MALDI double-focusing

Exact mass measurement at high resolution is an important tool along with other spectroscopic methods to help confirm the structure of novel flavonoids. It is used as structural proof when elemental analysis is not possible, e.g., when studying minor components. When EI-MS can be used, 1 to 10 pmol samples are required for one measurement however, when FAB-MS is used, 0.1 to 1 nmol is normally required. The use of ESI on a double focusing mass spectrometers and MALDI-TOF-MS requires smaller amounts of sample, and subpi-comole amounts of flavonoids may be adequate. 

The main advantage of the FAB technique compared to ESI and MALDI is, in fact, not the ionization method itself but the type of mass analyzer it works with. FAB ion sources are usually attached to magnetic sector, double-focusing mass spectrometers, enabling accurate mass measurements and, which is even more important, allowing the study of high-energy collision-induced dissociation (CID) reactions which provide information about the structure of the studied ion (see next section). 

These types of detectors have a limited resolving power in discriminating between small mass differences. In such cases, double focusing mass spectrometers are used. Fourier-transform ion cyclotron resonance (FTICR) analyzers interfaced with ESI or MALDI are used in the high molecular weight polymer characterization [44]. 

Double-focusing magnetic sector instruments represent the standard in FAB-MS, because they combine a suitable mass range with the ability to perform high-resolution and accurate mass measurements. Until the advent of ESI and MALDI, FAB-MS/MS on magnetic four-sector instruments was the method of choice for... 

The early appUcations of MALDI for pharmaceutical compounds have focused primarily on the characterization of compounds not amenable to ESI-MS determinations. MALDI has been utilized for the simple identification and accurate mass determination of the antihypertensive agent prazosine and its synthetic analogs [98], the antimalarial double deuterium-labeled ferrochloroquine [99], and a glucuronide metaboUte of an HI antihistamine, dimethindene [100]. 

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