Bonded Phases - The Preparation of an Oligomeric Bonded Phase
The Preparation of an Oligomeric Bonded Phase
The apparatus design and method of operation of the fluidized bed makes it ideal for processes that are carried out in sequential synthetic steps and consequently, should be perfect for the production of oligomeric stationary phases. This specific application of the fluidized bed technique was investigated by Akapo (20) and Akapo et al. (21) who carried out a ten-stage synthesis of a reverse phase, using octylmethyldichlorsilane as the reagent. Using the appropriate sequence of reactions, this reagent could deposit a layer of octylmethylsilyl oligomers on the silica gel surface such that each oligomer was attached to a single surface hydroxyl group.
The silica is placed in the fluidizing apparatus and is first subjected to hydrothermal treatment by passing steam through the bed at 120oC for three hours. This procedure, as previously explained, will enlarge some of the pores but more importantly, seal some of the smaller pores that would not otherwise be accessible to the dichlorsilane reagent. The silica is then dried at 250oC in a dry nitrogen stream that removes the majority of the strongly bound or hydrogen bonded water from the surface. The silica is then reacted with the dichlorsilane in the same manner as that described above.
After the initial reaction is complete the residual chlorsilane is removed, the silica bed flushed with steam from the steam generator and this is continued until hydrochloric acid gas is no longer evolved. The steam treatment rapidly converts the methyloctylmono-chlorsilane groups (which have been attached to each available hydroxyl group on the silica surface) to methyloctylmono-hydroxysilane groups. The product is then dried again in dry nitrogen and treated a second time with the dichlorsilane. This treatment, firstly with the dichlorsilane and secondly with water, is repeated until the required number of aliphatic chains have been bonded to the silica. Finally, after the last group has been attached, the product is dried and reacted with trimethylchlorsilane to end cap the final methyloctylmonohydroxysilane groups that were generated on the surface. Employing this procedure, Akapo et al (21) synthesized a ten-unit methyloctyl oligomeric bonded phase.
Synthesis of Bonded Phases by Surface Cross-Linking
In contemporary LC, the most popular bonded phases are the "brush' polymeric type with C8 and C18 straight chain reverse phases. These two bonded phases my well account for over 80% of all LC analyses. However, reverse phases carrying shorter chains are also available, (e.g. the C4 and the C6 reverse phases) and their expanding popularity has an interesting origin. As a result of the increasing activity in the biotechnology field there has been a rising demand for the separation of biological macro-molecules (e.g. proteins and polypeptides. However, it was quickly found that chains longer than C4 often caused proteins to be seriously denatured during the elution process. As a consequence, there arose a demand for reverse phases having shorter chain lengths.