Isomeric Differentiation Using Tandem Mass Spectrometry

2 Isomeric Differentiation Using Tandem Mass Spectrometry 

It is important to note that the principal fragment ions generated during sequential MS experiments within an ion trap should not be confused with fragment ions generated in the ionization source. For example, whilst the functional groups at C-16 were the most labile under MS (ion trap) conditions, the principal fragmentation within the APCI source involves losses from the C-8 position [57,58]. 

3 Novel Diterpene Alkaloid Identification Application of Tandem Mass Spectrometry 

The MS/MS product ion spectrum of m/z 637 (i.e. the MS/MS/MS of m/z 697) and the subsequent MS/MS/MS product ion spectrum of m/z 637 (i.e., the MS of m/z 697) indicated two additional methoxy groups at C-1 and C-6. The 1,6-di-methoxy, 14-hydroxy, 16-acetate compound was given the name bearline [56]. The unidentified alkaloid at m/z 739 corresponded to the 14-0-acetyl derivative of bearline. The MS/ MS/MS product ion spectra (m/z 739— m/z 679 m/z 647 m/z 615) were analogs to bearline. Subsequently, both compounds were isolated and their structural identities were confirmed using NMR spectroscopy [68]. 

The increasing accessibility of bench-top LC-MS systems to researchers of all disciplines, combined with the tandem and high-resolution mass spectrometry capabilities of such instruments, will only increase the number of applications to which LC-MS can be directed. The examples documented in this chapter illustrate some of the diversity and power of the techniques, including analytical applications for known analytes in various matrices, metabolomic analysis, the tentative structural identification of novel compounds, and the screening of extracts for minor, and perhaps novel, components of the alkaloidal profile of plants. 

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Hulst, A.G. and Kientz, C.E. (1996) Differentiation between the isomeric amino acids leucine and isoleucine using low-energy collision-induced dissociation tandem mass spectrometry. J. Mass Spectrom., 31 (10), 1188-90. 

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