Multiply charged ions isotopic patterns

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. 

With external ion sources it became feasible to interface any ionization method to the QIT mass analyzer. [171] However, commercial QITs are chiefly offered for two fields of applications i) GC-MS systems with El and Cl, because they are either inexpensive or capable of MS/MS to improve selectivity of the analysis (Chap. 12) and ii) instruments equipped with atmospheric pressure ionization (API) methods (Chap. 11) offering higher mass range, and some 5-fold unit resolution to resolve isotopic patterns of multiply charged ions (Fig. 4.47). [149,162,172,173]... 

The most effective technique to deal with complex spectra due to multiply charged ions is to achieve the full separation between signals corresponding to different charge states and to resolve their isotopic patterns. Beyond a molecular weight of about 2000 u this requires high-resolving mass analyzers. 

ESI mass spectrum showing the charge state of a multiply charged ion of recombinant human insulin (miz 1162) directly by the resolution (at M/AM < 20,000) of the isotope peaks on an FT-ICR instrument. Bottom trace measured spectrum top trace predicted isotope pattern. (Courtesy of W. J. Simonsick, Jr., Du Pont Marshall Laboratory, Philadelphia.)... 

Confidence in elemental assignment can be assured by comparison of the intensities of signals that can be ascribed to other isotopes of the element of interest, i.e. these intensity ratios should match the natural abundances isotopes listed in Appendix A.2, unless, of course, isotopic enrichment of one form or another has occurred (as is the case in nuclear reactions, extra-terrestrial samples, and so on) or has been applied (isotopic implantation). In the case of mono-isotopic elements such as Fluorine and Aluminum, molecular patterns, or even multiply charged ions, can be used to confirm assigmnents. 

---