Precursors selection

In the following sections some of the most typical polymeric precursors are discussed. 

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Both collisional activation (in ion traps) and post source decay (in curved field reflectron TOF analyzers) have been used successfully to obtain sequence ions from peptides prepared in situ on the sample holder. Single Dalton mass windows are advantageous for precursor selection, as are realized in ion-trap and trap-TOF configurations. Publicly available search algorithms can be used... 

With linked scan MS/MS, another analyzer is not required, but this mode gives either limited precursor selectivity with unit product-ion resolution, or unit precursor selection with poor product-ion resolution. Four-sector instruments can give very high quality data, but are costly, very space demanding, and complex. Hybrids like sector-TOF or sector-QFT are good (and cheaper) alternatives. High-energy CID MS/MS spectra are very reproducible (see Chapter 3). 

A related IMDA key step, however with exo geometry, was apphed in Mulzer s synthesis of ehsabethin A (22) [32]. According to the retrosynthetic plan (Scheme 14) the elisabethane carbon skeleton would be assembled via an IMDA cychzation of quinone 86 which would be generated by oxidation of the corresponding hydroquinoid precursor. Selective hydrogenation, base-catalyzed... 

Formation of THF from dihydrofolate (DHF) is catalyzed by the enzyme dihydrofolate reductase. DHF is made from folic acid, a vitamin that cannot be synthesized in the body, but must be taken up from exogenous sources. Most bacteria do not have a requirement for folate, because they are capable of synthesizing folate, more precisely DHF, from precursors. Selective interference with bacterial biosynthesis of THF can be achieved with sulfonamides and trimethoprim. 

The second instrument must thus scan according to a function that depends on the precursor selected in the first analyser. In practice, this can only be carried out through automatic computer control. 

TOF instruments are similar to sector instruments because of the need to have n +1 analyzers for every n stage of structural interrogation (i.e., precursor selection, excitation, and product analysis). 

Salleh WNW, Ismail AF, Matsuura T, Abdullah MS. Precursor selection and process conditions in the preparation of carbon membrane for gas separation A review. Sep PurifRev 2011 40 261-311. 

With these precursors, selective deposition generally can be achieved on metals in the presence of metal oxides under certain conditions. ... 

Mass analyzers capable of MS/MS experiments have been classified (Johnson 1990) as either tandem-in-space or tandem-in-time . Tandem-in-space experiments are performed using ion beam instruments (typically a triple quadrupole or a QqTOF) in which the ions are transported from one analyzer or other device to another, each of which is responsible for one of the precursor selection, ion fragmentation and product ion analysis functions. In contrast, ion traps conduct precursor ion selection plus fragmentation and product ion analysis by a tandem-intime sequence of events all within one spatial trapping region. 

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