Cationized molecular ions

By intentionally adding inorganic salts to the solution used for FD, cationated molecular ions can be produced in abundance. Equation 5.3 illustrates how addition of NaCl can give rise to [M + Na] ions. 

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. 

Ionization Methods/Processes. The recent development of several new ionization methods in mass spectrometry has significantly improved the capability for the analysis of nonvolatile and thermally labile molecules [18-23]. Several of these methods (e.g., field desorption (FD), Californiun-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser-desorption (LD), SIMS, and fast atom bombardment (FAB) or liquid SIMS) desorb and ionize molecules directly from the solid state, thereby reducing the chance of thermal degradation. Although these methods employ fundamentally different excitation sources, similarities in their mass spectra, such as, the appearance of protonated, deprotonated, and/or cationized molecular ions, suggest a related ionization process. 

The second process involves the formation of protonated or cationized molecular ions, i.e., [M+H]+ or [M+C]+, where the cationizing species C is usually a metal ion from the substrate, matrix, or an impurity. The basic fragmentation process... 

The LD spectrum of the tridecapeptlde neurotensin (Figure 4), shows several differences. Here fragmentation results almost exclusively from small neutral losses. A protonated molecular ion is still one of the major ions observed (m/z = 1673) and some cationized (K+) fragment species are apparent as well as a small amount of cationized molecular ion (<5%). Resolution has begun to be degraded to some extent, partially due to the decrease in overall ion signal which necessitates transformation of fewer data points to maximize signal to noise ratio. 

Methods have been developed for the analysis of hydrocarbon polymers (e.g. styrene, butadiene and isoprene) by MALDI-TOF-MS, through the attachment of Ag(acac) to matrices of tran5-3-indoleacrylic acid or l,4-bis(2-(5-phenyloxazolyl))benzene . SUver-cationized molecular ions were produced for polymers of styrene, butadiene and isoprene up to mass 125,000 Da. For lower-mass styrene polymers, the resolved oligomer molecular ions provide information concerning the end group. This technique permits the analysis of many commercially important materials such as acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile, styrene-methyl methacrylate and styrene-isoprene copolymers. The use of the salts of transition metals other than Ag, Cu or Pd as the cationizing agents fails to cationize polystyrenes in MALDI. The ability of MALDI to reduce metals to the oxidation state 4-1 is critically important to polystyrene cationization, as without this reduction MALDI tends to fail to form polystyrene-metal cations. Cu(acac)2 was used for the verification of the above . 

The rapid vaporization of macromolecules from biological samples prevents, or minimizes, their dissociation and fragmentation and thus provides a way to obtain molecular and structural information. Also, MALDI provides a way of producing different types of cationized molecular ions with LP, NaL CuL and Ag+, in addition to the protonated species usually formed in atmospheric pressure ionization processes. The most serious drawbacks of using MALDI are the price of a suitable laser and the complexity and limited applicability to certain types of solid samples. Although early studies were made with MALDI-MS with the ion source region under vacuum, the demonstration of atmospheric pressure MALDI with MS encouraged the development of MALDI-IMS. 

Cationized molecular ions can easily be formed through the addition of a few microliters of a dilute alkali halide solution to the sample/matrix solution on the FAB probe tip. Addition of lithium, sodium or potassium produces [M -b cation] ions with corresponding shifts of 6,22 and 38 daltons compared with the mass of the protonated molecular ion [M-bH]. In some samples higher mass species are observed, owing to the incorporation of additional cations through cation/ hydrogen exchange reactions. 

Bombick et al. [3] presented a simple, low cost method for producing thermal potassium metal ions for use as Cl reagents. All studies were performed on a commercial gas chromatography-mass spectrometiy (GC-MS) system. Thermionic emitters of a mixture of silica gel and potassium salts were mounted on a fabricated probe assembly and inserted into the Cl volume of the ion source through the direct insertion probe inlet. Since adduct ions (also referred to as cationized molecular ions or pseudomolecular ion ) of the type (M + K)+ have been observed, molecular weight information is easily obtained. The method is adaptable to any mass spectrometer with a Cl source and direct inlet probe (DIP). In addition, the technique is compatible with chromatographic inlet systems, i.e., GC-MS modes, which will provide additional dimensions of mass spectral information. 

Rapid-heating (or flash evaporation) techniques have been explored with ionization methods such as El and Cl in the analysis of thermally labile compounds [5,6], Their principles are based upon that if the analyte is rapidly heated, intact molecules may evaporate before decomposition takes place. Davis et al. reports that, by this technique, alkali metal attachment (cationization) of sodium benzoate and sodium acetate occirrs, giving rise to cationized molecular ions (e.g., [M + Na]+) and other cluster ions similar to those produced by desorption ionization processes [7, 8], In these ejqreriments, the molecular ions were formed by electron impact of salt molecules or clusters of sample molecules and salts in the gas phase. 

ESI can be considered a complementary method to MALDI. As with MALDI, electrospray ionization of biomolecules yields protonated or cationized molecular ions with little or no fragmentation, and it is also referred to as a soft ionization source. A particular advantage of ESI compared to MALDI is that the analyte is sampled from the solution phase. Under these conditions, ESI is readily coupled to high performance liquid chromatography (HPLC) or capillary electrophoresis (CE) separation systems. Such combinations permit online LC-MS or CE-MS experiments. 

Nonpolar organic-soluble polymers, such as hydrocarbons, have been addressed in a variety of ways. Bahr et al. " analyzed polystyrene (PS) with nitrophenyloctylether doped with silver trifluoroacetate to produce silver-cationized molecular ions. Danis et al. used silver acetoacetonate with the matrices lAA or l,4-di-(2-(5-phenyloxazolyl))benzene (POPOP) for the analysis of various hydrocarbons, PS, polybutadiene (PBD), and cis-... 

Fast atom bombardment. FAB mass spectrometry of fatty acid alkanolamides shows only protonated and cationized molecular ions in the positive ion mode. Fatty acid impurities are seen in the negative ion mode (29,85). 

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