Ionization methods, mass adductions

Another factor to consider in the choice of ionization method is the formation of adducts with sodium or other metal ions, in addition to the protonated molecule. Although these may help confirm the molecular mass, they tend to lower the signal-to-noise ratio of the protonated molecule, resulting in higher detection limits in trace analysis and causing problems in quantitative analysis. Sodium... 

We saw that numerous ionization techniques exist that yield radical cations or radical anions, protonated or deprotonated molecules, and various adducts. These ions yield fragments with an even number of electrons (closed shell) or with an odd number of electrons (open shell). Even though the radical cations derived from electron ionization sources retain a privileged status in common mass spectrometry, the other ionization methods become increasingly common. Electron ionization is not possible for many categories of molecules. Therefore, we will not limit the discussion to radical cations. 

The molecular mass of the analyte can be determined from the m/z of the molecular ion in El, if it is observed in the mass spectmm. With a soft-ionization method, next to, or even instead of, the protonated molecule in the positive-ion mode or the deprotonated molecule in the negative-ion mode, a variety of adducts ions may occur in the mass spectrum (Table 2.2). The presence of adduct peaks can be often helpful in assigning the correct molecular mass. The use of both the positive-ion mode, resulting in m/z of the [M+H] ion, and the negative-ion mode, resulting in m/z of the [M-H] ion, if applicable, also leads to an unambiguously molecular-mass determination for an unknown compound. 

Selva and coworkers - reported on their experiences to apply various mass spectrometric techniques to the analysis of -carotene and carotenoids and their adducts formed in aqueous solution. El mass spectrometry and field desorption (FD) mass spectrometry were applied to aqueous mixtures of -carotene and /J-cyclodextrin, and the polyene was found to be detectable . Tandem mass spectrometry can be applied to identify fi-carotenone as a minor component in complex carotenoid mixtures. EI/MIKE spectrometry of the molecular ion m/z 600) was used in this case . A previous study was focused on the characterization of. vecw-carotcnoids using EI/MIKE and CID spectrometiy . The more recent ionization methods, viz. MALDI and its variant working without a matrix, laser desorption/ionization (EDI), as well as electrospray ionization (ESI) mass spectrometry, have also been applied to this topic. MALDI and LDI mass spectrometry were used to analyse mixtures of -carotene and y-cyclodextrin in aqueous solution. Adduct ions were not observed using these mctbods. ... 

Both quantitative approaches have drawbacks. For instance, the current mass spectrometric method does not distinguish between the various isomers of IsoK/LG adducts and thus provides no information on the pathway leading to adducts. Because complete proteolyis is required, it is also not possible to specifically determine the identity of the adducted protein. The method is also very time-consuming and limited to labs with electrospray ionization tandem mass spectrometers and associated expertise. In contrast to mass spectrometry methods, antibody based methods are relatively easy and inexpensive however. 

This ionization method is soft in that it does not cause the analyte molecule to fragment into multiple ions as older ionization methods often do. The use of the unfragmented, or molecular ion, greatly simplifies the resulting mass spectra, which greatly facilitates interpretation of the data. As ESI depends on preformed ions being solution, ion adducts are seen in the case of positive ion formation. In most cases these are proton (H" ") adducts, but sodium (Na" ") and potassium (K ") are seen also. This form of ionization works with protic and polar analytes that readily form protonated species such as primary amines. 

Product-ion scan The first quadrupole is set to transmit a selected precursor ion falling within a mass window of usually 1 Da. These ions enter the second quadrupole and are fragmented by collision induced dissociation. The fragment ions are mass resolved by the third quadrupole operated in the full-scan mode. The result is an electron impact-like mass spectrum obtained indirectly from a molecular-adduct ion or other characteristic ion formed in the ion source by a soft ionization method. Also known as a daughter ion scan... 

Various types of analytical information about the analyzed molecule can be obtained using mass spectrometry. The determination of the molecular weight is one of the most common goals. The observation of molecular species (molecular ions, molecular adducts, or ions formed by a loss of specific neutrals from the analyzed and ionized molecule) are often sufficient proof for the presence of the desired molecule. Accurate mass measurements of molecular species provide information about the elemental composition of ions and their precursor molecules consequently, mass spectrometry has largely replaced traditional elemental analysis. Mass spectrometry is a powerful technique for both qualitative and quantitative analyses. GC, HPLC, TEC, and CE are separation techniques compatible with mass spectrometry. When combined with such chromatographic methods, mass spectrometry becomes a unique method for the identification of submicromolar quantities of analytes. 

The formation of complexes between analytes and metal (-containing) ions is commonly observed and used in other methods of ionization. ESI mass spectra of organic compounds very often display adducts with Na and K+ ions. Solvents, glassware, and impure chemicals are common sources of these ions. Intentional doping of solutions by alkali-metal ions in ESI is widely used to improve the ionization of (non-polar) analytes, and to determine the number of mobile hydrogen atoms in molecules. 

Some synthetically important radical reactions have been studied by API-MS methods. The transient radicals were unambiguously detected and characterized by MS/MS methods. Hess and coworkers [39] reported the degradation products resulting from modified Fenton reactions with the nitroaromatic compounds trinitrotoluene (TNT) and trinitrobenzene (TNB) through electrospray ionization tandem mass spectrometry (off-line ESI-MS"). Several hydroperoxide adducts were tentatively identified as initial, one-electron reduction products of TNT, and their stmcture was confirmed by tandem mass spectrometry (Scheme 5.1). 

Owing to its compatibility with solution samples, ESI is preferred over other ionization methods in many MS fields. Applications of metal ion adducts have been reported for ESI [55-57]. For example, ESI can be used to produce alkali-metal adducts of antibiotics that do not form abundant [M+H]+ ions. Informative adducts between alkali-metal ions and peptides have been observed under a variety of conditions of electrospray ionization mass spectrometry (ESI-MS). It should be noted, however, that the presence of salt ion adducts cause the signal suppression and interference with the interpretation of the mass spectra, particularly in analytical MS of proteins and other biological molecules. 

Observations of alkali-metal ion adducts of the type [M+Li]+ [M+Na]+ etc. are common in the desorption ionization (DI) mass spectra of a variety of polar molecules. In fact, alkali-metal ion association reactions are observed with FD ionization, FAB ionization, Cf plasma desorption (PD), secondary ion mass spectrometry (SIMS), MALDI, and ESI. Ion yields can be greatly enhanced by addition of alkali-metal salts to the sample. Particularly for the MALDI analysis of synthetic polymers, metal cations are often intentionally added to enhance signals. A qualitative description of the current understanding of formation mechanism of alkali-metal ion complexes from the condensed phase was presented [75]. Knowledge of the ionization mechanisms is important and helpful from the perspective of increasing the analytical utility of the method. 

Soft ionization methods like FAB, FD, ESI, and MALDI often cause cationization by Na", K", Cs", and Ag". Especially Na", adducts are almost omnipresent. Searching a spectrum for those peak distances reveals the true molecular mass. 

Schriemer and Li" used matrix assisted laser desorbtion/ionization (MALDI) mass spectrometry to detect high molecular weight narrow polydisperse polystyrene with molecular weights up to 1.5 x 10 Daltons. The method is rapid and agrees well with results obtained by chemical methods. Retinoic acid was used as the organic matrix for the laser absorption procedure and the samples were analysed as their silver cation adducts. 

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