Exploring Fluorous Affinity by Liquid Chromatography

Spherical fluorinated covalent organic polymer for highly efficient and selective extraction of fipronil and its metabolites in soil

Covalent organic polymers (COPs) have garnered considerable attention as promising adsorbents of online solid phase extraction (online SPE). Morphology modulation provides an appealing solution to enhance adsorption efficiency and reduce back-pressure in the absorbent. However, the synthesis of COPs with regular geometric shapes and specific adsorption selectivity remains challenging. In this study, a uniform spherical fluorinated COP (F-sCOP, average diameter: 2.14 μm) was successfully synthesized by Schiff base reaction of 1,3,5-triformylphoroglucinol (TP) and 2,2'-bis(trifluoromethyl)benzidine (TFMB). The F-sCOP had a large surface area (BET: 346.2 m2 g-1), remarkable enrichment capacity (enrichment factors: 186-782), high selectivity toward fipronil and its metabolites (adsorption efficiency >93.1%), and admirable service life (>60 times). Based on the adsorbent, a novel μ-matrix cartridge extraction-online-μ-solid phase extraction-high performance liquid chromatography-mass spectrometry (μ-MCE-online-μ-SPE-HPLC-MS) method was constructed and used to track trace fipronil and its metabolites in soil. The proposed method exhibited a wide linear range (0.05-1000 ng g-1), low quantitation limits (LOQs: 0.0027-0.011 ng g-1), high recoveries (90.1-119.6%) and good repeatability (RSD ≤10.5%, n = 3) for fipronil analysis. This study paves the way for pesticide analysis in soil risk assessment.

Chemical Information Processing by a Responsive Chemical System

Nature has an extraordinary capacity to precisely regulate the chemical reactivity in a highly complex mixture of molecules that is present in the cell. External stimuli lead to transient up- and downregulation of chemical reactions and provide a means for a cell to process information arriving from the environment. The development of synthetic chemical systems with life-like properties requires strategies that allow likewise control over chemical reactivity in a complex environment. Here, we show a synthetic system that mimics the initial steps that take place when a natural signal transduction pathway is activated. Monophosphate nucleosides act as chemical triggers for the self-assembly of nanoreactors that upregulate chemical reactions between reagents present at low micromolar concentrations. Different nucleotides template different assemblies and hence activate different pathways, thus establishing a distinct connection between input and output molecules. Trigger-induced upregulation of chemical reactivity occurs for only a limited amount of time because the chemical triggers are gradually removed from the system by enzymes. It is shown that the same system transiently produces different output molecules depending on the chemical input that is provided.

Surface-bubble-modulated liquid chromatography: an experimental strategy for identification of molecular processes of solute retention in reversed-phase separation systems

Molecular level understanding of the chemistry at the aqueous/hydrophobe interface is crucial to separation processes in aqueous media, such as reversed-phase liquid chromatography (RPLC) and solid-phase extraction (SPE). Despite significant advances in our knowledge of the solute retention mechanism in these reversed-phase systems, direct observation of the behavior of molecules and ions at the interface in reversed-phase systems still remains a major challenge and experimental probing techniques that provide the spatial information of the distribution of molecules and ions are required. This review addresses surface-bubble-modulated liquid chromatography (SBMLC), which has a stationary gas phase in a column packed with hydrophobic porous materials and enables one to observe the molecular distribution in the heterogeneous reversed-phase systems consisting of the bulk liquid phase, the interfacial liquid layer, and the hydrophobic materials. The distribution coefficients of organic compounds referring to their accumulations onto the interface of alkyl- and phenyl-hexyl-bonded silica particles exposed to water or acetonitrile-water and into the bonded layers from the bulk liquid phase are determined by SBMLC. The experimental data obtained by SBMLC show that the water/hydrophobe interface exhibits an accumulation selectivity for organic compounds, which is quite different from that of the interior of the bonded chain layer, and the overall separation selectivity of the reversed-phase systems is determined by the relative sizes of the aqueous/hydrophobe interface and the hydrophobe. The solvent composition and the thickness of the interfacial liquid layer formed on octadecyl-bonded (C₁₈) silica surfaces are also estimated from the bulk liquid phase volume determined by the ion partition method employing small inorganic ions as probes. It is clarified that various hydrophilic organic compounds as well as inorganic ions recognize the interfacial liquid layer formed on the C₁₈-bonded silica surfaces as being different from the bulk liquid phase. The behavior of some solute compounds exhibiting substantially weak retention in RPLC or the so-called negative adsorption, such as urea, sugars, and inorganic ions, can rationally be interpreted with a partition between the bulk liquid phase and the interfacial liquid layer. The spatial distribution of solute molecules and the structural properties of the solvent layer on the C₁₈-bonded layer determined by the liquid chromatographic methods are discussed in comparison to the results obtained by other research groups using molecular simulation methods.

Superhydrophobic fluorinated microspheres for fluorous affinity chromatography

Fluorous affinity chromatography has received growing attention in separation and purification of fluoro compounds, but the wettability of the fluorinated stationary phases is seldom noticed. Here, we construct a series of micro-sized fluorine-containing microspheres by solvothermal precipitation polymerization. The fluorinated microspheres could be obtained with narrow size distribution at even high monomer loading of 15 wt%. Through alternating fluoro monomer, both the particle size and the wettability of the microsphere array could be tuned. Among them, the poly(divinylbenzene -dodecafluoroheptyl methacrylate), P(DVB-DFHMA), microsphere (6.1 μm) arrays displays superhydrophobicity with 153.2° water contact angle. The P(DVB-DFHMA) fluorinated microspheres (7.58% fluorine content) can be packed into steel-less columns as stationary phase for high-performance liquid chromatography. The retention mechanism of the fluorinated column is proven to be the specific fluorine-fluorine interaction. Compared to the commercial C18 silica column, the fluorinated column can completely separate fluorine-containing compounds under high water content mobile phase, including small fluoro molecules and fluoro macromolecules, at much lower back pressure by fluorous affinity.

Intrinsic difference between phenyl hexyl- and octadecyl-bonded silicas in the solute retention selectivity in reversed-phase liquid chromatography with aqueous mobile phase

For choosing an optimal column for a particular separation by reversed-phase liquid chromatography (RPLC), it is essential to quantitatively understand the effects of the chemical structure of hydrophobic bonded layer derived onto silica particles on the distribution equilibrium of a solute compound at the interface between the aqueous mobile phase and the packing material. However, there is still a lack of understanding of the solute distribution equilibrium in RPLC separation due to the complexities of the chemistry at the interface between the mobile phase and the bonded layer. We successfully determined the distribution coefficients of various organic compounds concerning to their accumulation onto the water/bonded layer interface and into the bonded layer from bulk water using surface-bubble-modulated liquid chromatography with octadecyl- and phenyl hexyl-bonded silica columns. The water/phenyl hexyl-bonded layer interface accumulates organic compounds much less than the water/octadecyl-bonded layer interface due to its lower interfacial tension, and this result suggests that phenyl hexyl group orient their benzene ring facing toward water. On the other hand, aromatic moiety of phenyl hexyl group enhances partitioning of the organic compounds into the bonded layer. Experimental findings in the present work demonstrated that the water/bonded layer interface and the bonded layer itself have independent contributions to the solute distribution and the water/phenyl hexyl-bonded layer interface shows quite different solute retention selectivity from the water/octadecyl-bonded layer interface.

Shedding light on mechanisms leading to convex-upward van Deemter curves on a cellulose tris(4-chloro-3-methylphenylcarbamate)-based chiral stationary phase

An unusual convex-upward van Deemter curve was observed for the more retained enantiomer of a chiral sulfoxide (2-(benzylsulfinyl)benzamide) on a cellulose tris(4-chloro-3-methylphenylcarbamate)-based chiral stationary phase (CSP), prepared on silica particles of 1000 Å pore size. In contrast, the firstly eluted enantiomer of the same molecule exhibited the traditional convex-downward van Deemter curve. A detailed kinetic and thermodynamic investigation has revealed that this unusual phenomenon, which however has already been observed in chiral chromatography, originates when the adsorption of the compound is very strong and the solid-phase diffusion negligible. Experimentally, the intraparticle diffusion of the more retained enantiomer of the sulfoxide was found to be one order of magnitude smaller than that of the first eluted one. Overall, this translates into very little longitudinal diffusion (b-term of van Deemter curve) accompanied by high solid-liquid mass transfer resistance (c-term). Finally the comparison with another, differently-substituted chiral sulfoxide (whose enantiomers both exhibit traditional van Deemter curve behavior) has allowed to correlate these findings to the specific characteristics of the molecule.

Fluoro-functionalized stationary phases for electrochromatographic separation of organic fluorides

Fluorous affinity means remarkably specific interaction between highly organic fluorides. This work aims to explore the potential of fluoro-functionalized stationary phase for the separation of organic fluorides by means of fluorous-fluorous interaction. Here, by using the Michael addition strategy between 1H,1H,2H,2H-perfluorodecanethiol (PFDT) and polydopamine (PD), a novel fluoro-functionalized stationary phase was synthesized for open-tubular capillary electrochromatography (OT-CEC). The PFDT@PD was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray Photoelectron Spectrometer (XPS). The PFDT@PD@capillary exhibited outstanding separation performance towards neutral compounds (such as alkylbenzenes and chlorobenzenes) and organic fluorides (such as fluorobenzenes and perfluoroalkyl methacrylates etc.) with high resolution and high separation efficiency by hydrophobic interaction and fluorous-fluorous interaction. In addition, the column shows good stability and reproducibility. The relative standard deviations (RSDs) of the retention time for intra-day (n = 5) and inter-day (n = 3) runs and between columns (n = 3) are less than 0.39%, 1.22% and 3.87%, respectively. This novel type of fluoro-functionalized stationary phase represents a great application potential in organic fluorides separation field.

Modeling the nonlinear behavior of a bioactive peptide in reversed-phase gradient elution chromatography

The thermodynamic behavior of octreotide, a cyclic octapeptide with important pharmaceutical functions, has been simulated under reversed-phase gradient elution conditions. To this end, adsorption behavior was firstly investigated in isocratic conditions, under a variety of water/acetonitrile + 0.02% (v/v) trifluoroacetic acid (TFA) mixtures as mobile phase by using a Langmuir isotherm. Organic modifier was varied in the range between 23 and 28% (v/v). Adsorption isotherms were determined by means of the so-called Inverse Method (IM) with a minimum amount of peptide. The linear solvent strength (LSS) model was used to find the correlation between isotherm parameters and mobile phase composition. This study contributes to enlarge our knowledge on the chromatographic behavior under nonlinear gradient conditions of peptides. In particular, it focuses on a cyclic octapeptide.

Direct analysis of chiral active pharmaceutical ingredients and their counterions by ultra high performance liquid chromatography with macrocyclic glycopeptide-based chiral stationary phases

In this work the simultaneous separation of chiral active pharmaceutical ingredients (API) in salt form from their counterions has been performed by using different high-efficiency macrocyclic glycopeptide-based chiral stationary phases (CSPs). Not only a new zwitterionic vancomycin-based CSP has been prepared (similarly to what was done for teicoplanin) but macrocyclic selectors have also been bonded to sub-2 μm fully porous silica particles through traditional ureidic linkage to obtain versions of CSPs suitable for ultra-high performance applications. The direct separation of chiral APIs and counterions is particularly attracting since it simplifies the workflow traditionally used with reduction of analysis time and costs. The wide selection of macrocyclic antibiotics CSPs now available has allowed to manage different cases that can happen in the simultaneous separation of APIs and their counterions (either cations or anions). Indeed, while inorganic cations are retained on traditional vancomycin- and teicoplanin-based CSPs, inorganic anions are almost unretained (due to Donnan's effect). On the other hand, cations and anions can be both retained on the zwitterionic versions of these CSPs. Afterwards, zwitterionic CSPs allowed the separation of other compounds including N-derivative amino-acids, profens, polyols, sugar anomers, oligosaccharides and inorganic anions/cations opening new perspectives in the use of this family of CSPs.

Investigations into the separation behaviour of perfluorinated C8 and undecanoic acid modified silica hydride stationary phases

In this study, the surface charge properties of perfluorinated C8 (PerfluoroC8) and undecanoic acid (UDA) modified silica hydride stationary phases have been investigated. The zeta potential values of these stationary phases were measured in aqueous/acetonitrile mobile phases of different pH, buffer concentrations and acetonitrile contents. The retention behaviour of several basic, acidic and neutral compounds were then examined with these two stationary phases, with U-shaped retention dependencies evident with regard to the organic solvent content of the mobile phase. Plots of the logarithmic retention factor versus buffer concentration revealed slopes ≥ -0.41 for both stationary phases, indicating the involvement of mixed mode retention mechanisms with contributions from both ionic and non-ionic interactions. Using a linear solvation energy relationship approach, the origins of these interactions under different mobile phase conditions were differentiated and quantified. The PerfluoroC8 stationary phase exhibited stronger retention for basic compounds under high acetonitrile content mobile phase conditions, whilst stronger retention was observed for all compounds with the UDA stationary phase under high aqueous content mobile phase conditions. The more negative zeta potentials of the UDA stationary phase correlated with higher total charge density, surface charge density and charge density at the beta plane (the outer plane of the double layer) compared to the PerfluoroC8 stationary phase. With mobile phases of low buffer concentrations, more negative zeta potential values were unexpectedly observed for the PerfluoroC8 stationary phase with slight increases in the C descriptor value, reflecting also the greater accessibility of the analytes to the stationary phase surface. Comparison of the retention behaviours on these phases with other types of silica hydride stationary phases has revealed different patterns of selectivity.