Singly Charged Molecules

Singly charged ions encompass radical ions, protonated/deprotonated molecules, products of alkali ion additions, or complex ions with other charge carriers. In the case of singly charged radical ions, the molecular weight of an analyte molecule approximately equals to the m/z value of that ion (one electron affects the measurement by only 0.00055 u). In the case of protonated or deprotonated molecules, the m/z values are expressed as m -I- 1 or m - 1, respectively. Alkali metal adducts are also commonly observed in MS for example, m -l- 23 (sodium adducts) or m -i- 39 (potassium adducts). The alkali ions are mostly contaminants, which are very difficult to remove from sample vials, solvents, or sample plates. However, some analytes such as carbohydrates can only be ionized by association with alkali ions [5,6]. 

The isotopic distribution of ions can be predicted based on the empirical formula and natural abundances of every isotope (listed in Table 9.1). The mass of a species is the sum of the mass of every isotope, whereas the abundance is the product of the natural abundance of every isotope multiplied by the number of all possible structural combinations ( ) of the given molecule. For example, the monoisotopic mass of a molecule with an atomic composition of is  

Consider protonated arginine ([C6Hi5N402] ) as an example its monoisotopic mass is 175.122 Da while the natural abundance of its most abundant isotopic ion is 91.7%. The second most abundant isotopic ion of arginine is [ C C5Hi5N402] , with one of the (12.000 Da) atoms replaced by (13.003 Da). It has a molecular weight of  

Notably, the abundance of this isotopic ion is 6.1% of the most abundant isotopic ion  

The factor of 6 in the last term is the number of all possible carbon atom positions which can be replaced by C. The third most abundant isotopic form of protonated arginine is [ C6Hi5 N N402], which has a mass of 176.119 and an abundance of 1.5% that of the most abundant isotope. Notably, most mass spectrometers cannot resolve 176.125 and 176.119, resulting in a single combined feature near 176.122 and an abundance of roughly 7.6%. Abundances of the other isotopic ions can be estimated in the same way. 

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For all three compounds, the captured electron(s) become distributed over all the H atoms, which each takes up about 0.10 electrons in the singly charged molecules and 0.2 electrons in the doubly charged molecules (Mulliken values). This systematic uptake of electrons occurs for all H atoms in all the isomers and is little dependent on die local environment around the H atom. It is interesting to note that the B-H and C-H distances are not drastically affected by the electron uptake they increase by between 0.01 and 0.03 A. 

Another example of the usefulness of microcalorimetry to study the solid-liquid interface has been reported by Draoui et al. [118]. They analyzed the adsorption of Paraquat on different minerals, which are part of the soil, to study the retention process of this pesticide by soils. In the particular case of silica they concluded that the main driving force for adsorption is electrostatic. They found a linear decrease of the adsorption enthalpy in the range of -25 to -20 kJ/mol. Nevertheless, the Paraquat molecule has two charges the heat evolved during adsorption is of the same order as in the case of other single-charged molecules. This fact is explained assuming that the... 

In the case of the singly charged molecule, the middle of the molecule is almost undimerized, with a bond-length alternation of only 0.006 A. For the doubly-doped DP4 molecule, two reversals of the bond alternation pattern clearly are observed. As in the case of DP7 doped with two sodium atoms, the bond-length alternation pattern is fully reversed between the two solitons, with respect to the undoped situation. 

A possible scheme for the evolution of the electronic structure of DP4 upon succesive sodium doping is as follows. At the first doping stages, mainly singly charged molecules are... 

For a singly charged molecule with a molecular weight of 100 u, moving inside a 7 T magnet,/c can be calculated using Equation 3.34 q in coulombs B in teslas m in kilograms) ... 

High mass resolution is important for detection of isotopic distributions (Fig. 2.7a). A majority of polyatomic compounds in biological specimens are composed of different isotopes of the same combination of atoms. For a singly charged molecule, a set of signals (peaks) differing by 1 m/z unit will appear in the mass spectra. The first peak in the distribution corresponds to a... 

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