Ionization methods, mass isotope patterns

A third source of error is associated with the fragmentation pattern caused by dissociation of the molecular ions formed in the source region of the spectrometer. Under severe conditions these processes may proceed with substantial isotopic fractionation, and this obscures the measurements of isotopic composition at the collector. To some extent careful standardization of the instrumental conditions may ensure that errors from fragmentation are systematic, and thus cancel (at least to some extent). Alternatively, softer ionization methods can be used to prevent most or all of the fragmentation. The bottom spectrum in Fig. 7.7 illustrates this approach it shows the mass spectrum of chlorobenzene obtained by photoionization. Only the parent molecular ions are observed. It should be kept in mind, however, that softer ionization usually yields smaller ion currents and consequently statistical counting errors increase. 

In order to successfully interpret a mass spectrum, we have to know about the isotopic masses and their relation to the atomic weights of the elements, about isotopic abundances and the isotopic patterns resulting therefrom and finally, about high-resolution and accurate mass measurements. These issues are closely related to each other, offer a wealth of analytical information, and are valid for any type of mass spectrometer and any ionization method employed. (The kinetic aspect of isotopic substitution are discussed in Chap. 2.9.)... 

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]... 

These same six samples were also analyszed by inductively coupled plasma-mass spectrometry (ICP-MS) to determine if the same element patterns detected in the EMPA analysis could be observed with a less labor intensive method. ICP-MS works in a similar manner as TIMS, expect the sample is dissolved and injected into a plasma flame in order to atomize and ionize the sample. Isotopes ratios can be determined in this manner, but generally the precision is less than with TIMS. 

Both the reliable determination of isotope ratios and the quantitation of trace elements generally accompanied by complete destmction of all molecular entities. Opposed to what is normally desired in organic and bio-organic mass spectrometry, ionization methods in inorganic mass spectrometry therefore use conditions which erase any molecular stmcture, i.e., inorganic mass spectrometry particularly employs high temperature plasmas to achieve atomization prior to ionization and mass analysis [27]. Furthermore, ion-neutral reactions need to be suppressed or their products destroyed in order to ensure isotopic patterns free from interferences with isobars [28,29]. 

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