Repeller-induced fragmentation

Usually little structural information is immediately available and repeller-induced fragmentation or MS-MS is required to generate this. Spectra generated by repeller-induced fragmentation, because fragmentation of all ionic species generated in the interface occurs, are often difficult to interpret. 

Normally, the repeller was operated at 225 V in the positive ion mode and 0 V in the negative ion mode. Single quadrupole repeller-induced fragmentation spectra were obtained when this repeller was operated at 400 V and inserted 4 mm into the ion chamber. 

Since two ions per compound at most are generated in the positive ion mode, little structural information is obtained from these spectra. Consequently, three options to the single quadrupole positive ion thermospray mode were tested (1) NCI initiated by discharge (2) daughter ion CAD in the MS/MS mode of the (M+NH4)+ ions and (3) repeller-induced fragmentation in the positive ion single quadrupole mode. 

Table IV presents the repeller-induced fragmentation mass spectra for eight acid herbicides. Again, possible ions are assigned for those peaks greater than IS percent relative abundance. The limits of detection for the herbicides in this mode were approximately 1-5 ng, while similar limits under MS/MS CAD conditions were approximately 250-1000 ng.

TABLE IV. Positive Ion Repeller-induced Fragmentation Spectra of Chlorinated Acid Herbicides... 

The two examples discussed show the bioanalytical applicability of LC/MS, both in qualitative analysis and in target compound analysis. Although the moving belt interface owing to its El capabilities is ideally suited for the identification of unknown compounds and is used often for that purpose, identification problems can sometimes also be solved with thermospray LC/MS, for instance by applying repeller-induced fragmentation, and LC/MS/MS. The PSS approach is an example that indicates that LC/MS still can be improved. A more elaborate discussion on the so-called multidimensional approaches in LC/MS is given elsewhere (14). 

Schematic diagram of a thermospray ion. source. This source, of current design, also incorporates (a) a discharge electrode so that the source can be operated in plasmaspray mode and (h) a repeller electrode to induce fragmentation. The vaporizer itself can be used as a discharge electrode.

Schematic diagram of a thermospray ion. source. This source, of current design, also incorporates (a) a discharge electrode so that the source can be operated in plasmaspray mode and (b) a repeller electrode to induce fragmentation. The vaporizer -4tself can be used as a discharge electrode.----------------------------------------------------------------------...

Fragmentation occurs because the repeller voltage increases the kinetic energy of the ions, not only making collision-induced dissociation (CID) more likely but also allowing endothermic ion-molecule and solvent-switching reactions to occur. 

The TSP/MS performance, the repeller potential and the vaporizer temperature have been optimized with standard solutions of nitrazepam and aminonitrazepam in the usual way described above. In the TSP spectrum of aminonitrazepam three peaks appear the protonated molecule at m/z=252, a fragment peak at m/z=222, which is due to the loss of formaldehyde, and a fragment peak at m/z=213, which is most probably a thermally induced hydrolysis product. About 100 ng per injection (20 yl) was necessary to obtain a spectrum with a satisfactory signal-to-noise ratio. 

The general lack of fragmentation under thermospray conditions has led to the more extensive application of MS/MS instrumentation as well as to research into the possibilities of collision-induced dissociation of ions in the ion source by means of high voltages on the repeller electrode. The latter showed nice perspectives in fundamental studies, with mass spectra quite similar to those observed in MS/MS but they proved to lead to a signal reduction that was too large for successful use in real-life applications. 

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