Adsorption oligomers

There are different reasons to discard a column a column can be damaged by irreversible adsorption of reactive polymer samples. Small amounts of styrene oligomers are known to permanently elute from styrene-divinylbenzene materials with tetrahydrofuran as the eluent, which means a continuous shear degradation of the separation material and consequently a decrease of the packing quality this observation is very important if fractions are collected and used for further analyses, e.g., for the determination of infrared (IR) spectra. One can presume that similar effects are present with other organic materials too. 

Transition metal colloids can also be prevented from agglomeration by polymers or oligomers [27,30,42,43]. The adsorption of these molecules at the surface of the particles provides a protective layer. In the interparticle space, the mobility of adsorbed molecules should be reduced decreasing the entropy and thus increasing the free energy (Fig. 2). 

Surfactants are separated according to adsorption or partitioning differences with a polar stationary phase in NPLC. This retention of the polar surfactant moiety allows for the separation of the ethylene oxide distribution. Of all the NPLC packings that have been utilized to separate nonionic surfactants, the aminopropyl-bonded stationary phases have been shown to give the best resolution (Jandera et al., 1990). The separation of the octylphenol ethoxylate oligomers on an amino silica column is shown in Fig. 18.4. Similar to the capabilities of CE for ionic surfactants, the ethylene oxide distribution can be quantitatively determined by NPLC if identity and response factors for each oligomer are known. 

CITP employs two buffers in which the analyte zone is enclosed between. Either anions or cations can be analyzed in sharply separated zones. In addition, the analyte concentrations are the same in each zone thus, the length of each zone is proportional to the amount of the particular analyte. And finally, CGE, which is analogous to gel filtration, uses gel-tilled capillaries to separate molecules on the basis of relative differences in their respective molecular weight or molecular size. It was first used for the separation of proteins, peptides, and oligomers. Advantages of the gel include decreasing the electro-osmotic flow and also reducing the adsorption of protein onto the inner wall of the capillary (von Brocke et al. 2001). 

Later, the method of liquid adsorption chromatography, both in the isocratic and the gradient separation modes, was applied to investigate the FTD of oligomers, mainly of the first and the third types, with —OH, —COOH, —C=N and —SH functional groups. 

In general, it should be stated that for very long molecules the adsorption proceeds as a phase transition. Therefore, in order to work with long molecules (M > 105) at critical conditions, careful chromatographic experiments are necessary. In the case of oligomers, no such problems arise. 

Depending on the fit, the number and strength of contacts, and other factors, a monomer-oligomer equilibrium will exist. For example, in the case of Concanavalin A the tetramer is in equilibrium with dimer and monomer under normal conditions. In the case of insulin the monomer, dimer, and hexamer are all in equilibrium. In hemoglobin the dimer and tetramer are in equilibrium. In the case of the adsorption of such proteins, one must not only know what is the aggregation state of the protein in solution, but must be able to deduce the adsorbed state. Do the molecules adsorb as dimers 21) Do they adsorb as dimers and then associate as tetramers or hexamers on the surface, etc. ... 

There are numerous examples for isocratic oligomer separations by molar mass on polar adsorbents, i.e., under the conditions of adsorption chromatography. One of the first examples reported 14) is shown in Fig. 3. 

The ability of separating species differing in configuration at only one in 50 C atoms is a fine demonstration of the separation power of adsorption chromatography. Oligomers from methyl methacrylate with OH endgroups were reasonably retained... 

APMS and AAPS isotherms was similar. It was observed that the precision of a single adsorption experiment is very good, but the reproducibility of the entire isotherm is poor. The lack of reproducibility is again probably due to the wide variety of reactions undergone by the silanes. The partial formation of oligomers also makes the hydrolysis and adsorption process rather complicated. 

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