Charged molecular ions

If we had any means of reducing the electrostatic repulsion without, at the same time, affecting the quantum-mechanical attraction, we should have the possibility of forming such doubly charged molecular ions. Now a polar solvent has just the required properties the alignment of the solvent dipoles greatly reduces the electrostatic repulsion, but the quantum-mechanical forces of attraction arise from the rapid motion of... 

Figure 4.20 Product-ion MS-MS spectrum from the doubly charged molecular ion of the peptide glu-fibrinogen B generated during electrospray ionization. From applications literature published by Thermofinnigan, Hemel Hempstead, UK, and reproduced with permission.

At what m/z value would the singly charged molecular Ion of a compound with molecular formula C284H432N84O79S7 be observed ... 

Karas M., Gluckmann M., and Schafer J. (2000), Ionization in matrix-assisted laser desorption/ionization singly charged molecular ions are the lucky survivors, J. Mass Spectrom. 35, 1-12. 

Example The El mass spectrum of Qo also shows an abundant doubly charged molecular ion, at m/z 360 with its isotopic peaks located at 0.5 u distance and a signal at m/z 240 of very low intensity (Fig. 3.28). [22] The isotopic pattern remains the same for all of them. As a consequence of the compressed m/z scale, the doubly charged fragment ion is detected at m/z 348. More examples of multiply charged ions can be found throughout the book. 

Definition The molecular ion has the same empirical formula as the corresponding neutral molecule. The neutral and its molecular ion only differ by one (or more) electron(s). A singly charged molecular ion can either be a positive radical ion, M" , or a negative radical ion, M" (not in case of El). The mass of this ion corresponds to the sum of the masses of the most abundant isotopes of the various atoms that make up the molecule (with a correction for the elec-tron(s) lost or gained. Chap. 3.1.4). [8,9]... 

Karas, M. Gluckmann, M. Schafer, J. Ionization in MALDI Singly Charged Molecular Ions Are the Lucky Survivors. J. Mass Spectrom. 2000,35, 1-12. 

Figure 11 The Isotopic Pattern of a Peptide with Singly and Doubly Charged Molecular Ions Obtained from an Electrospray Quadrupole Mass Spectrometer 31...

The purpose of the MS techniques is to detect charged molecular ions and fragments separated according to their molecular masses. Most flavonoid glycosides are polar, nonvolatile, and often thermally labile. Conventional MS ionization methods like electron impact (El) and chemical ionization (Cl) have not been suitable for MS analyses of these compounds because they require the flavonoid to be in the gas phase for ionization. To increase volatility, derivatization of the flavonoids may be performed. However, derivatization often leads to difficulties with respect to interpretation of the fragmentation patterns. Analysis of flavonoid glycosides without derivatization became possible with the introduction of desorption ionization techniques. Field desorption, which was the first technique employed for the direct analysis of polar flavonoid glycosides, has provided molecular mass data and little structural information. The technique has, however, been described as notorious for the transient... 

Generally, the tendency is to represent the molecular ion with a delocalized charge. Djerassi s (1967) approach is to localize the positive charge on either a 7r bond (except in conjugated systems), or on a heteroatom. Whether or not this concept is totally rigorous, it is at the least a pedagogic tour de force. We shall use such locally charged molecular ions in this book. 

Figure 16.20—Multiply charged molecular ions. An electrospray spectrum of horse cytochrome c, a protein of molecular weight 12360 Da is shown. Between two consecutive peaks in the molecular ion cluster, the charge state varies by one unit. The second spectrum corresponds to a high-resolution spectrum in the 772-774 m/z range. In this isotopic cluster, all ions carry the same number of charges. It is possible from either of these spectra to calculate the approximate molecular weight and the number of charges carried by the ions (spectra reprinted with permission from F. W. McLafferty et al.. Anal. Chem., 1995, 67, 3802-5. Copright 1995 American Chemical Society).

Mass spectral data have frequently been used in the structural determination of boron heterocycles. One paper has been devoted to the mass spectra of some six-membered boron-nitrogen systems. It was concluded that the spectra could be interpreted analogously to their hydrocarbon counterparts. In all cases the molecular peak was the base peak of the spectrum (68T6755). Doubly charged molecular ions, a feature typical of aromatic compounds, are often encountered. It should be noted, however, that some certainly non-aromatic aminoboranes give such doubly charged ions as well. 

Figure 3-11 Matrix-assisted laser desorption / ionization time-of-flight (MALDI-TOF) mass spectrum of bovine erythrocyte Cu-Zn superoxide dismutase averaged over ten shots with background smoothing. One-half pi of solution containing 10 pmol of the enzyme in 5 mM ammonium bicarbonate was mixed with 0.5 pi of 50 mM a-cyanohydroxycinnamic acid dissolved in 30% (v / v) of acetoni-trile-0.1% (v / v) of trifluoroacetic acid. The mixture was dried at 37° C before analysis. The spectrum shows a dimer of molecular mass of 31,388 Da, singly charged and doubly charged molecular ions at 15,716, and 7870 Da, respectively. The unidentified ion at mass 8095.6 may represent an adduct of the matrix with the doubly charged molecular ion. Courtesy of Louisa Tabatabai.

For the trifluoroacetylation of 2-substituted thiophenes, furans, and pyrroles in C2H4C12, 75°, the p values are —7.4, —10.3, and ca. —4.5, respectively.259 The value for substituted benzenes is not known. In the gas phase ionization of substituted furans, thiophenes, selenophenes, and pyrroles,264 a reaction proceeding through a positively charged molecular ion taken to be analogous to the Wheland intermediate for electrophilic substitution, the p values are reported to be —20.2, —16.5,... 

The mass spectra of benzofused pseudoazulenes 65 and 80 -83 show the mass ion as the base peak, and intense peaks are observed that correspond to the doubly charged molecular ion. The authors197 have attributed this fact to the aromaticity of these systems. This conclusion, however, cannot be drawn for all pseudoazulenes since the relative intensity of the mass ion for 26b is only 5.5%,55 for 4-methyl-4H-cyclopenta[b]quinoline 64%, 14 for 5-methyl-5/f-cyclopenta[c]quinoline7.0%,123andfor2-phenacyl-9-benzoyl-2H-indeno[2,1-fo] pyridine 18%.105... 

Today, the two most common LC/MS interfaces are atmospheric pressure ionization interfaces, electrospray (ESI) and ion spray (ISI). Electrospray (Fig. 15.8) and its subtype, nanospray, are recommended for use with proteins and highly polar or ionized compounds. They are very soft ionization, concentration-dependent techniques that result in very little fragmentation and often produce multiply charged molecular ions. 

---