Gradient separations temperature dependence

Separation selectivity in LC depends on various factors. The most important is the choice of the stationary and mobile phases (Chen et al., 2004 Guo et al., 1987). In addition, the separation temperature (Hancock et al., 1994) and gradient slope (Chloupek et al., 1994) have also been shown to have a moderate impact on LC selectivity. 

The position of the solvent resonances depend on pH, solvent compositions and temperature. Especially in gradient separations the exact solvent composition in which the sample is dissolved is unknown. In addition, variation in the solvent position (which also depends on variations in other parameters) is strong. Depending on the absolute composition and temperature, a solvent change of 1 % can already shift the acetonitrile signal by more than 15 Hz. The complexity of the dependency of the solvent signals on the conditions makes it absolutely necessary to adjust the parameters for each sample individually. The adjustment procedure consists of the following steps ... 

The dynamic characteristics of adsorbed molecules can be determined in terms of temperature dependences of relaxation times [14-16] and by measurements of self-diffusion coefficients applying the pulsed-gradient spin-echo method [ 17-20]. Both methods enable one to estimate the mobility of molecules in adsorbent pores and the rotational mobility of separate molecular groups. The methods are based on the fact that the nuclear spin relaxation time of a molecule depends on the feasibility for adsorbed molecules to move in adsorbent pores. The lower the molecule s mobility, the more effective is the interaction between nuclear magnetic dipoles of adsorbed molecules and the shorter is the nuclear spin relaxation time. The results of measuring relaxation times at various temperatures may form the basis for calculations of activation characteristics of molecular motions of adsorbed molecules in an adsorption layer. These characteristics are of utmost importance for application of adsorbents as catalyst carriers. They determine the diffusion of reagent molecules towards the active sites of a catalyst and the rate of removal of reaction products. Sometimes the data on the temperature dependence of a diffusion coefficient allow one to ascertain subtle mechanisms of filling of micropores in activated carbons [17]. 

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