Simultaneous chromatographic separation of the anomers of saccharides on a polymer sulfobetaine zwitterionic stationary phase.

Simultaneous chromatographic separation of the anomers of saccharides was achieved by using a polymer zwitterionic stationary phase functionalized by acrylamide-type sulfobetaine. By optimization of separation parameters including column temperature, pH, and flow rate, the column operated in hydrophilic interaction chromatography mode exhibited excellent separation selectivity toward five monosaccharides and their anomers (including ribose, xylose, galactose, glucose, and arabinose) and two disaccharides (lactose and maltose). Baseline separation could be achieved at mild operation conditions such as 20-30°C of column temperature or typical mobile phase composition (85% acetrontrile-15% 20 mM ammonium formate [NH4 FA]) with wide pH tolerance range of 2-8. This offers a rapid way to determine the configuration of α or ß anomer of the saccharides.

Amino-functional magnetic covalent organic framework as an effective adsorbent for the determination of neonicotinoids in food samples.

Recently, covalent organic frameworks have gained popularity in sample pretreatment. However, the application of covalent organic frameworks in the enrichment of hydrophilic compounds remains a challenge. Thus, a functionalized magnetic covalent organic framework equipped with amino groups was constructed using a bottom-up functionalization strategy. Considering the advantages of this novel adsorbent such as high porosity, large adsorption capacity, and hydrophilic surface, a sensitive magnetic solid-phase extraction coupled with high-performance liquid chromatography-tandem mass spectrometry method was established for the effective determination of neonicotinoids. This method exhibited good linearities with correlation coefficients ranging from 0.9983 to 0.9995, low detection limits in the range 0.003-0.009 ng g-1 and 0.001-0.013 ng mL-1, and limits of quantification in the range 0.010-0.031 ng g-1 and 0.004-0.044 ng mL-1. Furthermore, satisfactory repeatability with relative standard deviations ≤ 6.7% and spiked recoveries between 82.3 and 99.8% were obtained. This work not only provided a promising adsorbent for the sensitive determination of trace-level neonicotinoids but also represented a unique insight for effective enrichment of super hydrophilic hazards.

Gas-Free Continuously Regenerated Impurity Removal Device for Ion Chromatography.

A gas-free continuously regenerated-anion impurity removal device (CR-ARD) is described for ion chromatography (IC). It is a sandwiched configuration consisting of three channels. The central eluent channel is isolated from two outer regenerant channels by stacked anion exchange membranes (sAEM) and a bipolar membrane (BPM) plus stacked cation exchange membranes (BPM-sCEM), in which the anion exchange side of BPM is facing the central channel. The sAEM side is an anode with respect to the cathode of the BPM-sCEM side. The central channel is packed with strongly basic anion exchange resins in hydroxide form. The application of an electrical voltage to the device causes enhanced water splitting at the interface of BPM to occur and the potential drives the hydroxide ions through the resin phase toward the anode, thereby enabling continuous regeneration of the resin and avoiding any off-line regeneration. Meanwhile, nonhydroxide anionic impurities (basically carbonate) are transported toward the anode through the sAEM. It does not produce gas in the eluent channel, and no degasser is required afterward. It shows a strong ability to remove anionic impurities, as indicated by effective removal of 5 mM CO32- injected sample plug (25 µL, 125 pmol) or continuous removal of 600 µM KNO3 solution at 1 mL/min (10 nmol/s) over 300 min. Much improved peak integration at the gradient mode and a higher signal-to-noise ratio at the isocratic mode can be achieved by CR-ARD. To our knowledge, this is the first account of gas-free CR-ARD.

A dissolved inorganic carbon measurement method featuring self-calibration function via an electrodialytic generator.

A simple dissolved inorganic carbon (DIC) measurement method featuring self-calibration function via an electrodialytic bicarbonate eluent generator (cEDG) is described. It is based on gas diffusion flow analysis system that uses conductometric detection for sensing the resultant conductivity changes of the effluent caused by CO2 penetration. The standard carbon sources with concentration ranging from 0.1 to 6 mM produced online by cEDG are for DIC calibration, eliminating manual preparation.

A strong anion exchanger of poly(glycidyl methacrylate-divinylbenzene) substrate functionalized with cationic quaternary ammonium monomer.

A strong anion exchanger applied for ion chromatography is described by grafting methacrylamido propyl trimethyl ammonium chloride onto hydrolyzed poly(glycidyl methacrylate-divinylbenzene) substrate. The anion exchanger has been in detail characterized and the final capacity of the anion exchanger can be readily manipulated by different monomer amounts. It exhibited obvious temperature-sensitive properties and large tolerance to the samples with high ionic strength. The obtained anion exchanger demonstrated effective separation ability towards the common seven inorganic anions within 12 min and high separation efficiency, for example, 58 000/m of theoretical plate count for chloride.

Electrodialysis Pump Based on Enhanced Water Dissociation of Bipolar Membrane.

We describe a novel micropump mode-electrodialysis pump (EDP), which is based on the recombination of hydrogen and hydroxyl ions generated by enhanced water dissociation of bipolar membrane (BPM). The pump is in a sandwich-like configuration in which the central production channel is spatially isolated from two outer regenerant chambers by a BPM, respectively. Both BPMs are put at the same direction, in which the anion exchange membrane (AEM) side is facing the anode with respect to the cation exchange membrane (CEM) side facing the cathode. Pure water as the feed solution flows through the regenerant chambers at a conventional flow rate (e.g., 0.2 mL/min). Under the electric field, enhanced water dissociation at the junction layer of BPM will occur, generating hydroxyl and hydrogen ions. Their electrodialytic migration into the central channel will recombine water and its flow rate is correlated with the applied current. The pump shows near-ideal Faradaic efficiency and at least 0.8 µL/min can be achieved by controlling the current. The produced water is near neutral and obvious enrichment of trace impurity anions is observed in the production channel.

A weak cation exchanger by encapsulating silica with maleic anhydride-modified polyvinyl alcohol.

A novel weak cation exchanger is described by encapsulating silica with a (or multiple) layer(s) of maleic anhydride-modified polyvinyl alcohol coating(s). The preparation method is facile and fast, which is simply performed by dipping silica particles into maleic anhydride-modified polyvinyl alcohol solution as-synthesized, then filtering and curing thermally, and finally generating a thin coating onto silica particles. Multiple layers of maleic anhydride-modified polyvinyl alcohol coatings can be generated by repeating above steps, offering an easy way to manipulate the capacity. The obtained weak cation exchanger demonstrated high separation efficiency and good selectivity toward common inorganic cations, for example, high plate count of 81 000 per meter was obtained for NH4 + . Simultaneous separation of alkali and alkaline earth metals could be achieved within 14 min under isocratic conditions.

Fabrication of a two-membrane configured electrodialytic methanesulfonic acid generator for ion chromatography.

An electrodialytic methanesulfonic acid (MSA) generator with a two-membrane configuration is described for ion chromatography (IC). It is fabricated by using a stacked anion exchange membrane (sAEM) and a bipolar membrane (BPM) plus stacked sAEMs (BPM-sAEM) to respectively isolate the central eluent channel from two outer regenerant chambers bearing the MSA feed solution. The cation exchange membrane (CEM) side of a BPM is facing the central channel (the cathode direction) and the sAEM is at the cathode side with respect to the anode of the BPM-sAEM side. Under an electric field, the hydronium ion produced at the junction layer of BPM via enhanced water splitting will electromigrate through CEM associated with BPM into the central channel, meanwhile methanesulfonate from the MSA feed solution at the cathode migrates into the central channel to combine with the hydronium ion together to form a solution of MSA. Such a device can produce a gas-free MSA eluent and no gas removal device is required. The produced MSA concentration was highly dependent on the applied current and up to at least 60.5 mM MSA could be produced. The device showed good running stability, as indicated by the retention time of RSD 0.15% and the peak area of RSD 1.7%.

Online Gas-Free Electrodialytic KOH Eluent Generator for Ion Chromatography.

An online gas-free KOH electrodialytic eluent generator (EDG) with two-membrane configuration is described for ion chromatography (IC). A central eluent channel is separated from two outer regenerant channels bearing KOH solution (or one is water) by stacked cation-exchange membranes (sCEMs) and a bipolar membrane (BPM) plus stacked CEMs (BPM-sCEMs), in which the anion-exchange membrane (AEM) of BPM is facing the central channel (the anode direction). Independent fluid input or output ports address all channels. One platinum screen electrode is put in each outer channel, and the sCEM side is anode with respect to the cathode of the BPM-sCEM side. Under the electric field, enhanced water splitting at the intermediate layer of BPM will occur, offering hydroxide and hydronium; hydroxide electromigrates through AEM into the central channel, while the potassium of KOH feed solution at the anode migrates into the central channel to combine with hydroxide to form a solution of KOH. Because the central eluent channel is spatially isolated from both electrodes, the generated KOH solution is gas-free and no gas-removal device is required. More important, the AEM side of BPM is contactless with the alkaline solution, nearly avoiding possible AEM degradation when immersed into concentrated alkaline solution. This ensures long-term running stability of the EDG and high purity of the produced KOH solution (as indicated by the typical suppressed background conductance of 0.28 µS/cm). The EDG has a pressure tolerance (at least 21 MPa), and the produced KOH concentration is up to 101 µeq/min with near-unity faradaic efficiency. When operated in both isocratic and gradient modes, the EDG demonstrates excellent reproducibility, as indicated by the retention time of RSD ≤ 0.08% and the peak area of RSD ≤ 0.6%. To the best of our knowledge, this is the first description of gas-free EDG matched conventional IC system.

Low-Bleed Silica-Based Stationary Phase for Hydrophilic Interaction Liquid Chromatography.

Bleeding of hydrophilic interaction liquid chromatographic (HILIC) phases is a major problem. A hydrophobic silica surface exhibits low bleeding but does not behave as a HILIC phase. A hydrophilic coating, such as that of poly(vinyl alcohol) (PVA) on a hydrophobic particle should result in a low-bleed hydrophilic phase. However, a silica particle functionalized in a hydrophobic manner is not wetted by PVA and cannot be coated with it. Coating with PVA becomes possible if one region of the hydrophobic functionality is polar. We describe a low-bleed HILIC stationary phase, PVA-coated benzylthioethyl-silica. The benzyl groups shield the silica from water erosion, while the thiol group provides sufficient local polarity for PVA to wet the material. The new stationary phase demonstrated good chromatographic performance and typical HILIC retention behavior. The measured silica concentration in the effluent was ∼80-fold lower than that from a bare silica column. The new stationary phase exhibited a lower background level, lower background noise, and lower background drift under gradient conditions than benchmark commercial columns.

Two-dimensional solid-phase extraction strategy for the selective enrichment of aminoglycosides in milk.

An orthogonal two-dimensional solid-phase extraction strategy was established for the selective enrichment of three aminoglycosides including spectinomycin, streptomycin, and dihydrostreptomycin in milk. A reversed-phase liquid chromatography material (C18 ) and a weak cation-exchange material (TGA) were integrated in a single solid-phase extraction cartridge. The feasibility of two-dimensional clean-up procedure that experienced two-step adsorption, two-step rinsing, and two-step elution was systematically investigated. Based on the orthogonality of reversed-phase and weak cation-exchange procedures, the two-dimensional solid-phase extraction strategy could minimize the interference from the hydrophobic matrix existing in traditional reversed-phase solid-phase extraction. In addition, high ionic strength in the extracts could be effectively removed before the second dimension of weak cation-exchange solid-phase extraction. Combined with liquid chromatography and tandem mass spectrometry, the optimized procedure was validated according to the European Union Commission directive 2002/657/EC. A good performance was achieved in terms of linearity, recovery, precision, decision limit, and detection capability in milk. Finally, the optimized two-dimensional clean-up procedure incorporated with liquid chromatography and tandem mass spectrometry was successfully applied to the rapid monitoring of aminoglycoside residues in milk.

Poly(vinyl alcohol) Modified Porous Graphitic Carbon Stationary Phase for Hydrophilic Interaction Liquid Chromatography.

We report a poly(vinyl alcohol) (PVA)-coated porous graphitic carbon (PGC, Hypercarb) packing as a novel stationary phase for hydrophilic interaction liquid chromatography (HILIC). The exterior and the pores of the PGC particles are coated with a thin layer of PVA by soaking the particles in a PVA solution, filtering, and thermally cross-linking the PVA. Such PVA coated PGC particles (5.7 µm diameter), hereinafter called PVA-PGC are stable at least through pH 1.0-12.7, can be made in <2 h, and exhibit different selectivity relative to six commercial HILIC phases and bare PGC. To our knowledge, this is the first fully pH-stable, completely neutral HILIC phase. Excellent efficiency stable is observed for polar analytes (∼70 000 and 118 000 plates/m for cytosine and resorcinol, respectively). Retention closely resembles standard HILIC behavior. Other substances can also be easily incorporated in the PVA layer; an anion exchange column can be readily made by incorporating diallyldimethylammonium chloride in the PVA coating solution. The ease of preparation without the requirement of synthetic skills or paraphernalia and the possibility of incorporating a variety of modifiers makes this a particularly versatile approach.

A hyperbranched polyethylenimine functionalized stationary phase for hydrophilic interaction liquid chromatography.

A hyperbranched stationary phase for hydrophilic interaction liquid chromatography (HILIC) has been prepared by grafting polyethylenimine (PEI) onto silica gel (termed as PEI-Sil). Rich primary, secondary, and tertiary amino groups associated with PEI render its good hydrophility. More importantly, the hyperbranched structure of PEI molecule is greatly helpful in improving interaction with polar analytes. For several kinds of model polar compounds, including organic acids, nucleosides, nucleic acid bases, amino acids, cephalosporins, and non-reducing sugars, PEI-Sil demonstrated excellent separation performance in terms of running stability, reproducibility, and separation efficiency (e.g., plate count ~74,000/m). In addition, PEI-Sil also exhibited much better separation selectivity toward inorganic anions when operated in the mode of ion chromatography relative to a commercial amino propyl-bonded column.

Separation analysis of macrolide antibiotics with good performance on a positively charged C18HCE column.

The separation of basic macrolide antibiotics suffers from peak tailing and poor efficiency on traditional silica-based reversed-phase liquid chromatography columns. In this work, a C18HCE column with positively charged surface was applied to the separation of macrolides. Compared with an Acquity BEH C18 column, the C18HCE column exhibited superior performance in the aspect of peak shape and separation efficiency. The screening of mobile phase additives including formic acid, acetic acid and ammonium formate indicated that formic acid was preferable for providing symmetrical peak shapes. Moreover, the influence of formic acid content was investigated. Analysis speed and mass spectrometry compatibility were also taken into account when optimizing the separation conditions for liquid chromatography coupled with tandem mass spectrometry. The developed method was successfully utilized for the determination of macrolide residues in a honey sample. Azithromycin was chosen as the internal standard for the quantitation of spiramycin and tilmicosin, while roxithromycin was used for erythromycin, tylosin, clarithromycin, josamycin and acetylisovaleryltylosin. Good correlation coefficients (r(2) > 0.9938) for all macrolides were obtained. The intra-day and inter-day recoveries were 73.7-134.7% and 80.7-119.7% with relative standard deviations of 2.5-8.0% and 3.9-16.1%, respectively. Outstanding sensitivity with limits of quantitation (S/N ≥ 10) of 0.02-1 µg/kg and limits of detection (S/N ≥ 3) of 0.01-0.5 µg/kg were achieved.

A polyvinyl alcohol-coated silica gel stationary phase for hydrophilic interaction chromatography.

Multiple layers of polyvinyl alcohol (PVA) coating are generated onto silica gel by thermal immobilization to form a stationary phase applied for hydrophilic interaction liquid chromatography (HILIC). It offers an easy way to manipulate the thickness of PVA coating and the obtained stationary phase demonstrated high efficiency and high chemical stability.

An arginine-functionalized stationary phase for hydrophilic interaction liquid chromatography.

An arginine-functionalized stationary phase for hydrophilic interaction liquid chromatography (HILIC) has been prepared for the first time by clicking arginine onto silica gel. It offers an efficient separation of organic acids, nucleotides and sugars. More interestingly, it exhibited excellent selectivity and enrichment toward acidic glycopeptides, even at a ratio of 1 : 150 to non-glycopeptides.

Facile preparation of polyvinyl alcohol coated SiO2 stationary phases for high performance liquid chromatography.

A facile method to prepare a polar stationary phase for hydrophilic interaction chromatography (HILIC) was proposed by coating polyvinyl alcohol onto silica particles (PVA-Sil). A water or organic solvent-insoluble permanent PVA coating on the silica particle surface can be formed simply by dipping silica particles into a hot PVA solution and then settled from this solution, leaving a thin layer of PVA coating and frozen in a freezer. The PVA-Sil shields the silica core from solution erosion to some degree and the pH tolerance range was extended to 9.5 from 8.0 for bare silica. PVA-Sil demonstrated a good hydrophilic property for several kinds of polar compounds and ∼57000 m(-1) of plate count was achieved. This method can also be extended as a universal method to prepare various stationary phases with exchangeable functionalities by doping the desired ingredient in a PVA solution.

Heat-shrink tubing as a solid-phase microextraction coating for the enrichment and determination of phthalic acid esters.

Heat-shrink tubing, which shrinks in one plane only (its diameter) when heated, commonly used for sealing protection in electrical engineering, was found to be able to function as a solid-phase microextraction coating. Its utility was demonstrated for the determination of phthalic acid esters in an aqueous solution combined with high-performance liquid chromatography equipped with a UV absorbance detector. The preparation procedure was rather simple and only ∼10 min was needed. The fiber cost is extremely low (∼10 cent each). The parameters affecting the extraction were optimized. Heat-shrink tubing fiber exhibited a significant enrichment effect for the three examined phthalic acid esters and up to 931-fold enrichment factor was obtained. The limit of detection was <10 µg/L for all analytes. The operation repeatability and fiber-to-fiber reproducibility were 1.2-8.3 and 5.4-9.1%, respectively. It was successfully applied for the analysis of bottled drinking water with recoveries ranging from 90.1-100.5%.

Highly selective separation of aminoglycoside antibiotics on a zwitterionic Click TE-Cys column.

Hydrophilic interaction liquid chromatography has emerged as a valuable alternative approach to ion-pair chromatography for the separation of aminoglycoside antibiotics in recent years. However, the resolution of structurally related aminoglycosides is a great challenge owing to the limited selectivity. In this work, a cysteine-based zwitterionic stationary phase (named Click TE-Cys) was utilized and compared with five commonly used hydrophilic interaction liquid chromatography columns. Click TE-Cys displayed much better selectivity for structurally similar aminoglycosides. The retention behaviors of aminoglycosides were investigated in detail, revealing that low pH (2.7 or 3.0) and high buffer concentration (≥50 mM) were preferable for achieving good peak shape and selectivity. Effective resolution of ten aminoglycosides including spectinomycin, dihydrostreptomycin, streptomycin, gentamicin C1, gentamicin C2/C2a, gentamicin C1a, kanamycin, paromonycin, tobramycin, and neomycin was realized at optimized conditions. Additionally, spectinomycin and its related impurities were successfully resolved. The results indicated the great potential of the Click TE-Cys column in the separation of aminoglycoside mixtures and related impurities.

Fabrication and application of a continuously regenerated cationic impurity removal device for ion chromatography.

A continuously regenerated cationic impurity removal device (CR-CRD) has been fabricated and applied for ion chromatography (IC). The removal of cationic impurities is realized by electrodialytically replacing the cationic impurities with hydronium ions. The device is configured in a sandwich structure and the central eluent channel is respectively isolated from both electrodes by stacked cation exchange membranes and a bipolar membrane (BPM) plus stacked anion exchange membranes. The eluent channel is packed with cation exchange resins in hydronium form and their continuous regeneration can be achieved by electrodialysis. A desirable feature of the device is gas-free, and no degasser is required. It showed sufficient ability to remove cationic impurities, as indicated by > 99.9 % removal of 10 mL of 1 mM LiOH solution injected (∼10 µmol) or continuous removal of 1 mM LiOH solution at the flow rate of 1 mL/min (1 µmol/min). A useful application was for sample pretreatment in nuclear power industry, by eliminating strong matrix interference of the sample containing LiOH (1 mM) and boric acid (2000 mg/L) with trace anion analysis.