Coiled planar capillary chromatography columns (0.9 mm I.D. × 60 cm L) were 3D printed in stainless steel (316L), and titanium (Ti-6Al-4V) alloys (external dimensions of ~5 × 30 × 58 mm), and either slurry packed with various sized reversed-phase octadecylsilica particles, or filled with an in situ prepared methacrylate based monolith. Coiled printed columns were coupled directly with 30 × 30 mm Peltier thermoelectric direct contact heater/cooler modules. Preliminary results show the potential of using such 3D printed columns in future portable chromatographic devices.
The idea of combining both anion-and cation-exchange groups in a single particle of ion-exchange resin for the purpose of improvement of ion-exchange selectivity appeared shortly after the invention of polymeric ion-exchange resins. Stach 1 synthesised a zwitterionic ion-exchange resin containing both sulfonic acid and quaternary ammonium functional groups in 1951. This material was obtained by copolymerization of styrene and vinylchloride, followed by the introduction of quaternary ammonium functional groups by reaction of chloro-groups with trimethylamine, and then sulfonation with sulfuric acid to give sulfonic acid groups. However this first zwitterionic exchange resin was not fully characterized and its physico-chemical properties and performance were not evaluated. Originally, phenolformaldehyde polycondensation type resins were very popular and the occurrence of anion-exchange functional groups and residual phenol groups provided some zwitterionic properties for this material in alkaline media where the phenol group was dissociated. These resins had a monotonic distribution of functional groups over the entire volume of the resin. Since 1951 the approaches to the synthesis and design of zwitterionic ion-exchangers were directed towards localization of the oppositely charged groups in a bonded layer on the surface of the particle or on attached molecules.Zwitterionic ion-exchangers are considered to include only those materials that contain the charged groups in a single particle, rather than mixed-bed ion-exchangers where a mixture of cation-and anion-exchangers is used. However, agglomerated ion-exchangers containing a layer of electrostatically retained microbeads around a core particle of opposite charge fall under the scope of this review. In some sources of literature the terms amphoteric and bipolar ion-exchangers have been also used. 2The distribution of the oppositely charged groups can vary considerably and the following types of zwitterionic ionexchangers can be identified: (i) Polyampholyte resins having a continuous distribution of oppositely charged groups in the whole volume of the ionexchange particle. (ii) Pellicular resins containing a layer of opposite charge on a charged core particle, with this layer comprising microparticles, latex microbeads, a polymer film, or a simple treated outer surface of the core (e.g. sulfonated), for the purpose of adding functional groups of opposite charge sign to the functional groups on the core. Zwitterionic ion-exchangers represent a new direction in the development of stationary phases for different modes of high-performance liquid chromatography. The combination of positively and negatively charged sites in a single particle, or within the functional groups of a single molecule attached to the surface of an adsorbent, provides unique opportunities to vary the selectivity of separation. The classification of zwitterionic ion-exchangers based on their structure, distribution of oppositely charged groups, and their applications is considered.
This review covers the latest developments and applications of nano-materials in stationary phase development for various modes of high-performance liquid chromatography. Specific attention is placed upon the development of new composite phases, including the synthetic and immobilisation strategies used, to produce either encapsulated nano-particles, or surface attached nano-particles, layers, coatings and other structures. The resultant chromatographic applications, where applicable, are discussed with comment upon enhanced selectivity and/or efficiency of the nano-particle modified phases, where such effects have been identified. In the main this review covers developments over the past five years and is structured according to the nature of the nano-particles themselves, including carbonaceous, metallic, inorganic, and organopolymer based materials.
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