Fragments, definition libraries

To be able to use MS/MS spectra library searching for general unknown screening, it is necessary to use an automatic process, called data dependent acquisition or information-dependent acquisition, to select the parent ions of interest, totally unexpected by definition, and to dissociate them and monitor their fragments. 

The definition of fragment varies, but usually refers to molecules with a molecular weight less than 200-300 Daltons and consisting of fewer than 15-20 heavy atoms.9,10 A small ( 50 000 member) diverse library is generally sufficient for high throughput screening to yield a low affinity hit. 

Although elution time from a LC separation provides valuable information for a particular species, definitive identification of the lipid species has to be performed based on its product-ion mass spectrum. For those researchers who are using LC-MS for identification and quantification of lipid species, familiar with the fragmentation patterns of lipid classes, therefore deriving the product-ion mass spectra of individual lipid species, is very important although a few databases and/or libraries (see Chapter 5) can be used to aid the identification. Resolving complex biological lipids into a... 

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