Polymer choice, effect selectivity

A composite polymer membrane has also been used as an effective amperometric detector for ion exchange chromatography [42] and showed detection limits similar to those obtained with a conductivity detector. An advantage of the amperometric detector based on micro-ITIES over the conductometric detector is that selectively can be tailored by proper choice of the ionophore. For instance, the selectivity of the membrane toward ammonium in the presence of an excess of sodium could be substantially increased by introducing an ammonium-selective ionophore (such as valinomycin) in the gel membrane [42]. 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

Theta solvents. Selection of a poor solvent for a polymer is desirable when making solution property measurements because it permits the use of higher concentrations and minimizes the effects of nonideality. The most suitable choice is a theta solvent (73). Table 12 lists the theta solvents and the corresponding theta temperatures which have been found for PTHF. 

Both organic and inorganic polymer materials have been used as solid supports of indicator dyes in the development of optical sensors for (bio)chemical species. It is known that the choice of solid support and immobilization procedure have significant effects on the performance of the optical sensors (optodes) in terms of selectivity, sensitivity, dynamic range, calibration, response time and (photo)stability. Immobilization of dyes is, therefore, an essential step in the fabrication of many optical chemical sensors and biosensors. Typically, the indicator molecules have been immobilized in polymer matrices (films or beads) via adsorption, entrapment, ion exchange or covalent binding procedures. 

Table 2.6 shows that some stationary phases show exactly the same Rohschneider constants. The fact that these phases also show identical chromatographic selectivity in practice forms an indication for the validity of the Rohrschneider approach. Any of these phases can be selected, but it would be a waste of time to investigate the selective effect of more than one of them. The choice will now be based on secondary considerations, such as stability, temperature range, availability and cost. Indeed, one of the first consequences of the Rohrschneider characterization scheme was for some manufacturers to reconsider the program of available stationary phases and to remove obsolete ones (see e.g. ref. [212], p.62). It is seen that not all non-polar stationary phases are identical, and that minor differences in selectivity may be anticipated from the use of Apiezon L instead of one of the silicone polymers. 

To be able to attribute the binding of an MIP to an imprinting effect it is of utmost importance to show that specific recognition sites have been formed due to the presence of the print molecules during the polymerisation. This is usually done by comparisons with appropriate reference polymers. Polymers prepared without print molecules are not always the best choice, since the physical properties (surface area, porosity, etc.) of these polymers often differ from those of imprinted polymers. Reference polymers prepared with the optical antipode or a racemic mixture as the print species are preferred. The selectivity will be reversed when using the optical antipode and a racemic mixture will give a polymer incapable of separating the two enantiomers (unless the monomer(s) is/are chiral). 

A dramatic impact on chemical properties is achieved by the appropriate choice of counterion. The counterion employed during synthesis can have a marked effect on the anion-exchange selectivity series of conducting polymers.54 Table 3.4 shows the anion-exchange selectivity series obtained for both polypyrrole chloride and polypyrrole perchlorate. As illustrated, polypyrrole salts do not behave similar to conventional ion-exchange resins. 

---