Capillary electrophoresis of neurotransmitters using in-line solid-phase extraction and preconcentration using a methacrylate-based weak cation-exchange monolithic stationary phase and a pH step gradient

A novel approach for in-line solid-phase extraction capillary electrophoresis (SPE-CE) for basic analytes was developed. The method is based on the use of a weak cation-exchange monolith synthesised in situ in the front end of the CE capillary via photoinitiated polymerization to form poly(methacrylic acid-co-ethylene glycol dimethacrylate), which was used to create the SPE phase in-line with the CE separation capillary. The monolithic SPE material exhibited a surface area of 23.1 m2/g and a capacity of 403 nM for dopamine. Adsorption of the analytes as protonated, cationic species onto the SPE phase was achieved using an electrolyte of 6 mM phosphate and 12 mM sodium ion, buffered at pH 7.0, which is above the pKa of the monolith but below the pKa of the analytes. Elution of the analytes from the SPE phase was achieved using an electrolyte with a pH below that of the pKa of the monolith, namely 12 mM phosphate and 12 mM sodium ion, buffered at pH 3.0. Due to the discontinuous electrolyte combination, analytes were simultaneously eluted and focused as the electrophoretically mobilised pH step boundary moved through the SPE monolith, after which the analytes were separated by conventional CZE in the remainder of the capillary. Quantitative extraction from a solution of 0.5 microg/ml dopamine and epinephrine was achieved when flushing up to 15 column volumes of sample through the capillary. The limits of detection (S/N=3) for dopamine and epinephrine were 3.7 and 4.3 ng/ml, and this method provided a sensitivity enhancement for dopamine of 462 times compared to CZE using hydrodynamic injection. The developed method was used to preconcentrate a test mixture of neurotransmitters comprising dopamine, epinephrine, 5-hydroxytryptamine, metanephrine and also histamine. The applicability of this approach to real life samples was demonstrated by using a urine sample from a healthy person to detect dopamine at sub-ppm levels.     

Separation of Nile Blue-labelled fatty acids by CE with absorbance detection using a red light-emitting diode

The separation of fatty acids derivatised with Nile Blue (NB) by CE with detection using a red light-emitting diode (LED) was examined. NB was selected as the derivatisation agent due to its high molar absorption coefficient of 76,000 M(-1) cm(-1) at 633 nm, making it well suited for sensitive absorbance detection using a red 635 nm LED. NB-labelled fatty acids were separated by both MEKC using SDS micelles, i-PrOH and n-BuOH and by NACE in a number of solvents including MeOH, EtOH and ACN. The sensitivity of NACE was superior to MEKC, with detection limits of 5x10(-7)-7x10(-7) M obtained for each acid, approximately 20 times lower than the MEKC method. The NACE detection limits are approximately 100 times lower than previous reports on the separation of fatty acids by CE using indirect absorbance detection, ten times lower than using indirect fluorescence detection and are inferior only to those obtained using precapillary derivatisation and direct fluorescence detection. The efficiency of the NACE method was also superior to MEKC and allowed the separation of unsaturated fatty acids to be examined, although it was not possible to baseline-resolve linoleic (C18:2) and linolenic (C18:3) acids in a reasonable time. The method was used to analyse the fatty acid profile of two edible oils, namely sunflower and sesame oils, after alkali hydrolysis, where it was possible to identify both the saturated and unsaturated fatty acids in each sample.     

Convex sweeping process in the framework of measure differential inclusions and evolution variational inequalities

In this paper, we analyze and discuss the well-posedness of two new variants of the so-called sweeping process, introduced by Moreau in the early 70s (Moreau in Sém Anal Convexe Montpellier, 1971) with motivation in plasticity theory. The first new variant is concerned with the perturbation of the normal cone to the moving convex subset \(C(t)\) , supposed to have a bounded variation, by a Lipschitz mapping. Under some assumptions on the data, we show that the perturbed differential measure inclusion has one and only one right continuous solution with bounded variation. The second variant, for which a large analysis is made, concerns a first order sweeping process with velocity in the moving set \(C(t)\) . This class of problems subsumes as a particular case, the evolution variational inequalities [widely used in applied mathematics and unilateral mechanics (Duvaut and Lions in Inequalities in mechanics and physics. Springer, Berlin, 1976]. Assuming that the moving subset \(C(t)\) has a continuous variation for every \(t\in [0,T]\) with \(C(0)\) bounded, we show that the problem has at least a Lipschitz continuous solution. The well-posedness of this class of sweeping process is obtained under the coercivity assumption of the involved operator. We also discuss some applications of the sweeping process to the study of vector hysteresis operators in the elastoplastic model (Krej?? in Eur J Appl Math 2:281–292, 1991), to the planning procedure in mathematical economy (Henry in J Math Anal Appl 41:179–186, 1973 and Cornet in J. Math. Anal. Appl. 96:130–147, 1983), and to nonregular electrical circuits containing nonsmooth electronic devices like diodes (Acary et al. Nonsmooth modeling and simulation for switched circuits. Lecture notes in electrical engineering. Springer, New York 2011). The theoretical results are supported by some numerical simulations to prove the efficiency of the algorithm used in the existence proof. Our methodology is based only on tools from convex analysis. Like other papers in this collection, we show in this presentation how elegant modern convex analysis was influenced by Moreau’s seminal work.     

Thermal Convection in a Darcy Porous Medium with Anisotropic Spatially Varying Permeability

We consider the problem of thermal convection in a saturated porous material of Darcy type. We allow a non-zero inertia term and also allow the permeability to be an anisotropic tensor. For a particular numerical result we consider the case of permeability tensor \(\mathbf{K}=\operatorname{diag} \{ {K_{\amalg},K_{\amalg},K_{\perp}(z) } \}\) . Linear instability results are calculated numerically and it is proved that the nonlinear energy stability bound is the same as the linear one.     

A reusable zirconium(IV) Schiff base complex catalyzes highly efficient synthesis of quinoxalines under mild conditions

A zirconium(IV) complex of a bidentate Schiff base (ZrL₂Cl₂) has been synthesized by the reaction of (z)-N-benzylidene-2-hydroxypropane-1-amine (HL) and ZrCl₄. Spectroscopic data and elemental analyses are consistent with a monomeric complex with a ligand:Zr ratio of 2:1. The catalytic activity of ZrL₂Cl₂ has been investigated for the efficient synthesis of a wide variety of quinoxaline derivatives under mild conditions. The employment of ethanol as an environmentally benign solvent in this high yield method, along with high turnover numbers and reusability of the catalyst providing ready scalability, makes it appropriate for practical applications.     

On-line simultaneous and rapid separation of anions and cations from a single sample using dual-capillary sequential injection-capillary electrophoresis

A novel capillary electrophoresis (CE) approach has been developed for the simultaneous rapid separation and identification of common environmental inorganic anions and cations from a single sample injection. The method utilised a sequential injection-capillary electrophoresis instrument (SI-CE) with capacitively-coupled contactless conductivity detection (C⁴D) constructed in-house from commercial-off-the-shelf components. Oppositely charged analytes from a single sample plug were simultaneously injected electrokinetically onto two separate capillaries for independent separation and detection. Injection was automated and may occur from a syringe or be directly coupled to an external source in a continuous manner. Software control enabled high sample throughput (17 runs per hour for the target analyte set) and the inclusion of an isolation valve allowed the separation capillaries to be flushed, increasing throughput by removing slow migrating species as well as improving repeatability. Various environmental and industrial samples (subjected only to filtering) were analysed in the laboratory with a 3 min analysis time which allowed the separation of 23 inorganic and small organic anions and cations. Finally, the system was applied to an extended automated analysis of Hobart Southern Water tap water for a period of 48 h. The overall repeatability of the migration times of a 14 analyte standard sample was less than 0.74% under laboratory conditions. LODs ranged from 5 to 61 μg L⁻¹. The combination of automation, high confidence of peak identification, and low limits of detection make this a useful system for the simultaneous identification of a range of common inorganic anions and cations for discrete or continuous monitoring applications.     

Ion-exchange and hydrophobic interactions affecting selectivity for neutral and charged solutes on three structurally similar agglomerated ion-exchange and mixed-mode stationary phases

The nature and extent of mixed-mode retention mechanisms evident for three structurally related, agglomerated, particle-based stationary phases were evaluated. These three agglomerated phases were Thermo Fisher ScientificIon PacAS11-HC – strong anion exchange, Thermo Fisher Scientific IonPac CS10 – strong cation-exchange PS-DVB, and the Thermo Fisher Scientific Acclaim Trinity P1silica-based substrate, which is commercially marketed as a mixed-mode stationary phase. All studied phases can exhibit zwitterionic and hydrophobic properties, which contribute to the retention of charged organic analytes. A systematic approach was devised to investigate the relative ion-exchange capacities and hydrophobicities for each of the three phases, together with the effect of eluent pH upon selectivity, using a specifically selected range of anionic, cationic and neutral aromatic compounds. Investigation of the strong anion-exchange column and the Trinity P1 mixed-mode substrate, in relation to ion-exchange capacity and pH effects, demonstrated similar retention behaviour for both the anionic and ampholytic solutes, as expected from the structurally related phases. Further evaluation revealed that the ion-exchange selectivity of the mixed-mode phase exhibited properties similar to that of the strong anion-exchange column, with secondary cation-exchange selectivity, albeit with medium to high anion-exchange and cation-exchange capacities, allowing selective retention for each of the anionic, cationic and ampholytic solutes. Observed mixed-mode retention upon the examined phases was found to be a sum of anion- and cation-exchange interactions, secondary ion-exchange and hydrophobic interactions, with possible additional hydrogen bonding. Hydrophobic evaluation of the three phases revealed log P values of 0.38–0.48, suggesting low to medium hydrophobicity. These stationary phases were also benchmarked against traditional reversed-phase substrates namely, octadecylsilica YMC-Pac Pro C18 and neutral μPS-DVB resin IonPac NS1-5u, yielding log P values of 0.57 and 0.52, respectively.     

Electrochemical impedance spectroscopy as a highly sensitive tool for a dynamic interaction study between heparin and antithrombin: a novel antithrombin sensor

Specific recognition between two biological partners is widely exploited in biosensors nowadays. To explore this avenue, a novel biosensor for antithrombin (AT) detection was constructed. Heparin was used as the affinity ligand.A well-known acrylic monomer (butyl methacrylate) was polymerized and grafted onto the heparin polysaccharide by the use of ceric ammonium nitrate as a redox initiator in aqueous nitric acid medium. Polymers were deposited as a thin layer onto surface of stainless steel electrode (SS316L).The obtained polymers were studied by Fourier transform infrared spectroscopy (FTIR) and analyzed by differential scanning calorimetry (DSC).Moreover, the films were characterized by electrochemical impedance spectroscopy (EIS), contact-angle measurements and AFM.EIS was used to study the biosensor affinity to AT and the relationship between functionalization growth of modified electrode and the response of the sensor.The proposed approach appears to be simple, sensitive and correlated with methods that analyse the detection of antithrombin.     

Retention behavior and selectivity of a latex nanoparticle pseudostationary phase for electrokinetic chromatography

The retention characteristics and separation selectivity of a novel latex nanoparticle (NP) pseudostationary phase (PSP) for electrokinetic chromatography have been characterized. The anionic NPs have very low or no affinity for cationic solutes, but show significant interactions and retention based on hydrophobic interactions. Retention factors of alkyl-phenyl ketones increase linearly with the concentration of the NPs and have zero or near zero y-intercepts as expected for electrokinetic chromatography with non-micellar PSPs. The retention factors of these solutes and representative pharmaceuticals decrease logarithmically with increases in the concentration of ACN in the background electrolyte, as expected for reversed-phase retention. Linear solvation energy relationship analysis indicates that the NPs are less cohesive than would be expected for polymeric PSPs with similar structure but that the overall separation selectivity can be expected to be similar to polymer PSPs with similar backbone chemistry. The results indicate that the hydrophobic core of the NPs is non-cohesive and is highly accessible to solutes, whereas the ionic head groups are not as accessible and do not contribute substantially to retention or selectivity.     

The nonclassical noncovalent interactions control: A case study of the crystal structure of 3,5-dibromo-2-amino-4,6-dimethylpyridinium tetrahalocuprate [3,5-DBr-2-A-4,6-DMPH]2CuX4 (X = Cl, and Br)

The compound [3,5-dibromo-2-amino-4,6-dimethylpyridinium]₂CuCl₄, (1), was prepared from acidic hydrochloric acetonitrilic medium containing CuCl₂ and 3,5-dibromo-2-amino-4,6-dimethylpyridinium bromide while [3,5-dibromo-2-amino-4,6-dimethylpyridinium]₂CuBr₄, (2), from acidic HBr/Br₂ ethanolic medium containing 4,6-dimethylpyridine and CuBr₂. The ring got dibrominated at 3 and 5 positions in situ. Single-crystal X-ray diffraction proved the compounds to be isomorphous in P − 1. A systematic study of the crystallographic parameters in dibrominated and nonbrominated 2-amino-4,6-dimethylpyridine salts will explain the role of nonclassical noncovalent interaction between aryl bromine and ionic metal bromide in the control of structure.