Two-dimensional liquid chromatography of synthetic polymers

Two-dimensional liquid chromatography, 2D-LC of synthetic polymers is critically assessed. Similarities and differences of 2D-LC of low-molecular-mass and polymeric substances are reviewed. The rationale of application of 2D-LC to macromolecular substances is discussed. Basic information on retention mechanisms in liquid chromatography of synthetic polymers is furnished. The principles, reasons, and significance of coupling of retention mechanisms are explained. The resulting separation processes are elucidated, and the technical concepts of the corresponding experimental arrangements are described. The benefits of 2D-LC are demonstrated together with numerous problems and shortcomings of the method.      

Chiral ligand-exchange chromatography of amino acids using porous graphitic carbon coated with a dinaphthyl derivative of neamine

In this paper, we describe the preparation and the evaluation of a porous graphitic carbon (PGC) column coated with a new dinaphthyl derivative of neamine for chiral ligand-exchange (LE) chromatography. It was shown that the graphitic surface/dinaphthyl anchor system efficiently (1.15 μmol/m²) and stably (three months of intensive use) adsorbs the neamine template onto the chromatographic support. The resulting coated PGC stationary phase showed appreciable LE-based enantioselective properties towards several native amino acids. Chromatographic separation of methionine enantiomers using a dinaphtyl neamine-based ligand-exchange chiral stationary phase     

Preparation and enantioseparation characteristics of three chiral stationary phases based on modified β-cyclodextrin for liquid chromatography

In order to study the effect of the nature and the length of the spacer, three mixed 10-undecenoate/phenylcarbamate derivatives of β-cyclodextrin have been prepared and linked to allylsilica gel by means of a radical reaction. The chiral recognition ability of the resulting materials, when used as liquid chromatography chiral stationary phases (CSPs), was evaluated using heptane and either 2-propanol or chloroform as organic mobile-phase modifiers. A large variety of racemic compounds have been separated successfully on these CSPs (mainly pharmaceuticals and herbicides). Optimization of these separations was discussed in terms of mobile-phase composition and structural patterns of the injected analytes. The efficiencies of the three prepared materials were compared to those of previously described perphenylated-β-cyclodextrin column and to analogous cellulose derivative-based CSPs. Schematic illustration of the b-cyclodextrin/mandelic acid inclusion complex     

Hydrophilic interaction liquid chromatography (HILIC) in proteomics

In proteomics, nanoflow multidimensional chromatography is now the gold standard for the separation of complex mixtures of peptides as generated by in-solution digestion of whole-cell lysates. Ideally, the different stationary phases used in multidimensional chromatography should provide orthogonal separation characteristics. For this reason, the combination of strong cation exchange chromatography (SCX) and reversed-phase (RP) chromatography is the most widely used combination for the separation of peptides. Here, we review the potential of hydrophilic interaction liquid chromatography (HILIC) as a separation tool in the multidimensional separation of peptides in proteomics applications. Recent work has revealed that HILIC may provide an excellent alternative to SCX, possessing several advantages in the area of separation power and targeted analysis of protein post-translational modifications. Figure Artistic impression of the HILIC separation mechanism     

Preparation and evaluation of a new synthetic polymeric chiral stationary phase for HPLC based on the trans-9,10-dihydro-9,10-ethanoanthracene-(11S,12S)-11,12-dicarboxylic acid bis-4-vinylphenylamide monomer

A new synthetic polymeric chiral stationary phase for liquid chromatography was prepared via free-radical-initiated polymerization of trans-9,10-dihydro-9,10-ethanoanthracene-(11S,12S)-11,12-dicarboxylic acid bis-4-vinylphenylamide. The new polymeric chiral stationary phase (CSP) showed enantioselectivity for many chiral compounds in multiple mobile phases. High stability and sample capacities were observed on this polymeric chiral stationary phase. Mobile phase components and additives affected chiral separation greatly. This new synthetic chiral stationary phase is complementary to two other related commercially available CSPs: the P-CAP and P-CAP-DP columns. Interactions between the chiral stationary phase and analytes that lead to retention and chiral recognition include hydrogen bonding, dipolar, and π–π interactions. Repulsive (steric) interactions also contribute to chiral recognition. Figure LC chromatograms showing the analytical (blue) and preparative (red) separations of N-(3,5-dinitrobenzoylleucine) enantiomers on a new synthetic polymeric chiral stationary phase     

Dicationic ionic liquid stationary phase for GC-MS analysis of volatile compounds in herbal plants

The seeming “dual nature” of ionic liquids (ILs) for separating both apolar and polar compounds suggests that ILs may have a great potential for complex samples like essential oils from herbal plants that contain a great variety of compounds. In the present work, a geminal dicationic IL, 1,9-di(3-vinylimidazolium)nonane bis[(trifluoromethyl)sulfonyl]imidate, was investigated for this purpose. To find the best way to achieve satisfactory separations simultaneously for the compounds in essential oils, the dicationic IL was used as the stationary phase for capillary gas chromatography (GC) in two ways, either in its pure state or as a mixed stationary phase with monocationic ILs and a polysiloxane diluent. Interestingly, it was found that the mixed stationary phase exhibited a much better selectivity for polar and nonpolar compounds than either the dicationic IL or the polysiloxane, suggesting that a kind of synergistic effect occurred when these stationary phases were combined in the way described. A comparison with two commercial stationary phases (polar and nonpolar) indicated that this novel mixed stationary phase behaved in a way closer to a polar stationary phase in terms of selectivity and elution order. The present work demonstrates that the mixed stationary phase is efficient and selective and can be an alternative choice for the GC analysis of samples of complex composition. Figure Divinyldiimidazolium-based ionic liquid stationary phase     

Gradient HPLC separation of dehydroepiandrosterone (DHEA) from its metabolites and biological congeners: role of tetrahydrofuran in the chromatographic mechanism

A three-step gradient reversed-phase high-performance liquid chromatography (RP-HPLC) method was developed for the separation of dehydroepiandrosterone (DHEA), its sulfate ester (DHEA-S), its three C7-oxidized metabolites (7αOH-DHEA, 7βOH-DHEA, 7-keto-DHEA), and its biosynthetic congeners (androstenedione, testosterone, estradiol, pregnenolone). This new method allows the quantitative characterization of DHEA metabolism and biosynthetic transformation under given physiological, pathological, or therapeutically influenced circumstances. Tetrahydrofuran probably acts as a proton acceptor coadsorbent, while isopropanol behaves as a proton donor during the separation of testosterone, estradiol, and the stereoisomers of 7-OH-DHEA. Figure Optimized gradient RP-HPLC results in full separation of DHEA from its biosynthetic congeners and metabolites     

Determination of antimicrobial residues and metabolites in the aquatic environment by liquid chromatography tandem mass spectrometry

Antimicrobials are used in large quantities in human and veterinary medicine. Their environmental occurrence is of particular concern due to the potential spread and maintenance of bacterial resistance. After intake by the organisms, the unchanged drug and its metabolized forms are excreted and enter wastewater treatment plants where they are mostly incompletely eliminated, and are therefore eventually released into the aquatic environment. The reliable detection of several antimicrobials in different environmental aqueous compartments is the result of great improvements achieved in analytical chemistry. This article provides an overview of the more outstanding analytical methods based on liquid chromatography tandem mass spectrometry, developed and applied to determine antimicrobial residues and metabolites present in surface, waste, and ground waters.      

Synthesis,characterisation and chromatographic evaluation of polyether dendrimer stationary phases

Supercritical carbon dioxide has attracted attention as a potential replacement for traditional organic solvents due to its simplified workup procedures and reduced environmental impact—providing a green chemistry approach for organic solvent-free functionalisation. In addition to the environmental benefits, the enhanced diffusivity observed in supercritical solvents can often enhance reaction rates. We have applied these valuable features to the preparation of silica-bonded stationary phases and examined their potential in liquid chromatography. We report the successful preparation and characterisation of polyether silica based on Frechet dendrimers—this significantly enhances the range of stationary-phase chemistries that can be prepared in supercritical fluids. First- and second-generation polyether silicas were prepared, characterised, end-capped and evaluated for use as stationary phases for liquid chromatography. Figure SRM1649 on 2nd generation polyether silica Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Dispersive liquid–liquid microextraction using an in situ metathesis reaction to form an ionic liquid extraction phase for the preconcentration of aromatic compounds from water

A novel microextraction method is introduced based on dispersive liquid–liquid microextraction (DLLME) in which an in situ metathesis reaction forms a water-immiscible ionic liquid (IL) that preconcentrates aromatic compounds from water followed by separation using high-performance liquid chromatography. The simultaneous extraction and metathesis reaction forming the IL-based extraction phase greatly decreases the extraction time as well as provides higher enrichment factors compared to traditional IL DLLME and direct immersion single-drop microextraction methods. The effects of various experimental parameters including type of extraction solvent, extraction and centrifugation times, volume of the sample solution, extraction IL and exchanging reagent, and addition of organic solvent and salt were investigated and optimized for the extraction of 13 aromatic compounds. The limits of detection for seven polycyclic aromatic hydrocarbons varied from 0.02 to 0.3 μg L⁻¹. The method reproducibility produced relative standard deviation values ranging from 3.7% to 6.9%. Four real water samples including tap water, well water, creek water, and river water were analyzed and yielded recoveries ranging from 84% to 115%.      

Critical evaluation of sample pretreatment techniques

Sample preparation before chromatographic separation is the most time-consuming and error-prone part of the analytical procedure. Therefore, selecting and optimizing an appropriate sample preparation scheme is a key factor in the final success of the analysis, and the judicious choice of an appropriate procedure greatly influences the reliability and accuracy of a given analysis. The main objective of this review is to critically evaluate the applicability, disadvantages, and advantages of various sample preparation techniques. Particular emphasis is placed on extraction techniques suitable for both liquid and solid samples. Figure Miniaturised extraction techniques allow sensitive analysis of also small sample volumes.     

Application of surface chemical analysis tools for characterization of nanoparticles

The important role that surface chemical analysis methods can and should play in the characterization of nanoparticles is described. The types of information that can be obtained from analysis of nanoparticles using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (TOF-SIMS), low-energy ion scattering (LEIS), and scanning-probe microscopy (SPM), including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), are briefly summarized. Examples describing the characterization of engineered nanoparticles are provided. Specific analysis considerations and issues associated with using surface-analysis methods for the characterization of nanoparticles are discussed and summarized, with the impact that shape instability, environmentally induced changes, deliberate and accidental coating, etc., have on nanoparticle properties.      

Prediction of retention times of polycyclic aromatic hydrocarbons and <Emphasis Type="Italic">n</Emphasis>-alkanes in temperature-programmed gas chromatography

We have developed an iterative procedure for predicting the retention times of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes during separations by temperature-programmed gas chromatography. The procedure is based on estimates of two thermodynamic properties for each analyte (the differences in enthalpy and entropy associated with movements between the stationary and mobile phases) derived from data acquired experimentally in separations under isothermal conditions at temperatures spanning the range covered by the temperature programs in ten-degree increments. The columns used for this purpose were capillary columns containing polydimethylsiloxane-based stationary phases with three degrees of phenyl substitution (0%, 5%, and 50%). Predicted values were mostly within 1% of experimentally determined values, implying that the method is stable and precise. Figure Predicted values were mostly within 1 % of experimentally determined values, thus implying that the method is stable and precise     

Evaluation of quantum dots applied as switchable layer in a light-controlled electrochemical sensor

Gold electrodes with switchable conductance are created by coating the gold surface with different colloidal quantum dots. For the quantum dot immobilization, a dithiol compound was used. By polarizing the electrode and applying a light pointer, local photocurrents were generated. The performance of this setup was characterized for a variety of different nanoparticle materials regarding drift and signal-to-noise ratio. We varied the following parameters: quantum dot materials and immobilization protocol. The results indicate that the performance of the sensor strongly depends on how the quantum dots are bound to the gold electrode. The best results were obtained by inclusion of an additional polyelectrolyte film, which had been fabricated using layer-by-layer assembly.      

Electrokinetic sample extraction and enrichment: a new method for the isolation of analytes from sludge-type matrices

Electrokinetic sample extraction and enrichment is introduced as a newly developed concept for the analysis of substances in sludge-type or paste-like matrices. It is based on electrokinetic transport phenomena as electromigration and electroosmosis occurring when an electrical field is applied to the fresh, wet samples. Problems usually associated to sample drying can be avoided, e.g., losses of volatile analytes or contamination. We have designed and built a suitable apparatus for electrokinetic sample extraction and enrichment. Appropriate operating conditions (field strength, buffer composition, concentration, and volume) were identified in experiments with an artificial sludge model and real-world lake sediments. A proof of principle of the method was provided by the electromigrative extraction and online enrichment on a solid-phase sorbent disk of an azo dye from a diatomaceous earth slurry. Electroosmotic extraction and enrichment of a cyanobacterial hepatotoxin at trace levels was finally investigated as an application example using lake sediments. Rather clean extracts were obtained even with high organic content sediment samples, as shown by high-performance liquid chromatography with diode array detection.      

Sensor technology and its application in environmental analysis

Environmental analysis is one of the fundamental applications of chemical sensors. In this review we describe different sensor systems for the gas and liquid phases that have been tested either with real-life samples or in the field during the last five years. Most field sensors rely either on electrochemical or optical transducers. In the gas phase, systems have been proposed for analysis of oxides of nitrogen, carbon, and sulfur in air, and volatile organic compounds. In the liquid phase, most detection systems used for real-life samples detect heavy-metal ions or organic contamination, for example pesticides, organic solvents and polycyclic aromatic hydrocarbons. Figure Chemical sensors for real-life environmental applications Dedicated to Professor Ulrich Nickel on the occasion of his 65th birthday.     

Two-dimensional electrophoresis on a microfluidic chip for quantitative amino acid analysis

Analysis of complex biological samples requires the use of high-throughput analytical tools. In this work, a microfluidic two-dimensional electrophoresis system was developed with mercury-lamp-induced fluorescence detection. Mixtures of 20 standard amino acids were used to evaluate the separation performance of the system. After fluorescent labeling with fluorescein isothiocyanate, mixtures of amino acids were separated by micellar electrokinetic chromatography in the first dimension and by capillary zone electrophoresis in the second. A double electrokinetic valve system was employed for the sample injection and the switching between separation channels. Under the optimized conditions, 20 standard amino acids were effectively separated within 20 min with high resolution and repeatability. Quantitative analysis revealed linear dynamic ranges of over three orders of magnitudes with detection limits at micromolar range. To further evaluate the reliability of the system, quantitative analysis of a commercial nutrition supplement liquid was successfully demonstrated. Figure       

Biosensing applications of clay-modified electrodes: a review

Two-dimensional layered inorganic solids, such as cationic clays and layered double hydroxides (LDHs), also defined as anionic clays, have open structures which are favourable for interactions with enzymes and which intercalate redox mediators. This review aims to show the interest in clays and LDHs as suitable host matrices likely to immobilize enzymes onto electrode surfaces for biosensing applications. It is meant to provide an overview of the various types of electrochemical biosensors that have been developed with these 2D layered materials, along with significant advances over the last several years. The different biosensor configurations and their specific transduction procedures are discussed.      

Liquid chromatography–electrospray tandem mass spectrometry investigations of fragmentation pathways of biliary 4,4′-methylenedianiline conjugates produced in rats

4,4′-methylenedianiline (DAPM) is the main building block for production of 4,4′-methylenediphenyldiisocyanate that has been widely used in the manufacturing of polyurethane materials including medical devices. Although it was revealed that damage to biliary epithelial cells of the liver and common bile duct occurred upon acute exposure to DAPM, the exact mechanism of DAPM toxicity is not fully understood. Both phase I and II biotransformations of DAPM, some of which generate reactive intermediates, are characterized in detail by liquid chromatography electrospray tandem mass spectrometry. The two most prominent metabolites found in rat bile (M2 and M7) implicated glutathione, glucuronic acid, and glycine conjugations (phase II) following hydroxylation, and N-oxidation (phase I). Their decomposition pathways, as evidenced by MS ⁿ experiments, have been elucidated in detail. Figure Proposed fragmentation pathways of a DAPM metabolite     

Diagonal chromatographic selection of cysteinyl peptides modified with benzoquinones

The derivatization of cysteine-containing peptides with benzoquinone compounds is rapid, quantitative and specific in acidic media. The conversion of cysteines into hydrophobic benzoquinone-adducted residues in peptides is used here to alter the chromatographic properties of cysteinyl peptides during liquid chromatography separation. The benzoquinone derivatization is shown to allow the accurate selection of cysteine-containing peptides of bovine serum albumin tryptic digest by diagonal reversed-phase chromatography, which consists of one primary and a series of secondary identical liquid chromatographic separations, before and after a cysteinyl-targeted modification of the peptides by benzoquinone compounds. Figure Diagonal chromatographic selection of cysteinyl peptides modified with benzoquinones Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.