A new thermally immobilized fluorinated stationary phase for RP-HPLC

Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

Stationary phase based on cellulose dodecanoate physically immobilized on silica particles for high-performance liquid chromatography

The chemical agent free preparation of a stationary phase using a natural macromolecule was the focus of this paper. Thermal immobilization of cellulose dodecanoate on silica particles was used for the preparation of a stationary phase without the use of chemical reagents. Cellulose modification was performed to produce a hydrophobic macromolecule with solubility in common organic solvents. The new stationary phase was characterized morphologically and physico-chemically, presenting as spherical particles immobilized with a thin cellulose dodecanoate layer. The degree of substitution of cellulose dodecanoate was 1.7, which resulted in a separation mechanism in reversed phase mode, but with lower hydrophobicity and higher steric selectivity, which are properties from cellulose. These characteristics resulted in a stationary phase with intrinsic selectivity that was able to separate mixtures of polar drugs, homologs of an anionic surfactant and omeprazole isomers, which are not well resolved in typical C₁₈ phases. Considering that cellulose is a natural polymer and the preparation method of stationary phase involves only physical processes of silica modification, the final material presents as a stationary phase with specific retention properties coming from both dodecanoate and cellulose.

Fluorinated Mesoporous Silica Nanoparticles for Binuclear Probes in 1H and 19F Magnetic Resonance Imaging

The development of molecular and cellular magnetic resonance imaging (MRI) procedures has always represented a challenge because of the fact that conventional MRI contrast agents are not directly detected in vivo; in proton MRI (e.g., with the nucleus ¹H), their local concentration is measured through the effect they exert on the signal of hydrogen protons present in their immediate vicinity. Because the contrast effects generated by conventional MRI probes superpose to and can often impede the anatomical information contained in ¹H MRI images, new probes based on a nucleus other than ¹H, are being developed. In this study, we report on the development of fluorinated mesoporous silica nanoparticles (MSNs), which could represent an interesting dual probe that allows two MRI modes: ¹H for high-resolution anatomical information and ¹⁹F for the detection of MSNs used as drug delivery agents. MSNs were synthesized and covalently functionalized either with fluorosilane (FMSNs) or polyfluorosiloxane (polyFMSNs) to enable their detection in ¹⁹F MRI. Then, gadolinium chelates were grafted on the particles to enhance their detectability in ¹H MRI. The physicochemical, textural, and relaxometric properties (¹H and ¹⁹F relaxation times) of the nanoparticles were measured and compared. The ¹⁹F relaxation properties were found to be dependent on the concentration of fluorine; they were also highly sensitive to the presence of gadolinium. The shortest relaxation times were obtained with polyFMSNs. At clinical magnetic field strengths, high ¹H relaxivities and low relaxometric ratios (r₂/r₁ = 1.45; 2.2 for nanoparticles entrapped in hydrogel) were found for both nanoparticle systems. Finally, the visibility of both systems was confirmed in ¹H, and the detectability of polyFMSNs was confirmed in ¹⁹F MRI. This physicochemical and relaxometric study opens the door to the applications of fluorinated silica nanoparticles as theranostic materials allowing dual MRI (¹H and ¹⁹F).