Metal-organic frameworks in chromatography

High Selectivity of A Novel Pillar[5]arene with Ester Units as a Gas Chromatographic Stationary Phase toward Aromatic Isomers

This work reports the first example of employing ester-functionalized pillar[5]arene (P5A-C10-OAc) stationary phase for gas chromatography (GC) separations. The as-fabricated P5A-C10-OAc column achieved improved column efficiency of 4270 plates/m and separation performance in contrast to the P5-C10-Br column. The P5A-C10-OAc column showed good separation performance for a wide range of analytes such as alkanes, bromoalkanes, ketones, fatty acid methyl esters, aldehydes, alcohols, halobenzenes, anilines, phenols, naphthalenes, and showed sharp and symmetrical peak shapes for analytes that are liable to peak-tailing in GC analysis. As testified by the challenging isomer mixtures (bromonitrobenzene, chloronitrobenzene, bromobenzaldehyde, chlorobenzaldehyde, nitrobenzaldehyde), the P5A-C10-OAc column exhibited comprehensively higher separation capability than the P5A-C10-Br, P5A-C10 and commercial HP-35 columns. This work demonstrates the great potential of pillararene-based stationary phases as a new type of stationary phases for GC separations.

A promising sensing platform for explosive markers: Zeolite-like metal-organic framework based monolithic composite as a case study

Preconcentration for on-site detection or subsequent determination is a promising technique for selective sensing explosive markers at low concentrations. Here, we report divinylbenzene monolithic polymer in its blank form (neat-DVB) and as a composite incorporated with sodalite topology zeolite-like metal-organic frameworks (3-ZMOF@DVB), as a sensitive, selective, and cost-effective porous preconcentrator for aliphatic nitroalkanes in the vapor phase as explosive markers at infinite dilution. The developed materials were fabricated as 18 cm gas chromatography (GC) monolithic capillary columns to study their separation performance of nitroalkane mixture and the subsequent physicochemical study of adsorption using the inverse gas chromatography (IGC) technique. A strong preconcentration effect was indicated by a specific retention volume adsorption/desorption ratio equal to 3 for nitromethane on the neat-DVB monolith host-guest interaction, and a 14% higher ratio was observed using the 3-ZMOF@DVB monolithic composite despite the low percentage of 0.7 wt.% of sod-ZMOF added. Furthermore, Incorporating ZMOF resulted in a higher percentage of micropores, increasing the degree of freedom more than bringing stronger adsorption and entropic-driven interaction more than enthalpic. The specific free energy of adsorption (ΔGS) values increased for polar probes and nitroalkanes, denoting that adding ZMOFs earned the DVB monolithic matrix a more specific character. Afterward, Lewis acid-base properties were calculated, estimating the electron acceptor (KA) and electron donor (KB) constants. The neat-DVB was found to have a Lewis basic character with KB/KA = 7.71, and the 3-ZMOF@DVB had a less Lewis basic character with KB/KA = 3.82. An increased electron-accepting nature can be directly related to incorporating sod-ZMOF into the DVB monolithic matrix. This work considers the initial step in presenting a portable explosives detector or preconcentrating explosive markers trace prior to more sophisticated analysis. Additionally, the IGC technique allows for understanding the factors that led to the superior adsorption of nitroalkanes for the developed materials.

Hydrophilic interaction liquid chromatography with methanol-water eluent on a zeolite

BACKGROUND

Hydrophilic interaction chromatography (HILIC) works with organic solvent-water mixtures as eluent and is based on the formation of a water enriched liquid phase on the surface of a hydrophilic stationary phase. Hydrophilic solutes are retained on that stagnant water-rich film depending on the difference of solvation compared to the mobile phase composition. However, the enhancement of selectivity by increasing the fraction of organic cosolvent is coupled with a limitation the analyte solubility, and the improvement of the HILIC principle by new hydrophilic stationary phases is the remaining option.

RESULTS

Y-zeolite (faujasite, FAU type) in the Na⁺-form with an average particle diameter of 5 μm was used as packing material in a 125 mm long HPLC column. The chromatographic response of the column was tested in methanol-water mixtures as eluent after injection of several aliphatic alcohols, polyols and monosaccharides with eluent conditions where no separation occurs on diol functionalized silica. On the zeolite the retention time increases according to ethylene glycol < glycerol < erythritol < sorbitol < inositol. The separation principle is explained to be superposed by two effects: firstly, a partition equilibrium between the water-rich phase in the zeolite micropores exists, and secondly, selective interactions with the inner crystalline pore surface and fixed-position Na⁺ ions, both serving to enhance the selectivity. Furthermore, arabinose and fructose monosaccharides could be separated into their tautomeric forms. Only upon increasing the temperature from 20 to 60 °C the tautomeric pattern merges into a single peak.

SIGNIFICANCE AND NOVELTY

Instead of the stagnant water rich surface layer, zeolite micropores now take over that function. As a result, the selectivity among polyols and between α/β-arabinopyranose and β-fructopyranose/β-fructofuranose tautomers is extraordinary superior towards conventional hydrophilic interaction liquid chromatography (HILIC).

Chiral metal-organic frameworks and their composites as stationary phases for liquid chromatography chiral separation: A minireview

Chiral metal organic frameworks (CMOFs) are a kind of crystal porous framework material that has attracted increasing attention due to the customizable combination of metal nodes and organic ligands. In particular, the highly ordered crystal structure and rich adjustable chiral structure make it a promising material for developing new chiral separation material systems. In this review, the progress of CMOFs and their different types of composites used as chiral stationary phases (CSPs) in liquid chromatography for enantioseparation are discussed. The characteristics of CMOFs and their composites are summarized, aiming to provide new ideas for the development of CMOFs with better performance and further promote the application of CMOFs materials in enantioselective high-performance liquid chromatography (HPLC).

A monolithic composite based on zeolite-like metal-organic framework@divinylbenzene polymer separates azeotropic fluorocarbon mixture efficiently

Organic monolithic columns are mainly used to separate macromolecules; however, many attempts to extend their performance toward small molecules were examined by incorporating micro- and nanoparticles. The incorporation technique enabled utilizing organic monoliths in gas chromatography (GC) for small molecules, which are still scarce. Here, we prepared a composite matrix of capillary monolithic columns of a zeolite-like metal-organic framework with a sodalite topology (sod-ZMOF) and Divinylbenzene polymer (DVB) for GC separations under 0.5 MPa. Relatively short DVB monolithic columns (18 cm long × 0.25 mm i.d.) incorporated with a tiny amount of sod-ZMOF nanoparticles (0.7 and 1.17 wt%) with an average particle size of 225 nm were successfully fabricated and used to separate linear alkanes and polar probes mixtures with increasing resolution up to 3.7 and 5.1 times, respectively, compared to a blank DVB monolithic column. A high-performance separation of linear alkanes series mixture (methane to decane) was exhibited in less than 2 min. McReynolds constants revealed that sod-ZMOF provided the composite monolith with a nonpolar character yielding a negative average polarity value smaller than the standard squalene column. An Excellent retention time of pentane and octane day-to-day reproducibility was achieved during 16 days and over more than 500 runs with RSD% of 2.25% and 3.3% using a composite monolithic column with 5 mg mL^-1 sod-ZMOF (5-ZMOF@DVB). In addition, a qualitative determination of the gas mixture content of three commercially available Lighter gas cartridges was performed via the 5-ZMOF@DVB column. Finally, successfully separating an azeotropic freon mixture of difluoromethane (R-32) and pentafluoroethane (R-125) was achieved with a selectivity of up to 4.84. A further thermodynamic study related the preferential adsorption of R-125 to entropic factors rather than enthalpic while trapping inside ZMOF pores. This work sheds light on utilizing the infinite diversity of MOFs and combining their properties with high permeability and easily fabricated organic monoliths for GC separations of light molecules and gasses. Furthermore, the study highlights the role of GC as an easy and fast approach for the preliminary evaluation of the separation efficiency of porous polymers.

Mixed Metal-Organic Framework Stationary Phases for Liquid Chromatography

Strategic design of the stationary phase in liquid chromatography (LC) is crucial for modern separation science. Herein, a design approach using mixed metal-organic frameworks (MOFs) as tunable LC stationary phases is proposed. Three MOFs with an isostructural pillared-layer structure are employed, with pore sizes tuned by the systematic design of the constituent ligands, using 1,4-benzenedicarboxylate (bdc), 1,4-naphthalenedicarboxylate (ndc), and 9,10-anthracenedicarboxylate (adc). Packed columns filled with the MOFs and their mixed-particle/solid-solution stationary phases are prepared and examined for the retention capability of polyethylene glycol (PEG) in LC. While the MOF-packed columns filled with binary mixtures of different MOF particles provide good control of the retention with respect to the particle mixing ratio, the columns filled with mixed-linker solid-solution MOFs show a significant multicomponent effect on the retention behavior. Specifically, mixed-linker solid-solution MOFs consisting of bdc/ndc binary ligands are found to show a strong retention that surpasses even their parent MOFs, namely, pure bdc- and ndc-MOF stationary phases. The retention behavior on the MOF-packed columns is explained by the specific nanostructures of the solid-solution MOFs, which affects the balance between substrate affinity and adsorption kinetics into the MOF pores, dictating the total retention capability. The results provide an extra dimension for stationary phase design using MOFs as a promising recognition medium for LC.

Evaluation of the Adsorption-Accessible Surface Area of MIL-53(Al) using Cannabinoids in a Closed System

Cannabinoids are important industrial analytes commonly assayed with high-pressure liquid chromatography (HPLC). In this study, we evaluate the suitability of MIL-53(Al), a commercially available metal-organic framework (MOF), as a stationary phase for cannabinoid separations. The suitability of an MOF for a given separation is hypothesized to be limited by the ability of a given molecule to enter the pore of the MOF. To evaluate the extent of possible adsorptive interactions between cannabinoids and the interior surface area of MIL-53(Al), the radii of gyration (R_g) and solvent-accessible surface areas were calculated for three cannabinoids, namely, cannabidiol, cannabinol, and Δ9-tetrahydrocannabinol, as well as the MOF. These values were used to calculate the theoretical adsorption capacity of the MOF, using four competing adsorption models. The R_g of cannabinoids (4.1 Å) is larger than one MOF pore aperture dimension (4.0 × 5.0 Å). The adsorption capacity was measured by relating a decrease in the cannabinoid concentration in acetonitrile when exposed to 100 mg of MOF. The cannabinoid uptake by the MOF was estimated using the relative standard deviation (RSD) of the soaking solution assay, as the decomposition-corrected RSD as uptake (DCRU). The DCRU was calculated as 0.007 ± 0.004 μg_cannabinoids/mg_MOF. These findings indicate that most of the MOF surface area was inaccessible for adsorption by cannabinoids due to size-exclusion effects. The implication of this work is that the suitability of an MOF for adsorptive separations, such as liquid chromatography, must have an upper limit for the size of the analyte. Additionally, MOFs may generally be more suitable for separations in the gas phase, where adsorbates are not hindered by the presence of a solvation shell.

Preparation of poly(ionic liquid@MOF) composite monolithic column and its application in the online enrichment and purification of tectochrysin in medicinal plants

Herein, a poly(ionic liquid@MOF) composite monolithic column was prepared via in situ radical polymerization using ionic liquid (1-allyl-3-methylimidazolium hexafluorophosphate) and MOF (derivatized UIO66-2COOH) as copolymer monomers. The composite monolithic column was characterized via scanning electron microscopy (SEM), nitrogen adsorption-desorption isotherms and mercury intrusion porosimetry. Subsequently, the composite monolithic column combined with high performance liquid chromatography (HPLC) was used as a solid-phase extraction (SPE) absorbent for online purification and enrichment of tectochrysin in medicinal plants. The results indicated that the addition of the ionic liquid and MOF not only increased the surface area but also increased the adsorption capacity of the monolith for tectochrysin. The method showed good linearity in the concentration range of 0.01-500 μg mL^-1. The calibration equation was y = 2154.6x - 8.3785 and the limit of detection (LOD, S/N = 3) and the limit of quantification (LOQ, S/N = 10) were 3.33 ng mL^-1 and 10 ng mL^-1, respectively. The relative standard deviation (RSD) of the intra-day and inter-day precision was less than 2.62%, the RSD of inter-column was less than 3.16%, and the recoveries ranged from 100.58% to 105.00%. Thus, results showed that this method is simple, accurate and convenient for the online enrichment and purification of tectochrysin from medicinal plants.

On the use of metal-organic frameworks for the extraction of organic compounds from environmental samples

The determination of trace metals and organic contaminants in environmental samples, such as water, air, soil, and sediment, is until today a challenging process for the analytical chemistry. Metal-organic frameworks (MOFs) are novel porous nanomaterials that are composed of metal ions and an organic connector. These materials are gaining more and more attention due to their superior characteristics, such as high surface area, tunable pore size, mechanical and thermal stability, luminosity, and charge transfer ability between metals and ligands. Among the various applications of MOFs are gas storage, separation, catalysis, and drug delivery. Recently, MOFs have been successfully introduced in the field of sample preparation for analytical chemistry and they have been used for sample pretreatment of various matrices. This review focuses on the applications of MOFs as novel adsorbents for the extraction of organic compounds from environmental samples.

Preparation and properties of GC capillary column with hypercrosslinked stationary phase

For the first time, highly porous hypercrosslinked polystyrene layer was synthesized within a gas chromatography capillary column and successfully deposited onto the capillary walls generating porous layer open-tubular capillary column. Elaborated three steps synthetic procedure provides tightly bonded porous polymeric layer and ensues complete elimination of particle shedding and the needs for particle traps. Due to highly developed surface area, porous layer open-tubular column provides strong solute retention that is useful for the separation of various volatile solvents and light gas compounds including ethane, ethylene, ethyne. Aqueous injections will not harm the column.

Fluoroarene Separations in Metal-Organic Frameworks with Two Proximal Mg2+ Coordination Sites

Fluoroarenes are widely used in medicinal, agricultural, and materials chemistry, and yet their production remains a critical challenge in organic synthesis. Indeed, the nearly identical physical properties of these vital building blocks hinders their purification by traditional methods, such as flash chromatography or distillation. As a result, the Balz-Schiemann reaction is currently employed to prepare fluoroarenes instead of more atom-economical C-H fluorination reactions, which produce inseparable mixtures of regioisomers. Herein, we propose an alternative solution to this problem: the purification of mixtures of fluoroarenes using metal-organic frameworks (MOFs). Specifically, we demonstrate that controlling the interaction of fluoroarenes with adjacent coordinatively unsaturated Mg²⁺ centers within a MOF enables the separation of fluoroarene mixtures with unparalleled selectivities. Liquid-phase multicomponent equilibrium adsorption data and breakthrough measurements coupled with van der Waals-corrected density functional theory calculations reveal that the materials Mg₂(dobdc) (dobdc^4- = 2,5-dioxidobenzene-1,4-dicarboxylate) and Mg₂(m-dobdc) (m-dobdc^4- = 2,4-dioxidobenzene-1,5-dicarboxylate) are capable of separating the difluorobenzene isomers from one another. Additionally, these frameworks facilitate the separations of fluoroanisoles, fluorotoluenes, and fluorochlorobenzenes. In addition to enabling currently unfeasible separations for the production of fluoroarenes, our results suggest that carefully controlling the interaction of isomers with not one but two strong binding sites within a MOF provides a general strategy for achieving challenging liquid-phase separations.

Chiral porous organic frameworks and their application in enantioseparation

Porous organic frameworks (POFs) are a kind of porous material with a network structure composed of repeated monomers, which have excellent physical and chemical properties, such as a high surface area, high porosity, uniform pore sizes and structural diversity, and which have aroused broad interest among researchers. With the rapid development of materials science, increasingly more porous materials have been developed and applied, especially metal organic frameworks (MOFs) and covalent organic frameworks (COFs), which have been widely applied in the fields of luminous materials, catalytic research, adsorption and drug transport. One of the most important applications for chiral porous materials is in chiral separation and these materials have become a research hotspot in the field of chromatographic separation and analysis in recent years. In this review, from the viewpoint of enantioseparation, the synthesis of chiral porous materials and their applications in high-performance liquid chromatography (HPLC), capillary electrochromatography (CEC), and gas chromatography (GC) are reviewed. The typical applications of MOFs in solid-phase microextraction (SPME) are also discussed.

Preparation of value-added metal-organic frameworks for high-performance liquid chromatography. Towards green chromatographic columns

This work applies the concepts of green chemistry, where polyethylene terephthalate (PET) bottles were used as the acid-dicarboxylic linker source for the synthesis of MIL-53(Al) metal organic frameworks (MOFs) and then used as a stationary phase for the separation of various solutes and compared with MIL-53(Al) synthesized from traditional terephthalic acid. Both synthesized MIL-53(Al) MOFs were characterized by scanning electron microscopy (SEM), FT-IR, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and specific surface area analysis. Eight groups of standard analytes in addition to real samples were tested to evaluate the separation performance of the MIL-53(Al) packed columns in HPLC under various chromatographic conditions. Based on elution order of the studied compounds and the effects of mobile phase composition, the working mechanism was reversed phase mode in the presence of size-exclusion effects for large molecules, which exceeded the dynamic diameter of MIL-53(Al) (~7.6 Å). The effects of stationary phase sieving, mobile phase flow rate and composition, injected sample mass, and temperature were investigated relative to the chromatographic behavior of MIL-53(Al). MIL-53(Al) particle sieving before packing reduced peak broadening and significantly enhanced the chromatographic performance of the prepared columns up to 2.26 times relative to the number of theoretical plates. The MIL-53(Al) packed columns offered high-resolution separation for all studied mixtures with R_s >2 and good stability and long-term durability. At optimal conditions, the prepared columns exhibited efficiencies between 5600-63200 plates m^-1. Higher efficiencies were observed for alkylbenzenes and polyaromatic hydrocarbons as the organic linker in the MIL-53(Al) structure, which improved retention and separation of aromatics through π-π interactions. Thermodynamic parameters including ΔH, ΔS, and ΔG for the transfer of analyte from the mobile phase to the MIL-53(Al) stationary phase were studied. Compared with previously cited MOFs packed columns, the present MIL-53(Al) columns gave comparable selectivity and much better efficiency for most of the studied chemicals at optimum conditions, indicating the feasibility of MIL-53(Al) as a stationary phase for HPLC applications.

Roles of metal, ligand and post synthetic modification on metal organic frameworks to extend their hydrophobicity and applicability toward ultra-trace determination of priority organic pollutants

Implementation of metal organic frameworks (MOFs) in the separation science has attracted many researchers attention. In this study, the role of metal, ligand, the reaction condition and modification on the extraction efficiency of some MOFs was investigated. Among the prevalent reported MOFs, some members of the MIL and MOF-5 families were chosen, and eleven MOF-based sorbents were prepared by changing the metal and ligand type, reaction condition, and/or functionality through post synthetic modification (PSM). MIL-101 and MIL-101-NH₂ based structures were initially synthesized based on the chromium and iron salts. Also, three zinc-based structures including MOF-5, [NH₂(CH₃)₂]₂ [Zn₃(C₆H₄(CO₂)₂)₄].DMF.H₂O and [NH₂(CH₃)₂]₂ [Zn₃((C₆H₄)₂(CO₂)₂)₄].5DMF were synthesized. The PSM hydrophobic-oriented products of MILs were obtained by their reactions with benzyl alcohol. The resulted MOFs were characterized by FT-IR, PXRD, SEM, BET, BJH, water contact angle and TG analyses. The extraction trends of these nanostructures were studied toward some priority environmental pollutants such as polycyclic aromatic hydrocarbons (PAHs), chlorobenzenes (CBs), and benzene homologs. The extraction procedures were performed via adapting a home-made headspace needle trap extraction (HS-NTE) setup, and determinations were followed by gas chromatography-mass spectrometry (GC-MS). Among all the synthesized nanostructures, the chromium-based PSM product of MIL-101-NH-CH₂C₆H₅ proves its improved extraction capability for most of the model compounds. Eventually, the superior MOF was applied as the extractive phase in a HS-NTE-GC-MS method for isolation and trace determination of PAHs, in tea, coffee, and some other environmental water samples. Under the optimized conditions, the linear dynamic range (LDR) was in the range of 1-1000 ng L^-1 (R² > 0.992) while the limits of detection (LOD) and limits of quantification (LOQ) values were 0.1-0.2 and 0.3-0.7 ng L^-1, respectively. Also, the extraction capability of the Cr-based MIL-101-NH-CH₂C₆H₅ was compared with commercial polydimethyl siloxane-divinyl benzene (PDMS-DVB) fiber coating. The intra-and inter-day relative standard deviations for three replicates at the concentration levels of 20 and 500 ng L^-1 were in the range of 4-7% and 5-10%, respectively. The needle-to-needle reproducibility was also found to be in the range of 6-10%. Acceptable relative recovery values at the concentration levels of 20 and 500 ng L^-1 ranged from 89 to 98%, showing no significant matrix effect.

Impact of the Preparation Procedure on the Performance of the Microporous HKUST-1 Metal-Organic Framework in the Liquid-Phase Separation of Aromatic Compounds

To date, metal-organic frameworks (MOFs) have been recognized as promising solid phases in high-performance liquid chromatography (HPLC). This research aimed to elucidate the role of the physico-chemical characteristics of the microporous HKUST-1 metal-organic framework in its operation as a selective adsorbent in HPLC. For this, the HKUST-1 samples were prepared by microwave-assisted synthesis and a solvothermal procedure. According to the chromatographic examinations, the HKUST-1 material synthesized in the microwave fields shows an efficient performance in the selective adsorption of aromatic compounds with different functionalities. This study revealed a significant impact of the preparation procedure on the mechanism of the liquid-phase adsorption on the HKUST adsorbents under conditions of the HPLC. An effect of the elution solvent with the different coordination ability to the Cu²⁺ sites in the HKUST-1 structure on the adsorption selectivity was observed.

Exploring the Parameter Space of p-Cresyl Sulfate Adsorption in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have high porosity and surface area, making them ideal candidates for adsorption-mediated applications. One high-value application is the removal of uremic toxins from solution for dialysis. Previous studies have reported adsorptive removal of the uremic toxin p-cresyl sulfate from solution via zirconium-based MOFs, but a specific analysis of parameters contributing to adsorptive uptake is needed to clarify differences in uptake performance between MOFs. We synthesized zirconium 1,3,5-benzenetricarboxylate (MOF-808) and an iron-based analog, MIL-100(Fe), and compared their adsorptive uptake with previously reported values of other zirconium-based MOFs. MIL-100(Fe) adsorbed three times more p-cresyl sulfate from solution on a per mass basis than MOF-808 and had a greater adsorption efficiency than 75% of previously reported Zr-based MOFs. We compared p-cresyl sulfate uptake by MOFs as a function of BET surface area, number of aromatic carbons in the organic linker, internal cage diameter, and pore window diameter. There is poor correlation between p-cresyl sulfate uptake and each of the variables considered, but the number of aromatic carbons of the MOF linker was a better predictor of uptake than BET surface area (R² = 0.7034 and 0.1430, respectively), and pore window aperture was a better predictor of uptake than the pore cage diameter (R² = 0.4780 and 0.0383, respectively). We hypothesize that the greater adsorptive capacity of MIL-100(Fe) compared to MOF-808 results from direct coordination of p-cresyl sulfate to vacant metal sites in the MOF, and the total adsorption may be accounted for by some combination of adsorptive interactions occurring at both metal and organic linker sites near to the exterior particle surface. The adsorptive uptake of p-cresyl sulfate by MIL-100(Fe) was observed to increase with p-cresyl sulfate content, mass of MIL-100(Fe), and volume of p-cresyl sulfate solution; the mass of MIL-100(Fe) had the greatest effect on total adsorption.

Metal-Organic Framework Stationary Phases for One- and Two-Dimensional Micro-Gas Chromatographic Separations of Light Alkanes and Polar Toxic Industrial Chemicals

Despite promising advances with metal-organic frameworks (MOFs) as stationary phases for chromatography, the application of MOFs for one- and two-dimensional micro-gas chromatography (μGC and μGC × μGC) applications has yet to be shown. We demonstrate for the first time, μGC columns coated with two different MOFs, HKUST-1 and ZIF-8, for the rapid separation of high volatility light alkane hydrocarbons (natural gas) and determined the partition coefficients for toxic industrial chemicals, using μGC and μGC × μGC systems. Complete separation of natural gas components, methane through pentane, was completed within 1 min, with sufficient resolution to discriminate n-butane from i-butane. Layer-by-layer controlled deposition cycles of the MOFs were accomplished to establish the optimal film thickness, which was validated using GC (sorption thermodynamics), quartz-crystal microbalance gravimetric analysis and scanning electron microscopy. Complete surface coverage was not observed until after ~17 deposition cycles. Propane retention factors with HKUST-1-coated μGC and a state-of-the-art polar, porous-layer open-tubular (PLOT) stationary phase were approximately the same at ~7.5. However, with polar methanol, retention factors with these two stationary phases were 748 and 59, respectively, yielding methanol-to-propane selectivity factors of ~100 and ~8, respectively, a 13-fold increase in polarity with HKUST-1. These studies advance the applications of MOFs as μGC stationary phase.

Extraction of Metal Ions with Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are crystalline porous materials composed of metal ions or clusters coordinated with organic linkers. Due to their extraordinary properties such as high porosity with homogeneous and tunable in size pores/cages, as well as high thermal and chemical stability, MOFs have gained attention in diverse analytical applications. MOFs have been coupled with a wide variety of extraction techniques including solid-phase extraction (SPE), dispersive solid-phase extraction (d-SPE), and magnetic solid-phase extraction (MSPE) for the extraction and preconcentration of metal ions from complex matrices. The low concentration levels of metal ions in real samples including food samples, environmental samples, and biological samples, as well as the increased number of potentially interfering ions, make the determination of trace levels of metal ions still challenging. A wide variety of MOF materials have been employed for the extraction of metals from sample matrices prior to their determination with spectrometric techniques.

Evolution and Evaluation of GC Columns

A chromatographic column is the fundamental element required for gas-chromatographic analysis. The separation of components coming from complex mixtures, prior to their detection was leading to a prominent revolution in different areas of science. Moreover, current advances in gas chromatographic (GC) columns technology and development have been providing almost unlimited possibilities for analysis employing diverse matrices. We aim through this review article to describe the evolution of chromatographic columns, by pointing the most important stages, as well as the new trends and future perspectives predicted for the new generation of GC columns. Furthermore, it was in our scope to present the main fundamentals regarding the theoretical relationships that describe the chromatographic separation, to introduce concepts related to columns selection in accordance with the required application as well as to discuss the available evaluation parameters for columns efficiency. Consequently, the early stages of first columns preparation up to the development of GC capillary columns used nowadays, together with examples of their applications are also reported and described in detail.

Evaluation of metal-organic framework NH2-MIL-101(Fe) as an efficient sorbent for dispersive micro-solid phase extraction of phenolic pollutants in environmental water samples

This work proposes an application of amine-functionalized metal-organic framework (NH₂-MIL-101(Fe)) as sorbent for dispersive micro-solid phase extraction (D-μSPE) of ten priority phenolic pollutants. The sorbent was simply synthesized under facile condition. The entire D-μSPE process was optimized by studying the effect of experimental parameters affecting the extraction recovery of the target analytes. The final extract was analyzed using high performance liquid chromatography with photodiode array detector. Under the optimum condition, the proposed procedure can be applied for wide linear calibration ranges between 1.25–5000 μg L⁻¹ with the correlation coefficients of greater than 0.9900. The limits of detection (LODs) and limits of quantitation (LOQs) were in the ranges of 0.4–9.5 μg L⁻¹ and 1.25–30 μg L⁻¹, respectively. The precision evaluated in terms of the relative standard deviations (RSDs) of the intra- and inter-day determinations of the phenol compounds at their LOQ concentrations were below 13.9% and 12.2%, respectively. High enrichment factors up to 120 were reached. The developed method has been successfully applied to determine phenol residues in environmental water samples. The satisfactory recoveries obtained by spiking phenol standards at two different concentrations (near LOQs and 5 times as high as LOQs) ranged from 68.4–114.4%. The results demonstrate that the NH₂-MIL-101(Fe) material is promising sorbent in the D-μSPE of phenolic pollutants.

Well-Defined Materials for High-Performance Chromatographic Separation

Chromatographic separation has been widely applied in various fields, such as chemical engineering, precision medicine, energy, and biology. Because chromatographic separation is based on differential partitioning between the mobile phase and stationary phase and affected by band dispersion and mass transfer resistance from these two phases, the materials used as the stationary phase play a decisive role in separation performance. In this review, we discuss the design of separation materials to achieve the separation with high efficiency and high resolution and highlight the well-defined materials with uniform pore structure and unique properties. The achievements, recent developments, challenges, and future trends of such materials are discussed. Furthermore, the surface functionalization of separation ma-terials for further improvement of separation performance is reviewed. Finally, future research directions and the challenges of chromatographic separation are presented.

Amphiphilic triptycene-based stationary phase for high-resolution gas chromatographic separations

This work reports a new type of triptycene-based amphiphilic stationary phase (TP-2IL) for gas chromatography (GC). It is an integration of the 3D π-rich triptycene framework with ionic liquids. Its capillary column showed the efficiency of 3880 plates/m determined by n-dodecane at 120 °C (k = 2.79) and exhibited good performance for analytes from apolar to polar nature. Particularly, it has outstanding capability for resolving critical pairs of anilines and phenols with good peak shapes and shows distinct advantages over its composing counterparts (TP-2BO and O-IL) and widely-used commercial columns, namely 35% phenyl methyl polysiloxane (DB-35) and polyethylene glycol (INNOWAX). Moreover, the TP-2IL column exhibited good repeatability and reproducibility with the values of relative standard deviation in the range of 0.02%-0.07% for run-to-run, 0.10%-0.35% for day-to-day and 2.9%-5.1% for column-to-column, respectively, and good thermal stability up to 300 °C. Furthermore, its applications for determining isomer impurities in real samples demonstrate its feasibility for practical GC analysis. This work presents a facile strategy for constructing triptycene-based stationary phases with amphiphilic selectivity and provides alternatives of highly selective stationary phases for chromatographic analysis.

Emerging porous materials in confined spaces: from chromatographic applications to flow chemistry

Porous materials confined within capillary columns/microfluidic devices are discussed, and progress in chromatographic and membrane separations and catalysis is reviewed.

Excess adsorption of acetonitrile and water on MIL-100(Fe) and its potential application in mixed-mode chromatography

Potential application of MIL-100(Fe) to reversed- and-normal phase mixed-mode chromatography was suggested.

Metal-Organic Framework Modified Glass Substrate for Analysis of Highly Volatile Chemical Warfare Agents by Paper Spray Mass Spectrometry

Paper spray mass spectrometry has been shown to successfully analyze chemical warfare agent (CWA) simulants. However, due to the volatility differences between the simulants and real G-series (i.e., sarin, soman) CWAs, analysis from an untreated paper substrate proved difficult. To extend the analytical lifetime of these G-agents, metal-organic frameworks (MOFs) were successfully integrated onto the paper spray substrates to increase adsorption and desorption. In this study, several MOFs and nanoparticles were tested to extend the analytical lifetimes of sarin, soman, and cyclosarin on paper spray substrates. It was found that the addition of either UiO-66 or HKUST-1 to the paper substrate increased the analytical lifetime of the G-agents from less than 5 min detectability to at least 50 min.

Highly CO2 Selective Microporous Metal-Imidazolate Framework-Based Mixed Matrix Membranes

Microporous metal-imidazolate framework (MMIF), a highly CO₂ selective MOF, was incoporated into a polymeric membrane for separation of CO₂ from CH₄ and N₂ for the first time. MMIF nanoparticles of 50-200 nm were synthesized using the sonication method and dispersed into Matrimid, a commercial polyimide, with MOF loading of 10% and 20% by weight to fabricate mixed matrix membranes (MMMs). Morphology, thermal behavior, and glass transition temperature of the membranes were characterized, and single and mixed gas permeation measurements at 35 °C and 4 bar feed pressure were carried out to reveal their separation performance. Both 10% and 20% MMIF containing Matrimid membranes exhibited enhanced gas permeabilities for all three gases. Contrary to expectations, ideal selectivity of membranes was not improved possibly due to the flexible framework of MMIF. On the other hand, mixed gas permeability measurements showed significant improvement in CO₂/CH₄ separation factor by 130% and CO₂/N₂ separation factor by 79% due to competitive adsorption favoring CO₂.