Liquid chromatography under limiting conditions of adsorption and limiting conditions of desorption for separation of complex polymers. The role of “flower-like” interactions of macromolecules

Abstract

Liquid chromatography of polymers under limiting conditions (LC-LC) of enthalpic, interactions employs different velocities of small (eluent or auxiliary liquid) molecules and large (sample) molecules within a HPLC column. Slowly moving small molecules of a liquid act as a “barrier” hindering fast progression of large molecules eluting with exclusion retention mechanism. Under specific (“limiting”) conditions the low molar mass barrier enables a molar mass independent polymer elution. Depending on the applied barrier mechanism (adsorption, partition, phase, separation) and on the experimental arrangement (eluent as a continuous barrier or narrow zone of auxiliary liquid as a local barrier) six different modes of LC-LC can be created. Here, we deal with the limiting conditions of adsorption (LC-LCA—eluent as a barrier) and the limiting conditions of desorption (LC-LCD—the barrier created by a zone of an auxiliary liquid), which utilize adsorption barrier mechanism. Reduced sample recovery was repeatedly observed in many LC-LCA systems, especially when working with the narrow—pore column packing. The fraction of macromolecules retained within the LC-LCA column raised with the decreasing pore size of the column packing and with the increasing molar mass of the polymer samples. With the same both polymers and column packing, LC-LCD yielded full sample recovery. This finding is explained by the difference in the liquid layer composition on the outer surface of the packing particles and on the walls of broader pores compared to that within the narrow pores. Local equilibrium within, the liquid phase seems not to be reached in the narrow packing pores. This means that adsorption promoting state is preserved within narrow pores of the LC-LCA column, which may lead to the “flower-like” adsorption of macromolecules. Polymer species penetrate (“reptate”) into these narrow packing pores to be strongly adsorbed by a multiple attachment. The desorption of the flower-like adsorbed macromolecules is slow and, consequently, irregularities of chromatographic bands and reduced sample recovery may appear. In contrast, the desorption promoting state remains in the narrow pores of the LC-LCD packing and it prevents entrapment of macromolecules. The flower-like adsorption of macromolecules is not feasible in LC-LCD and this results in high sample recovery. Our results indicate an important advantage of the LC-LCD over the LC-LCA method and may also help explaining some irregularities observed in liquid chromatography under critical conditions of adsorption.