Eluent gradient polymer HPLC

FIGURE 16.11 Schematic representation of eluent gradient polymer HPLC. Two polymer species A and B are separated. They exhibit different nature and different interactivity with the column packing (e.g., adsorp-tivity) or with the mobile phase (solubility). The linear gradient from the retention promoting mobile phase to the elution promoting mobile phase is applied. The focused peaks—one for each polymer composition/ architecture—are formed in the appropriately chosen systems. Each peak contains species with different molar masses. 

In conclusion, eluent gradient polymer HPLC represents a useful tool for separation of complex polymer system. It belongs to the important constituents of several two-dimensional polymer HPLC procedures. 

The latter solubility based methods do not directly belong to chromatography, however, they can be offline combined with polymer HPLC. Moreover, the solubility effects are directly employed in some coupled chromatographic methods (compare section 11.8.4, Eluent Gradient Polymer HPLC), while some of them employ the tendencies to phase separation rather than the complete precipitation processes (see section 11.8.6, Liquid Chromatography under Limiting Conditions of Enthalpic Conditions). 

More precise is the electronic control of piston movement, especially when the pistons are actuated with two otherwise independent motors. In the latter cases, the additional pulse dampeners that were common with classical pumps are less important. Often, a long capillary mounted between pump and sample injector is satisfactory. Its volume is to be considered in the eluent gradient polymer HPLC procedmes (see section 11.8.4). Special pumping systems commonly employed in gradient HPLC of low-molecular compounds are frequently also used in eluent gradient polymer HPLC. 

The concept of entropy-enthalpy compensation resulting in the critical conditions of enthalpic interactions and the molar mass independent sample retention turned out useful also for the understanding several other coupled methods of polymer HPLC. It is accepted [195,196] that the polymer species tend to elute at the critical conditions also when either eluent strength or quality change within the HPLC system in the course of the HPLC experiment that is in the continuous and local gradient methods (Sections 16.5.3, 16.5.4, and 16.5.6). Irrespective of the problems and limitations of LC CC, its concept belongs to the important breakthroughs in polymer HPLC. 

All above homopolymers are used also for the identification of suitable conditions for the coupled polymer HPLC techniques. Typical examples are liquid chromatography under critical (LC CC) and limiting (LC LC) conditions, and eluent gradient liquid chromatography (EG LC). For the development of latter methods, several defined statistical and block copolymers are available. 

Thermodynamic quality of mobile phase may be so poor that maeromolecules tend to precipitate within column. The occurence of phase separation is generally undesired in the isocratic coupled methods of polymer HPLC (see section 11.8). Controlled phase separation of maeromolecules within column represents a specific featme of certain procedures of polymer HPLC, which are as usual connected with application of the eluent gradient. 

The setnp for polymer HPLC is qnite similar to SEC systems with a few modifications. Liqnid Adsorption Chromatography (LAC) requires the adsorption and desorption on a stationary phase. Therefore, in most cases isocratic separation is not sufficient. Gradients with respect to pH valne, ionic strength, eluent composition, or temperature are applied. The most common approach is to use eluent composition gradients. In contrast to SEC where polymeric phases dominate, silica-based column packings are the most important stationary phase. Both normal phase and reversed phase separations have been described. A summary of different applications in copolymer separation is offered by Pasch [27]. Detectors used in gradient LAC are mainly UV/DAD detectors and ELSD. 

Eluent gradient adsorption chromatography is one of the main HPLC techniques used in polymer fractionation including deposition-dissolution, chromatography under critical conditions, temperature gradient adsorption LC and precipitation dissolution LC [24-30]. 

An ELSD converts the HPLC eluent into a particle stream and measures the scattered radiation. It offers universal detection for nonvolatile or semivolatile compounds and has higher sensitivity than the RI detector (in the low ng range) in addition to being compatible with gradient analysis. ELSD is routinely used in combinatorial screening. Response factors are less variable than that of other detectors. An ELSD consists of a nebulizer equipped with a constant temperature drift tube where a counter-current of heated air or nitrogen reduces the HPLC eluent into a fine stream of analyte particles. A laser or a polychromatic beam intersects the particle stream, and the scattered radiation is amplified by a photomultiplier. Manufacturers include Alltech, Polymer Laboratories, Shimadzu, Waters, Sedere, and ESA. 

As one very striking example of the capabilities of the high-temperature gradient HPLC system, the separation of random ethyleneA inyl acetate copolymers is presented in Fig. 23. On silica gel as the stationary phase and using decaline-cyclo-hexanone as the eluent, full separation of copolymers of different compositions was achieved. In addition, the homopolymers PE and PVAc were well separated from the copolymers. This was the first time that a chromatographic system was available that separates olefin copolymers irrespective of crystallinity and solubiUty over the entire range of compositions. Namely, the mobile phase components used are solvents for both PE and PVAc. The non-polar solvent, decalin, supports adsorption of PVAc on the silica gel, while the polar solvent, cyclohexanone, enables desorption and elution of the adsorbed polymer sample firom the column [155]. 

---