitrile. The quenched solution is fed directly into the ESI source. The transient cationic species... 

Fig. 6.2 Schematic of an Aerodyne aerosol mass spectrometer (AMS). Vaporized aerosol species are ionized and analyzed via mass spectrometry. This figure shows the ion attachment version of ionization methods. Other existing versions of the AMS that utilize several unique ionization methods and developments that are not shown in this schematic are discussed in the text. Extended drawing of flash vaporizer system shows that the particle beam first impacts on a vaporizer, and volatile aerosol components that vaporize are subsequently subjected to cationization. The unique feature of this detection scheme is the fact that a vaporizer is directly coupled into an ion attachment technique to enable a two-step particle vaporization and ionization process. The separation of the vaporization and ionization processes allows for quantitative detection of aerosol mass with the AMS. (Reprinted with permission from Ref [8]. 2007, John Wiley and Sons)...

Mass spectrometry (MS) in its various forms, and with various procedures for vaporization and ionization, contributes to the identification and characterization of complex species by their isotopomer pattern of the intact ions (usually cation) and by their fragmentation pattern. Upon ionization by the rough electron impact (El) the molecular peak often does not appear, in contrast to the more gentle field desorption (FD) or fast-atom bombardment (FAB) techniques. An even more gentle way is provided by the electrospray (ES) method, which allows all ionic species (optionally cationic or anionic) present in solution to be detected. Descriptions of ESMS and its application to selected problems are published 45-47 also a representative application of this method in a study of phosphine-mercury complexes in solution is reported.48... 

In mass spectrometry, a small sample of a compound is introduced into an instrument called a mass spectrometer, where it is vaporized and then ionized as a result of an electron s being removed from each molecule. Ionization can be accomplished in several ways. The most common method bombards the vaporized molecules with a beam of high-energy electrons. The energy of the electron beam can be varied, but a beam of about 70 electron volts (eV) is commonly used. When the electron beam hits a molecule, it knocks out an electron, producing a molecular ion, which is a radical cation—a species with an unpaired electron and a positive charge. 

Ionization of the analyte is the first crucial and challenging step in the analysis of any class of compounds by mass spectrometry. The key to a successful mass spectrometric experiment lies to a large extent in the approach to converting a neutral compound to a gas-phase ionic species. A wide variety of ionization techniques have become available over the years, but none has universal appeal. In some techniques, ionization is performed by ejection or capture of an electron by an analyte to produce a radical cation [M+ ] or anion [M ], respectively. In others, a proton is added or subtracted to yield [M - - H]+ or [M — H] ions, respectively. The adduction with alkali metal cations (e.g., Na+ and K+) and anions (e.g., Cl ) is also observed in some methods. The choice of a particular method is dictated largely by the nature of the sample under investigation and the type of information desired. Table 2.1 lists some of the methods currently in vogue. Some methods are applicable to the atomic species, whereas others are suitable for molecular species. Also, some methods require sample molecules to be present in the ion source as gas-phase species, whereas others can accommodate condensed-phase samples. The methods that are applicable to molecular species are the subject of the present chapter those applicable to atomic species are described in Chapter 7. 

In the latel980s, Fuji et al. developed a novel method [10, 11] for the detection of radical species in the gas phase by using Li+ ion attachment. It is called lAMS, which is a technique where a sample is ionized by a primary ion in an ion-molecule association reaction. This approach is similar to ion association (cationization) for detection of stable molecitles [2, 3]. They explore the mechanism of alkali-metal ion/molecitle association reaction and develop both the instrumentation for the attachment to free radical intermediate and to other species (which are sometimes not found imder ordinary conditions). The newly developed instrumentation exhibits several advantages over conventional mass spectrometers [12,13]. Section 5.2 details lAMS, smnmarizing analytical applications. 

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