Multiply charged ions

Several characteristics of molecular ions can be used to distinguish them from 

A molecular ion peak must have the highest mass of any peak in the 

Under normal operating conditions, the mean free path of the ions and molecules is great enough to preclude a significant number of bimolecular or ion-molecule collisions, and only by the transfer of atoms to a molecular ion during such a collision can a particle heavier than the molecular weight 

The converse, however, is not true The highest mass peak need not correspond to a molecular ion. 

A molecular ion may not be observed if one or more bonds are too weak to withstand the harsh ionization conditions of the electron beam. 

ESI has important characteristics for instance, it is able to produce multiply charged ions from large molecules. The formation of ions is a result of the electrochemical process and of the accumulation of charge in the droplets. The ESI current is limited by the electrochemical process that occurs at the probe tip and is sensitive to concentration rather than to total amount of sample. 

The ESI mass spectra of biological macromolecules normally correspond to a statistical distribution of consecutive peaks characteristic of multiply charged molecular ions obtained through protonation (M + zH)z+, or deprotonation (M - zll) , with minor if any contributions of ions produced by dissociations or fragmentations. However, as the measured apparent mass is actually m/z, to know m one needs to determine the number of charges z. 

Consider a positive ion with charge z whose mass-to-charge ratio is measured as being m Th, issued from a molecular ion with mass M Da to which z protons have been added. We then have 

Deconvolution of an ESI spectrum of a protein mixture. From Finnigan documentation. Reprinted, with permission. 

An ion separated from the first one by (J — 1) peaks, in increasing order of mass-to-charge ratio, has a measured ratio of m2 Th and a number of charges z - j, so that 

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Singly, doubly, triply, etc. charged ion. An atom, molecule, or molecular moiety that has gained or lost one, two, three, or more electrons. The term multiply charged ion is used to refer to ions that have gained or lost more than one electron where the number of electrons lost or gained is not designated. 

Multiply charged ions such as adsorb on an ionic surface more strongly than singly charged ions such as Cl. The addition of a solution of... 

Electrospray ionization, in contrast to the majority of other ionization methods employed in mass spectrometry, produces predominantly multiply charged ions of the intact solute molecule. This effectively extends the mass range of the mass spectrometer and allows the study of molecules with molecular weights well outside its normal range. 

Electrospray is unusual in that it produces almost exclusively multiply charged ions in a variety of different charge states. The way in which the molecular weight of an analyte may be calculated has been derived. In addition, the appearance of an electrospray spectrum may vary considerably with the conditions in the solution from which it has been generated. Eor this reason, the mechanisms leading to the production of ions using this technique have been described at some length. 

Multiply charged ion An ion with more than one charge. The electrospray spectra from compounds of high molecular weight contain exclusively multiply charged ions. 

Transformation The mathematical process of changing a raw electrospray spectrum containing a number of multiply charged ions into a mass spectrum plotted on a true mass scale. 

The main characteristics of electrospray ionisation are given in Table 6.21. Electrospray ionisation is a suitable technique for producing accurate molecular mass information on a wide range of low-MW samples. ESI-MS is particularly appealing for polar, high-MW samples (more than ca. 1000 Da), where the multiply charged ions formed have m/z values within the range of the spectrometer. However, ESI presents some problems in the identification of unknowns ... 

Main MS interferences Multiply charged ions Argides , dimers Argides , dimers Polyatomic ions... 

THERMOCHEMISTRY OF SINGLY AND MULTIPLY CHARGED IONS PRODUCED BY ELECTROSPRAY... 

As already mentioned, the experimental studies were restricted to singly charged ions. While singly charged ions are involved in the majority of the important condensed-phase ion chemistry, there are many important processes involving multiply charged ions. Thus, doubly charged ions such as Mg2+, Ca2+, Fe2+, Co2+, Ni2+, Cu2+, etc. are of paramount importance in condensed-phase chemistry and biochemistry. 

A. Charge Reduction (Charge Separation) on Desolvation of Multiply Charged Ion-Solvent Molecule Clusters... 

When a multiply charged ion solvated by several solvent molecules is subjected to heating or to vibrational excitation due to collisions with inert neutrals, ion desolvation occurs by successive loss of single solvent molecules, as illustrated below, for a double-charged ion and solvent molecules SI ... 

Equilibria Where a Neutral Molecule Is Exchanged. The difficulties discussed above for proton transfer and electron transfer equilibria involving multiply charged ions are not present when neutral molecules (ligands) which are complexed to a given ion are exchanged. Equation 43 is a typical example ... 

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