Polymeric stationary-phase composition

We think, therefore, that the conformation, chain and segment mobilities in the attached macromolecules can play a significant role in the shielding behavior of the polymeric stationary phase as well as in the processes of its formation of complexes with solutes. Obviously, the chromatographic studies relevant to composite supports suffer from a lack of information on the structure of the attached polymer. Nevertheless, we will attempt to point out some relevant data from independent studies on polymer adsorption and/or graft polymerization. 

The polymeric stationary phases with various contents of styrene and butadiene were characterized by gas chromatography. Physical blends of statistical and grafted copolymers and block copolymers were studied. It was found that the composition of the stationary phase can be determined from measured retention volumes, making use of a previously drawn standardization curve however one cannot distinguish between the blends of block or grafted copolymers of the same composition. 

FIGURE 9.20 Effect of mobile phase composition on shape selectivity with a polymeric octadecyl-polysiloxane stationary phase, column using (a) SRM 869a (b) triphenylene/o-terphenyl (c) chrysene/benzo[a]anthracene with column outlet pressure 20.0 MPa and flow rate 1 mL/min at pump head. (Reprinted from J. W. Coym, 1. G Dorsey,... 

The analysis of a technical poly(ethylene oxide) with respect to chemical composition and degree of polymerization has been performed by Pasch and Hiller [210]. This investigation was conducted under conditions which are common for HPLC separations, i.e. sufficiently high flow rate, moderate sample com-centration, and on-flow detection. Using an octadecyl-modified silica gel as the stationary phase and an eluent of acetonitrile/deuterium oxide 50 50 (v/v), the sample was separated into different functionality fractions (see Fig. 38). The major fraction of the sample eluting between 14 and 25 min exhibited a partial oligomer separation. 

Polymer libraries are covered according to their numerous applications, each described through a specific example. The reported examples include libraries of copolymers as liquid/solid supports with different compositions, libraries of biodegradable materials for clinical applications, libraries of stationary phases for GC/LC separations, libraries of polymeric reagents or catalysts, libraries of artificial polymeric receptors or molecularly imprinted polymers, and libraries of polymeric biosensors. The opportunities that could arise in the near future from novel applications of polymer libraries are also briefly discussed. 

A typical mobile-phase composition is an acetonitrile-water gradient with a fixed concentration of trifluoroacetic acid (TFA), formic, or acetic acid (typically 0.05-0.5%). TFA acts as an ion-pairing agent and masks secondary interactions with the silica-based stationary phase. TFA may significantly suppress the ESI response in positive-ion mode. To avoid this, either formic acid is preferred or a mixture of 0.02% TFA and 0.5% acetic acid can be used. Some silica-based RPLC materials can be used with a lower TFA concentration (PepMap ). Alternatively, poly(styrene-divinylbenzene) polymeric materials (PS-DVB) can be applied. With a monolithic PS-DVB column, only a small decrease in separation efficiency on the monolithic column was observed when the TFA concentration was reduced from 0.2%to0.05%[51]. 

Figure 2.9 Structure of polysiloxanes (silicones), and polyethylene glycols. An inventory of all the compositions of this type of phase, used either for impregnation or bonding, would be lengthy. Treatment of the internal wall of a silica column with tetradimethylsiloxane will obtain a stationary phase bounded, polymerized and later reticulated. (The bonding resembles the fixing of indelible colours in order to create a brightly tinted fabric the colour contains an active site with which is able to attach itself, for example, to the alcohol functionality of cellulose on cotton fibres).

Fig. 3. Elution of polystyrene as a function of the degree of polymerization P and the eluent composition stationary phase silica gel mobile phase chloroform-carbon tetrachloride. (From [8] with permission)...

Over the years thousands of substances have been used as stationary phases. For several reasons most of these have been abandoned in favor of a small number of liquids and adsorbents with favorable thermal stability and kinetic properties, complementary selectivity, reasonably well-defined and reproducible chemical composition, and if used in WCOT columns, the possibility of immobilization. Practical considerations dictate that liquid stationary phases should be inert, of low vapor pressure, have good coating characteristics, and have reasonable solubility in some common volatile organic solvent. The desirability of a wide temperature operating range tends to dictate that most common stationary phases are polymeric materials, although polymers are more likely to show greater composition variation than stoichiometric compounds. 

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