FAB-MS of Ionic Analytes

FAB is well suited for the analysis of ionic analytes. In positive-ion mode, the spectrum is usually dominated by the cationic species, C which is accompanied by cluster ions of the general composition [CnH-An-i] . Thus, the distance between these signals corresponds to the complete salt CA, i.e., yields its molecular weight. This behavior is perfectly analogous to FD (Chap. 8.5.3). In negative-ion FAB, the anion A will cause the base peak of the spectrum, and accordingly, cluster ions of the type [Cn.iH-AJ are formed in addition. Consequently, both cation and anion are usually identified from the same FAB spectrum, irrespective of the chosen polarity. Nonetheless, it is common practice to select the polarity of the more interesting ion for the measurement. 

Provided the salt is sufficiently soluble in the matrix, the signals normally exhibit high intensity as compared to those of the matrix. This result is consistent with the model of preformed ions in solution that only need to be desorbed into the gas phase. 

Example The positive-ion FAB spectrum of an immonium perchlorate, [Ci HieN] CIO4 , [97] dissolved in NBA is dominated by the immonium ion (C ) at m/z 198 (Fig. 9.10). The perchlorate counterion can well be identified from the cluster ions [2Ch-A], m/z 495, and [3Ch-2A], m/z 792. The signal at m/z 495 is shown expanded to demonstrate that chlorine is readily recognized from its isotopic pattern (Chap. 3.2.3). 

Example The Bunte salt [ 3( 2)158-803] Na yields a very useful negative-ion FAB spectrum from NBA matrix (Fig. 9.11). NBA forms [Ma-H] and Ma ions. The salt anion contributes the base peak at m/z 337.3. [Ch-2A] m/z 697.5, and [2C-I-3A] m/z 1057.6, cluster ions are observed in addition, their isotopic patterns being in good agreement with theoretical expectation. It is noteworthy that the matrix adduct at m/z 513.4 is a negative radical ion. 

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