Charge State and Interaction with Isotopic Patterns

8 Charge State and Interaction with Isotopic Patterns 

The effect of higher charge states are worth considering. As z increases from 1 to 2, 3 etc., the numerical value of m/z is reduced by a factor of 2, 3 etc., i.e., the ion will be detected at lower m/z than the corresponding singly charged ion of the same mass. In general, the entire m/z scale is compressed by a factor of z if z 1. 

Consequently, the isotopic peaks are then located at u distance. Vice versa, the charge state is obtained from the reciprocal value of the distance between adjacent peaks, e.g., peaks at V3 u correspond to triply charged ions. The reasons for the compression of the m/z scale are discussed later (Chap. 4). 

Example Ceo - El mass spectrum, also showing an abundant doubly charged molecular ion, at m/z 360 with its isotopic peaks located at Am/z = 0.5 distance and a Ceo signal at m/z 240 of very low intensity (Fig. 3.34) [29]. The isotopic pattern remains unaffected by the charge state. As a consequence of the compressed m/z scale, the doubly charged Csg fragment ion is detected at m/z 348. 

Note Isotopic distributions remain unaffected by the charge state of an ion as far as the relative intensities are concerned. However, the distance between the peaks is reduced by thus allowing multiply charged ions to be easily distinguished from singly charged ions and their charge state to be determined. 

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