Chromatographic selectivity study of 4-fluorophenylacetic acid positional isomers separation

Evaluation of RPLC stationary phases for tocopherol and tocotrienol positional isomer separation: Method development and profiling

Reversed-phase separation of tocopherols (Ts) and tocotrienols (Ts) using C18 stationary phases results in the coelution of β and γ positional isomers, leading to identification errors. This study investigates the potential of alternative stationary phase chemistries to effectively resolve tocochromanols, specifically focusing on the critical pair of β and γ positional isomers. Initial screening of seven different stationary phases (C18, C18-PFP, C30, PFP, 5PYE, πNAP, and RP-Amide) was conducted. Linear solvent strength (LSS) studies were performed to assess the impact of the organic modifier (methanol) and temperature on the chromatographic performance parameters. Five columns were found to be suitable for the tocochromanol separation and two different chromatographical conditions per column were proposed. Elution order of tocochromanols was unique for 5PYE, πNAP and C30 columns in comparison to RP-Amide and PFP. Method development for the quantitative analysis of four tocopherol and four tocotrienol homologues was performed. The optimised method employed the RP-Amide (150 × 4.6 mm, 2.6 μm dp) superficially porous particle column, mobile phase of methanol:water of 92:8, v/v, with a flow rate of 1.0 mL/min, column oven temperature of 40 °C and fluorescence detection (λex 295 nm, λem 330 nm). The analysis run time was 10.5 min with 13.6 MPa back pressure. The method was validated and the obtained LOQs were found to be 1.30-3.13 μg/mL. The method developed was successfully applied for the determination of tocochromanols in twenty samples with unique tocochromanol profiles. Principal component analysis illustrated three distinct groups based on the tocochromanol profile.

Influence of Metal Coordination on the Gas-Phase Chemistry of the Positional Isomers of Fluorobenzoate Complexes

The fragmentation behaviors of the o-, m-, and p-fluorobenzoate complexes of La³⁺, Ce³⁺, Fe³⁺, Cu²⁺, and UO₂²⁺ were investigated by electrospray ionization mass spectrometry, and the corresponding reaction mechanisms were explored by density functional theory (DFT) calculations. Fluoride transfer product La^IIIFCl₃^-/Ce^IIIFCl₃^- and decarboxylation product La^IIICl₃(C₆H₄F)^-/Ce^IIICl₃(C₆H₄F)^- were observed when the carboxylate precursors La^IIICl₃(C₆H₄FCO₂)^-/Ce^IIICl₃(C₆H₄FCO₂)^- were subjected to collision-induced dissociation. The variation in product ratios, which is not obvious in the meta and para cases, qualitatively follows the increasing overall energy barrier and reaction endothermicity of the two-step CO₂/C₆H₄ elimination mechanism, and this aligns with the increase in U-F distance in the ortho, meta, and para decarboxylation product isomers. In contrast, the mass spectra of Fe^IIICl₃(C₆H₄FCO₂)^-/Cu^IICl₂(C₆H₄FCO₂)^- are dominated by the reduction product FeCl₃^-/CuCl₂^- regardless of the fluorobenzoate isomer. DFT/B3LYP calculations show that the two-step CO₂/C₆H₄F elimination pathways are comparable in energy for all three positional isomers. It is energetically more favorable to give the reduction product than the fluoride transfer product, which is opposite to the lanthanum cases. Although the decarboxylation product was observed for all three U^VIO₂Cl₂(C₆H₄FCO₂)^- isomers, the ortho isomer behaves more similarly to La^IIICl₃(C₆H₄FCO₂)^-/Ce^IIICl₃(C₆H₄FCO₂)^- as evidenced by the formation of U^VIO₂FCl₂^-, and the appearance of U^VO₂Cl₂^- in the cases of the meta and para isomers indicates the similarity with Fe^IIICl₃(C₆H₄FCO₂)^-/Cu^IICl₂(C₆H₄FCO₂)^-. The shorter U-F distance in U^VIO₂Cl₂(o-C₆H₄F)^- causes the decrease in the fluoride transfer barrier and thus makes this process more favorable over o-C₆H₄F radical loss to give U^VO₂Cl₂^-.

Analysis of positional isomers of 2-3-4-alkoxyphenylcarbamic acid derivatives by a combination of TLC and IMS

In this study, we have demonstrated a separation of positional isomers of some derivatives of alkoxyphenylcarbamic acid. These compounds belong to drugs with local anesthetics activity. The low volatility compounds were analysed by a Thin Layer Chromatography (TLC) and Ion Mobility Spectrometry (IMS) using diode laser desorption for sample introduction to IMS. This combined approach allowed the identification of compounds. Also, we have carried out IMS studies of all compounds and determined their ion mobilities The ion mobilities were increasing with the geometry change from position ortho to para of alkoxy chain, which is in agreement with their different collision cross section (CCS).

Conformational equilibrium of phenylacetic acid and its halogenated analogues through theoretical studies, NMR and IR spectroscopy

This paper presents a study on the conformational preferences of phenylacetic acid (PA) and its halogenated analogues (FPA, CPA, BPA). To clarify the effects that rule these molecules' behaviour, theoretical calculations were used, for both the isolated phase and solution, combined with nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. Most conformations of phenylacetic acid and its halogenated derivatives are stabilized through the hyperconjugative effect, which rules the conformational preference. NMR analyses showed that even with the variation in medium polarity, there was no significant change in the conformation population. Infrared spectroscopy showed similar results for all compounds under study. In most spectra, two bands were found through the carbonyl deconvolution, which is in accordance with the theoretical data. It was possible to prove that variation in the nature of the substituent in the ortho position had no significant influence on the conformational equilibrium.

Reverse-phase high performance liquid chromatography separation of positional isomers on a MIL-53(Fe) packed column

The application of the metal–organic framework (MOF) MIL-53(Fe) as a novel stationary phase for reverse-phase high performance liquid chromatography (HPLC) separation of positional isomers is described for the first time.

Selectivity of Brij-35 in Micellar Liquid Chromatographic Separation of Positional Isomers

Implementation of Brij-35, a nonionic surfactant, as a mobile phase for separation of positional isomers is investigated. Chromolith C-18 SpeedROD is used as a stationary phase. The effect of surfactant and organic modifier (propanol) concentration on the separation of some selected isomers is studied and evaluated in terms of linear solvation energy relationship (LSER). Shape selectivity is assessed by α value of sorbic and benzoic acid, which is found to be 1.339 by using mobile phase composed of 0.5% aqueous solutions of Brij-35 and propanol in 9 : 1. Isomers of parabens, nitroanilines, nitrophenols, and quinolinols are successfully separated using mobile phases composed of various percentages of surfactant and propanol. System constants for nonionic MLC using LSER analysis show that hydrogen bond basicity and dipolarity may be major contributors to selectivity, while excess molar refraction helps fine-tuning the separation which also imparts unique selectivity to nonionic surfactants as compared to ionic ones.