Deprotonated molecules

A small percentage of deprotonated molecules indicates that the acid HA is very weak. 

The product of the desorption process is the formation of singly charged (protonated or deprotonated) molecules of the analyte, with dimensions ranging from a few hundred daltons to several hundred thousand daltons. It follows that, contrary to ESI, in the study of... 

If during the ionization the amount of energy deposited on the molecule is low, as occurs in soft techniques, i.e. Cl, ESI, DESI and MALDI, the mass spectrum is very simple. It is characterized by protonated/deprotonated molecules, and eventually few adduct ions but very few or no fragment ions. This implies that it is easy to obtain the molecular weight of the analyte under investigation, but structural information is missing. As an example, the ESI mass spectrum of a small molecule is reported in Figure 2.20. There are two main ions one at m/z 556 and another at m/z 578. As the mass spectrum has been obtained in positive... 

The DLI interface became the second commercially available LC/MS interface in 1981. The liquid eluent is introduced into the ion source through a capillary or a pinhole diaphragm. As the name implies, the analyte in DLI LC/MS is introduced from solution into the MS ion source.When sufficient energy is given to the solution, the preformed ions in solution such as protonated molecules, deprotonated molecules, cationized molecules and solvated ions can be desorbed into a mass spectrometer while the bulk solvent is vaporized and eliminated by the vacuum system. 

Fragmentation pathways of protonated and deprotonated molecules of C-glycosidic flavonoids obtained with CID tandem MS techniques have enabled important structural information to be obtained about different substitution patterns of sugars in this class of compounds.Fragment ions formed by the loss of water were more pronounced for... 

The i factor can be used to help determine the extent to which a substance is ionized in solution. For example, in dilute solution, HC1 has an i factor of 1 in toluene and 2 in water. These values suggest that HC1 retains its molecular form in toluene but is ionized in water. A weak acid that is 5% deprotonated in water would have an i factor of (0.05 X 2) + 0.95 = 1.05, because the deprotonated molecules produce two ions each. 

A small percentage of deprotonated molecules indicates that the solute consists primarily of the acid HA. The concentrations of H30+ ions and conjugate base ions must then be very low. 

Negative ions can be formed by electron capture due to the presence of thermalized electrons produced by Penning ionization or by surface Penning ionization. Negative ions can also be obtained by reactions of analyte molecules with negative ions formed from atmospheric water and oxygen to produce the deprotonated molecule. 

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. 

Scheme Representation of equilibria between Ti04 framework species and H2O2/H2O solutions a interaction with ionic species b interaction with molecular species and subsequent formation of deprotonated molecule on the Ti04. Adapted from [49] with permission. Copyright (2004) by ACS...

Ionization Mode. The first selection of ionization settings is the choice of ionization mode, i.e., positive ion mode or negative ion mode. This depends on the structures of analytes. For basic compounds (i.e., amines), a positive ion mode can be used to form a protonated or cationized molecule. For acidic compounds, a deprotonated molecule is formed in the negative ion mode. The negative ion MS offers selectivity and sensitivity since only limited... 

The term "molecular ion" by definition refers to a radical cation or anion of an intact molecule. Molecular ions are odd-electron ions, which may thus be generated by El. Unfortimately, the term molecular ion is also frequently used to indicate the even-electron ionic species produced by electrospray and APCl. This obviously is not correct. In the soft ionization techniques, predominantly even-electron protonated molecules are generated in positive-ion mode, and deprotonated molecules in negative ions. In addition, various adduct ions may be generated (Table 2.2). These all are even-electron ions, and should therefore not be referred to as molecular ions. Alternatively, the term protonated molecular ions is used, which again is incorrect one cannot protonate a radical cation ... 

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. 

Einding the optimum mobile-phase composition for RPLC of analytes that readily form protonated or deprotonated molecules in solution is not always straightforward. Next to RPLC, other approaches available for the separation of... 

The appearance of ESI mass spectra depends on the solvent conditions, especially on the extent of sodium contamination. Sodiated molecules were observed as the base peak for chlortoluron, isoproturon, diuron, linuron, and diflubenzuron [34], while protonated molecules and only weak sodiated molecules were observed for monuron, diuron, and neburon [14], Some phenylureas can also be analysed in negative-ion ESI, where deprotonated molecules as well as acetate or formate adducts can be observed, depending on the mobile-phase composition [9],... 

Chlorinated phenoxy acid (CPA) herbicides can analysed by GC-MS only after derivatization. For LC-MS, negative-ion ESI is the method of choice. Deprotonated molecules are detected as the most abundant ions under these conditions, often next to a phenolate fragment ion due to the loss of the acid side chain. Next to the deprotonated molecule, weak formic acid adducts [M+HCOO] were observed for MCPA, 2,4-D, MCPP, andMCPB [41]. 

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