Preparation and evaluation of monolithic poly(N-vinylcarbazole-co-1,4-divinylbenzene) capillary columns for the separation of small molecules

Abstract

Short-term polymerization or the so-called low-conversion polymerization was applied for the preparation of N-vinylcarbazole (NVC) and 1,4-divinylbenzene (DVB) monolithic capillary columns. The synthesis was carried out by thermally initiated free radical copolymerization under the influence of inert micro- (toluene) and macroporogen (1-decanol) and α,α'-azoisobutyronitrile (AIBN) as radical initiator. The morphological and porous properties were studied by scanning electron microscopy (SEM), nitrogen adsorption, and mercury intrusion porosimetry (MIP). The copolymerization process was studied by monomer conversion measurements. This approach led to increased porosity and specific surface area. A specific surface area above 400 m(2)/g of the monolith and a distinct bimodal pore size distribution were obtained. The chromatographic performance was determined in terms of theoretical plate heights and number of theoretical plates. The lowest plate height value was found to be 3.9 μm (corresponding to ≈256,000 plates per meter) applying methylparaben utilizing an 80 mm × 0.2 mm i.d. monolithic capillary. The developed NVC/DVB monolithic supports showed high separation efficiency towards small molecules, which was exemplified applying reversed-phase (RP) separation of alkylbenzenes, beta-blockers, flavanoids, parabens, and phenones. The loading capacity was analyzed for isocratic separation of seven alkylbenzenes and was found to be up to 77 ng total mass of alkylbenzenes. Furthermore, a long-term stability test of 1,000 consecutive runs was performed and resulted in a maximum variance of 0.97, 0.85, and 0.16 % RSD for resolution, peak width at half height, and retention times, respectively. The material was proven to have a high permeability of 1.11E-14 m(2), applying water as a mobile phase.

Current trends in the development of porous polymer monoliths for the separation of small molecules

“…We have demonstrated, that hypercross‐linked styrene‐based polymer monolith with grafted polar functionality do not change its properties even after 10 000 injections of test compounds . Also monolithic columns prepared by short polymerization reaction times with the most dominant effect of reaction kinetics provide repeatable and robust column properties . Actually, the vast majority of articles cited thorough this review provided a part devoted to columns preparation repeatability confirming that polymer‐based monoliths are very robust and stable stationary phases.…”

“…An early termination of a polymerization reaction is one of the simplest ways how to control both porous properties and related efficiency of polymer monoliths . Koeck et al utilized short‐term free‐radical polymerization and prepared poly(N‐vinylcarbazole‐ co‐ divinylbenzene) capillary columns for the separation of small molecules . Monomers conversion analysis revealed that while N‐vinylcarbazole is consumed linearly with longer polymerization time, divinylbenzene reaches conversion of 90% already after 90 min of polymerization reaction.…”

“…Additionally, column provided isocratic efficiency of 116 000 plates/m for benzene that doubled for methyl‐ and butylparaben. Prepared columns provided robust mechanical, chemical, and thermal properties over the span of 1000 injections .…”

“…At flow rate of 10 μL/min, analysis temperature 50°C, UV detection at 210 nm, Analytes–(1) benzene, (2) toluene, (3) ethylbenzene, (4) propylbenzene, (5) butylbenzene, (6) pentylbenzene, (7) hexylbenzene. Reprinted with permission from .…”

Polar and nonpolar organic polymer‐based monolithic columns for capillary electrochromatography and high‐performance liquid chromatography

“…Low conversion polymerization (i.e., short‐term polymerization) has been applied for the preparation of poly( N ‐vinylcarbazole [NVC]‐ co ‐1,4‐divinylbenzene [DVB]) using NVC as the functional monomer and DVB as the cross‐linker . Applying this approach in combination with variation in the radical initiator content proved to have a noticeable impact on the porous properties of the newly developed poly(NVC‐ co ‐DVB) stationary phase.…”