Preparation and evaluation of a hydrolytically stable amide-embedded stationary phase

A carbonylative coupling approach to alkyl stationary phases with variable embedded carbamate groups for high-performance liquid chromatography

A highly efficient carbonylative coupling method for the preparation of alkyl stationary phases with variable numbers of carbamate groups was established. The effectiveness of such method was verified through elemental analysis, X-ray photoelectron spectroscopy and solid-state ¹³C nuclear magnetic resonance spectroscopy of three as-synthesized stationary phases bearing different alkyl chains and different numbers of carbamate groups (octadecyl/one carbamate group, C18C; docosyl/three carbamate groups, C22C³; triacontyl/two carbamate groups, C30C²). The comparative evaluation of these stationary phases using a great variety of analytes, including three sets of isomers of alkylbenzenes, two sets of standard mixtures of polycyclic aromatic hydrocarbons (SRM 1647e and 869b), nine polychlorinated biphenyls, fiveteen N-substituted ureas, ten sulfonylureas, five xanthines and some other phytonutrients, revealed their remarkable applicability in reversed-phase liquid chromatography. Notably, the intercalated carbamate groups rendered the resultant stationary phases compatible with 100% aqueous mobile phase. The suppression of silanol activity was positively related to the number of polar groups embedded in the bonded selector, and the smallest peak tailing factor (1.14) for amitriptyline was obtained by C22C³. The molecular shape-related selectivity was found to be more closely related to the length of the selector's aliphatic chain, as supported by the lowest α_TBN/BaP value (0.31) by C30C². These carbamate-embedded alkyl stationary phases constituted another class of polar-embedded stationary phases possessing a single type of functional ligand.

Does phase ratio in reversed phase high performance liquid chromatography vary with temperature?

Phase ratio Φ for an high performance liquid chromatography (HPLC) column is a parameter defined as the ratio between the volume of the stationary phase V_s and the void volume of the column V_m. Together with the equilibrium constant K of the separation process, phase ratio is part of the retention factor k (k = KΦ). Although a considerable number of studies have been dedicated to the evaluation of V_s and V_m with the goal of obtaining the value for Φ, there are still debatable results regarding the true value of Φ, which for a column with a specific stationary phase may vary with the composition of the mobile phase. One route for the evaluation of the value of Φ uses the measurements of retention factors k on a specific column and mobile phase for two or more hydrocarbons for which the octanol/water partition coefficients log K_ow are known. This procedure has been applied in the present study for the evaluation of Φ for three commercially available C18 columns and two mobile phase compositions water/acetonitrile, in the temperature range 20 °C to 50 °C. It was found that phase ratio does change depending on the temperature, its "effective value" decreasing as the temperature increases which is in accordance with the decrease of retention times in reversed phase HPLC when the temperature increases. Besides other factors that may affect the correct calculation of thermodynamic functions, the change of phase ratio with temperature has implications regarding the possibility to calculate the enthalpy and entropy values from van't Hoff plots of the variation of log k as a function of 1/T, even when the retention process is dominated by unique hydrophobic type interactions.

Synthesis and chromatographic evaluation of a new stationary phase based on mild thiol-Michael addition reaction

Click chemistry has attracted increasing attention for the synthesis of novel stationary phases. Considering the advantage of click chemistry, a strategy based on thiol-Michael addition was developed for the preparation of a new stationary phase herein, and a phenyl vinyl sulfone stationary phase (M-PVS) was prepared. The resulting M-PVS bonded silica was characterized by elemental analysis, solid-state ¹³C cross-polarization/magic-angle spinning NMR and infrared spectroscopy, confirming the successful immobilization of phenyl vinyl sulfone on the silica support. The retention properties of M-PVS were investigated and exhibited unambiguous reversed phase retention characteristics. Moreover, shape selectivity and silanol activity were studied to reveal the diverse interactions of M-PVS, including hydrophobic, π-π, hydrogen bonding, and ion-exchange interactions. In addition, de-wetting tolerance and hydrophilic properties were evaluated and a pronounced "U" retention curves were obtained, indicating enhanced retention for polar analytes and transitions of different interaction modes. Selectivity differences between M-PVS column, phenyl column and conventional C₁₈ column were examined using series natural standards. The diverse interactions of M-PVS demonstrated its improved selectivity for the compounds with similar hydrophobic skeleton but different polar substituents.

A novel RP-HPLC refractive index detector method development and validation for determination of trace-level alcohols (un-sulfated) in sodium lauryl sulfate raw material

The main aim of the study is to develop and validate a simple and rapid liquid chromatographic analytical method for simultaneous determination of trace level of un-sulfated alcohol impurities in sodium lauryl sulfate using high-performance liquid chromatography with a refractive index detector. The chromatographic separation was achieved using flow rate of 3.0 ml/min with a Waters Symmetry C₁₈ (150 × 4.6 mm, 5 μm) column and 50°C as a column temperature. The mobile phase comprised milliQ water and acetonitrile in the ratio of 30:70 v/v respectively. The detection was performed using a refractive index detector at a sensitivity of 64. The resolutions among n-decanol, n-dodecanol, n-tetradecanol, n-hexadecanol, and n-heptadecanol were found to be >4. Regression analysis showed a correlation coefficient for the stated compounds of >0.999. The validated HPLC method was utilized effectively for the estimation of un-sulfated alcohols in quality control labs for commercial release of sodium lauryl sulfate.

A simple method for the synthesis of a polar-embedded and polar-endcapped reversed-phase chromatographic packing with low activity of residue silanols

Octadecyl bonded silica (ODS) is the most popular packing for reversed-phase chromatography. However, it generally demonstrates bad resolution for polar analytes because of the residue silanols and its poor stability in aqueous mobile phase. To address the problem, a new reversed-phase packing containing both polar-embedded and polar-endcapped moieties was proposed. It was prepared by a very simple method, in which the epoxide addition reaction of 3-glycidoxypropyltrimethoxysilane with 1-octadecanethiol proceeded simultaneously with the reaction of silane coupling onto silica particles. By controlling the molecular ratio of 3-glycidoxypropyltrimethoxysilane to 1-octadecanethiol higher than 1.0 (1.56 for the present study), both polar-embedded and polar-endcapped moieties were achieved onto the packing. The performance of the packing was evaluated in detail. The results demonstrated that neutral, acidic and basic analytes were well separated on the packing. The column efficiency for phenanthrene was 34,200 theoretical plates per meter. In addition, four nucleotides can be separated in 100% phosphate buffered saline solution with good reproducibility, which indicates the packing has good stability in aqueous mobile phase. Amitriptyline, a typical basic analytes, was eluted out with relatively symmetric peak shape (asymmetry factor of 1.36), which implies that the packing has not suffered from the negative effect of residue silanols significantly. Good stability in buffer solution of pH ranging from 2.0 to 10.0 was also documented for the packing.

Preparation of a dibenzylated 1,4-diazacyclohexane-derived strong anion-exchange stationary phase

This paper reports the preparation of a novel, silica-based, strong anion-exchange stationary phase from a 1,4-diazacyclohexane derivative. To prepare the difunctional strong anion-exchange stationary phase, activated silica beads were first bonded with 3-chloropropyltriethoxysilane and then reacted with 1-methylpiperazine followed by benzyl chloride. The silica beads, the strong anion-exchange stationary phase and its precusors were instrumentally characterized. Aromatic acids were separated with non-aqueous anion-exchange chromatography. After elution with eluant prepared in mixed solvents of water and methanol, the resulting 1,4-diazacyclohexane-derived, difunctional, strong anion-exchange stationary phase exhibited good separation and selectivity for the aromatic acids investigated. The effects of eluant pH, eluant ion concentration and solvent composites on the separations were investigated. Organic acids with different substituents were eluted in order of decreasing dissociation coefficients, with no observable peak shape differences.

Stationary phase characterization and method development

The properties of stationary phases and their characterization methods are reviewed. New and significant developments have occurred in the last few years, and new methods for stationary phase characterization have become available. The characterization methods are discussed, and the differences between the different methods are pointed out. In addition, method development approaches are reviewed, with special emphasis on recent developments that employ multiple parameters in parallel. Also, the renewed interest of temperature as a tool in method development is surveyed.

Development of a polar-embedded stationary phase with unique properties

This paper describes a new polar-embedded stationary phase that contains an internal sulfonamide functional group coupled with an ether linkage. The synthesis involves functionalization of spherical silica particles with ligands prepared in a multi-step synthesis. The resulting material contains 16.5% carbon, corresponding to a ligand coverage of 2.4mumol/m(2). Chromatographic evaluations indicates that the new stationary phase exhibits lower polarity than any other polar-embedded packings investigated, with additional features such as low silanol activity, excellent compatibility with 100% aqueous mobile phases, higher shape selectivity for polycyclic aromatic hydrocarbons, and strong affinity to nitro-containing compounds.