Mass doubly charged ions

Odd-mass doubly charged ions appear at a half-mass position. A doubly charged ion of mass 115 will appear at mk 57.5. 

Even-mass doubly charged ions are identified by the isotope peak and appear at a mass 0.5 Da greater than the P ion. For example, the doubly charged ion of mass 130 will appear at mk 65, with the isotope peak at mIz 65.5. 

The example spectrum is shown in Fig. 6.19. The precursor m/z is 741.0 Th and the charge is 2+. Brief examination of the spectrum shows that there are almost no abundant doubly charged ions. Also, the spectmm does not cover the mass range near the precursor m/z region. Therefore, we cannot begin with the same procedure as previously described. On the other hand, there are a number of very intense peaks with mass differences specific for particular amino acids. 

Doubly-charged ions exist because the potential of second ionization of many metals is relatively low with respect to the plasma thermal energy. For instance, 11.9 eV are needed to remove two electrons, in contrast with 6.1 eV for one electron, from a calcium atom. At 8000K, a little less than 0.1% of Ca would be in the Ca + form and overlap with Mg+ isotopes. Likewise, Ba is rather easily formed and overlaps with Zn+ isotopes. Isobaric interferences with doubly-charged ions are easily identified as odd-mass atoms will produce peaks at half masses, such as Ca at mass 21.5. 

Fig. 5.1. El mass spectrum of l,2,3,4,5,6-hexaliydrophenanthro[l,10,9,8-op<7ra]perylene. AH signals in the expanded inset correspond to doubly-charged ions. Adapted from Ref. [6] with permission. Elsevier Science, 2002.

Multiply charged species can be seen when using ESI. In positive ion mode, the number of charged species normally observed is determined by the number of basic sites on a molecule that can be protonated at low pH. Singly charged species will show isotopic peaks that differ by 1 mass unit doubly charged ions show isotopic peaks that differ by 0.5 mass units. 

Another way of distinguishing between the two cases is to look carefully at the separation between the masses of the two ions at, in this case, mIz 251. As seen in Figure 6.8, the doubly charged ion wiU have a separation between adjacent masses of only 0.5 Da, whereas it will be 1 Da for an MH ion. This way is simpler in some respects, but does need good instrument resolution. 

In general, a mass separation of 0.5-0.7 Da would be indicative of a doubly charged ion. Similarly, a separation of only 0.3 mass units would indicate a triply charged ion. 

The spectra of 2,3 -bipyridine, ° 2,2 6, 2"-terpyridine/ nem-ertelline (11)/° some substituted 2,3 -bipyridines/ ° and 4,4 -bipyridines ° have been briefly discussed, and the fragmentation patterns of hydroxy-and alkoxy-2,2 -bipyridines, ° 2,2 -bipyridine-5-carboxylic acid, 2,2 -bipyridine-5-sulfonic acid, and 6-halogeno-2,2 -bipyridines ° have been covered in detail. The doubly charged ion mass spectrum of 2,2 -bipyridine has recently been reported. °... 

In this section the solubilities of NaCl, CaCl2, Na2C03 and CaC03 are compared to investigate the effects of having doubly charged ions in a compound. The solubilities of the compounds are given in Table 3.17 in terms of mass % and molar concentration. Sodium and calcium cations are chosen for this comparison because they have almost identical ionic radii Na h 102 pm, Ca2+ 100 pm. 

As mentioned previously, most ions are singly charged, but double ionization does occur and this is indicated by peaks at half-mass units. These represent odd-numbered masses that carry a double charge. For example, a doubly charged ion of mass 89 gives rise to a peak at 89/2 or m/z 44.5. 

Some elements in the low mass range, such as Ce and Ba, have second ionisation potentials low enough to yield significant quantities of doubly charged ions. In general, mass resolutions between 2000 and 10000 are required to separate them. 

The singly charged peak tor an ion of relative molecular mass 10,000 would appear at m/z 10.000, while the doubly charged ion, M . would appear at miz 5000... 

The thiatriazacycl[3,3,2]azine derivatives 49 and 50a64 and selenatriazacycl[3,3,2]azines (50b)65 have been prepared in a way closely related to the synthesis of polyazacycl[3,3,3]azines described in Section V. The mass spectra are characteristic of aromatic compounds large molecular ion peaks and an abundance of doubly charged ions. Electrophilic substitution is predicted and found to occur in position 5 (49) and 6 (50a, 50b), respectively. 

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