Contribution to the elucidation of the mechanism of sugar retention on amine-modified silica in liquid chromatography

Hydrophilic interaction chromatography

“…2e and f). This is also the water concentration range where the stationary phases start to become saturated and the enrichment levels off on underivatized and aminopropyl silicas, according to Verhaar and Kuster [34] and Nikolov and Reilly [36]. This finding could very well be taken as an indication of liquid -liquid partitioning.…”

“…Whereas retention in conventional NPC is predominantly governed by surface adsorption phenomena, Alpert suggested that the retention mechanism for HILIC was a partitioning between the bulk eluent and a water-rich layer, partially immobilized on the stationary phase. His arguments for suggesting a partitioning mechanism were (a) that carbohydrate separations and water retention experiments by Verhaar and Kuster [34], Orth and Engelhardt [35], and Nikolov and Reilly [36] carried out on underivatized silica and silicas derivatized with various amines had all been shown to involve a partitioning mechanism, since the stationary phases retain a semi-immobilized layer of water-enriched mobile phase; (b) a similarity with the extension of Albertsson's [37] aqueous two-phase partition systems to chromatography by Müller and Kutemeier [38 -40], with respect to (i) increase of retention time with size for an oligothymidylic [pd(T) 12 -30 ] homologous series; (ii) retention for oligo(nucleic acids) following an opposite pattern compared to anion exchange; and (iii) extremely high sensitivity of the retention to eluent ionic strength. Let us now examine these fundaments a little closer, but before we embark on this journey we should briefly review the basic equations describing pure partitioning and adsorptive interactions: RP chromatography has its roots in liquid -liquid chromatography and retention is considered to be (ideally) controlled by partitioning only.…”

“…However, if we review these references more carefully, we find that Verhaar and Kuster [34] never made any experiments to actually assess the mechanism. Instead they referred to Samuelson [49], who pioneered separations of sugars on relatively weakly crosslinked gel-type ion-exchange materials.…”

“…It does not elucidate of which kind these interactions are. It is interesting to note that Verhaar and Kuster [34] saw a strong catalytic effect from aminopropyl silica on the mutarotation rate of sugars when used as a catalyst in stirred bulk experiments, and concluded that the amine groups were responsible for the high reaction rate. These results are hard to explain without the possibility of direct contact between the amino groups and the sugars, i. e., in the absence of surface interactions.…”

“…For commercial sources consult Table 1, where crossreferences to composition and applications are found for most of the works cited. It is natural to start the treatise on naked inorganic stationary phases, notably unmodified silica, since separations in the early days of "high aqueous NP" chromatography that can be considered as the foundation of HILIC were all based on underivatized [36] or aminopropyl-bonded silica [23,24,34,36].…”