Analytica Chimica Acta (v.821, #C)

Supercritical fluid chromatography as a tool for enantioselective separation; A review by Květa Kalíková; Tereza Šlechtová; Jiří Vozka; Eva Tesařová (1-33).
Supercritical fluid chromatography (SFC) has become popular in the field of enantioselective separations. Many works have been reported during the last years. This review covers the period from 2000 till August 2013. The article is divided into three main chapters. The first one comprises a basic introduction to SFC. The authors provide a brief explanation of general principles and possibilities of this method. The advantages and drawbacks are also listed. Next part deals with chiral separation systems available in SFC, namely with the commonly used chiral stationary phases. Properties and interaction possibilities of the chiral separation systems are described. Recent theoretical papers are emphasized in this chapter. The last part of the paper gives an overview of applications of enantioselective SFC in analytical chemistry, in both analytical and preparative scales. Separation systems and conditions are summed up in tables so that they provide a helpful tool for analysts who search for a particular method of analysis.
Keywords: Review; Supercritical fluid chromatography; Chiral stationary phase; Enantioselective separation; Enantiomers;

Electrochemical detection of benzo(a)pyrene and related DNA damage using DNA/hemin/nafion–graphene biosensor by Yongnian Ni; Pingping Wang; Haiyan Song; Xiaoyun Lin; Serge Kokot (34-40).
A novel electrochemical biosensor, DNA/hemin/nafion–graphene/GCE, was constructed to quantitatively study the DNA damage induced by the metabolite of benzo(a)pyrene in the presence of H2O2.Display OmittedA novel electrochemical biosensor, DNA/hemin/nafion–graphene/GCE, was constructed for the analysis of the benzo(a)pyrene PAH, which can produce DNA damage induced by a benzo(a)pyrene (BaP) enzyme-catalytic product. This biosensor was assembled layer-by-layer, and was characterized with the use of cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and atomic force microscopy. Ultimately, it was demonstrated that the hemin/nafion–graphene/GCE was a viable platform for the immobilization of DNA. This DNA biosensor was treated separately in benzo(a)pyrene, hydrogen peroxide (H2O2) and in their mixture, respectively, and differential pulse voltammetry (DPV) analysis showed that an oxidation peak was apparent after the electrode was immersed in H2O2. Such experiments indicated that in the presence of H2O2, hemin could mimic cytochrome P450 to metabolize benzo(a)pyrene, and a voltammogram of its metabolite was recorded. The DNA damage induced by this metabolite was also detected by electrochemical impedance and ultraviolet spectroscopy. Finally, a novel, indirect DPV analytical method for BaP in aqueous solution was developed based on the linear metabolite versus BaP concentration plot; this method provided a new, indirect, quantitative estimate of DNA damage.
Keywords: DNA damage; Polycyclic aromatic hydrocarbons; DNA/hemin/nafion–graphene/glassy carbon electrode; Differential pulse voltammetry; DNA biosensor;

Electrochemical assessment of water|ionic liquid biphasic systems towards cesium extraction from nuclear waste by T. Jane Stockmann; Jing Zhang; Anne-Marie Montgomery; Zhifeng Ding (41-47).
Display OmittedA room temperature ionic liquid (IL) composed of a quaternary alkylphosphonium (trihexyltetradecylphosphonium, P66614 +) and tetrakis(pentafluorophenyl)borate anion (TB) was employed within a water|P66614TB (w|P66614TB or w|IL) biphasic system to evaluate cesium ion extraction in comparison to that with a traditional water|organic solvent (w|o) combination. 137Cs is a major contributor to the radioactivity of spent nuclear fuel as it leaves the reactor, and its extraction efficiency is therefore of considerable importance. The extraction was facilitated by the ligand octyl(phenyl)-N,N′-diisobutylcarbamoylphosphine oxide (CMPO) used in TRans-Uranium EXtraction processes and investigated through well established liquid|liquid electrochemistry. This study gave access to the metal ion to ligand (1:n) stoichiometry and overall complexation constant, β, of the interfacial complexation reaction which were determined to be 1:3 and 1.6 × 1011 at the w|P66614TB interface while the study at w|o elicited an n equal to 1 with β equal to 86.5. Through a straightforward relationship, these complexation constant values were converted to distribution coefficients, δα , with the ligand concentrations studied for comparison to other studies present in the literature; the w|o and w|IL systems gave δα of 2 and 8.2 × 107, respectively, indicating a higher overall extraction efficiency for the latter. For the w|o system, the metal ion-ligand stoichiometries were confirmed through isotopic distribution analysis of mass spectra obtained by the direct injection of an emulsified water–organic solvent mixture into an electron spray ionization mass spectrometer.
Keywords: Ionic liquids for electroanalytical chemistry; Ion transfer at microinterfaces; Cesium extraction from spent nuclear fuel; Ligand in TRans-Uranium EXtraction processes; Mass spectrometry for stoichiometry;

Display OmittedThe 4-mercapto-4-methylpentan-2-one (4MMP) is a key aroma compound in wines, especially in Sauvignon Blanc ones. Its accurate quantification is quite difficult due to its traces levels and its reactivity in wine conferred by the thiol function. In this paper, we proposed a new method for its quantification in wine without any sample preparation, based on automated derivatization procedure by methoximation and SIDA–SPME–GC–MS/MS analysis. The derivatization procedure was adapted from a previously published method in order to decrease the amount of reagents and the volume of wine (only 3 mL are required). The use of SPME and the detection conditions have also been optimized to reach the best sensitivity as possible. The method was then validated according to the International Organization of Vine and Wine recommendations and exhibited excellent performances. Indeed, this method allowed us to quantify the 4MMP in wine at traces levels (LOD = 0.19 ng L−1) with reproducible results (RSD < 15%) and a very good accuracy (recovery = 102%).
Keywords: 4-mercapto-4-methylpentan-2-one; Wine; Solid phase micro extraction; Stable isotope dilution assay; Varietal thiol; Validation;

A fluorescence anisotropy method for measuring protein concentration in complex cell culture media by Radu Constantin Groza; Amandine Calvet; Alan G. Ryder (54-61).
Display OmittedThe rapid, quantitative analysis of the complex cell culture media used in biopharmaceutical manufacturing is of critical importance. Requirements for cell culture media composition profiling, or changes in specific analyte concentrations (e.g. amino acids in the media or product protein in the bioprocess broth) often necessitate the use of complicated analytical methods and extensive sample handling. Rapid spectroscopic methods like multi-dimensional fluorescence (MDF) spectroscopy have been successfully applied for the routine determination of compositional changes in cell culture media and bioprocess broths. Quantifying macromolecules in cell culture media is a specific challenge as there is a need to implement measurements rapidly on the prepared media. However, the use of standard fluorescence spectroscopy is complicated by the emission overlap from many media components. Here, we demonstrate how combining anisotropy measurements with standard total synchronous fluorescence spectroscopy (TSFS) provides a rapid, accurate quantitation method for cell culture media. Anisotropy provides emission resolution between large and small fluorophores while TSFS provides a robust measurement space. Model cell culture media was prepared using yeastolate (2.5 mg mL–1) spiked with bovine serum albumin (0 to 5 mg mL–1). Using this method, protein emission is clearly discriminated from background yeastolate emission, allowing for accurate bovine serum albumin (BSA) quantification over a 0.1 to 4.0 mg mL–1range with a limit of detection (LOD) of 13.8 μg mL–1.
Keywords: Fluorescence; Anisotropy; Total synchronous fluorescence spectroscopy (TSFS); Bioprocess; Protein;

Display OmittedWe report the first successful direct and unequivocal identification and quantification of four minor geometric (9-cis, 11-trans) 18:2, (9-trans, 11-cis) 18:2, (9-cis, 11-cis) 18:2 and (9-trans, 11-trans) 18:2 conjugated linoleic acid (CLA) isomers in lipid fractions of lyophilized milk samples with the combined use of 1D 1H-NMR, 2D 1H― 1H TOCSY and 2D 1H― 13C HSQC NMR. The significant sensitivity barrier has been successfully overcome under selective suppression of the major resonances, with over 104 greater equilibrium magnetization of the ―(CH2) n 1H spins compared to that of the 1H spins of the conjugated bonds of the CLA isomers. The resolution barrier has been significantly increased using reduced 13C spectral width in the 2D 1H― 13C HSQC experiment. The assignment was confirmed with spiking experiments with CLA standard compounds and the method does not require any derivatization steps for the lipid fraction. The proposed method is selective, sensitive and compares favorably with the GS-MS method of analysis.
Keywords: Conjugated linoleic acid; 1H― 13C heteronuclear single quantum coherence; 1H― 1H total correlated spectroscopy; Geometric cis/trans isomers;

A reference electrode based on polyvinyl butyral (PVB) polymer for decentralized chemical measurements by Tomàs Guinovart; Gastón A. Crespo; F. Xavier Rius; Francisco J. Andrade (72-80).
Display OmittedA new solid-state reference electrode using a polymeric membrane of polyvinyl butyral (PVB), Ag/AgCl and NaCl to be used in decentralized chemical measurements is presented. The electrode is made by drop-casting the membrane cocktail onto a glassy carbon (GC) substrate. A stable potential (less than 1 mV dec−1) over a wide range of concentrations for the several chemical species tested is obtained. No significant influence to changes in redox potential, light and pH are observed. The response of this novel electrode shows good correlation when compared with a conventional double-junction reference electrode. Also good long-term stability (90 ± 33 μV/h) and a lifetime of approximately 4 months are obtained. Aspects related to the working mechanisms are discussed. Atomic Force Microscopy (AFM) studies reveal the presence of nanopores and channels on the surface, and electrochemical impedance spectroscopy (EIS) of optimized electrodes show low bulk resistances, usually in the kΩ range, suggesting that a nanoporous polymeric structure is formed in the interface with the solution. Future applications of this electrode as a disposable device for decentralized measurements are discussed. Examples of the utilization on wearable substrates (tattoos, fabrics, etc) are provided.
Keywords: Reference electrode; Decentralized measurements; Solid-contact potentiometric sensors;

No more conventional reference electrode: Transition time for determining chloride ion concentration by Yawar Abbas; Derk Balthazar de Graaf; Wouter Olthuis; Albert van den Berg (81-88).
Display OmittedIon selective electrodes (ISE) are used extensively for the potentiometric determination of ion concentrations in electrolytes. However, the inherent drift in these measurements and the requirement of a stable reference electrode restrict the feasibility of this method for long-term in-situ applications. This work presents a chronopotentiometric approach to minimize drift and avoid the use of a conventional reference electrode for measuring chloride ion concentration. An anodic current pulse is applied to a Ag/AgCl working electrode which initiates a faradaic reaction that depletes the chloride ions near the electrode surface. The rate of change in potential at the Ag/AgCl electrode, due to chloride ion depletion, reaches an inflection point once the chloride ions deplete completely near the electrode surface. The moment of the inflection point, also known as the transition time, is a function of the chloride ion concentration and is described by the Sand equation. It is shown that the square root of the transition time is linearly proportional to the chloride ion concentration. Drift in the response over two weeks is negligible: 59 μM/day when measuring 1 mM of Cl ions using a 10 A m−2 current pulse. The transition time at a specific ion concentration can be tuned by the applied current pulse, e.g., in a solution containing 5 mM chloride ions, the transition times with current pulses of 10 and 20 A m−2 are 1.56 and 0.25 s, respectively. The moment of inflection determines the response, and thus is independent of the absolute potential of reference electrode. Therefore, any metal wire can act as a pseudo-reference electrode, enabling this approach for long-term and integrated-sensor applications such as measurement inside concrete structures.
Keywords: Chronopotentiometry; Chloride ion measurement; Transition time; Sand equation; Silver/silver chloride electrode; Pseudo-reference electrode;

Display OmittedStatistical design mixtures of acetone, chloroform, dichloromethane and ethanol were used to study the effects of different solvents and their mixtures on the quantities of coumarin and related metabolites extracted from Mikania laevigata samples harvested in each of the four seasons. RP-HPLC-DAD and both positive and negative modes of UPLC-MS analyses were used to determine relative quantities of coumarin, o-coumaric acid and melilotic acids in each season for all the mixture design extracts. The existence and measurement of the relative abundances of melilotic acid in Mikania laevigata have not been reported previously. Highest coumarin concentrations were encountered in the summer whereas its o-coumaric acid precursor and melilotic acid were most abundant in the spring. O-coumaric and melilotic acids concentrations were strongly correlated during the year. Also solvent effects were seen to be significant. Ethanol and 1:1 binary mixtures of ethanol and acetone extracted the largest quantities of coumarin whereas ethanolic binary and ternary mixtures with chloroform and dichloromethane provided the best yields of o-coumaric and melilotic acids. Statistical mixture models indicated that synergic binary interactions, especially those involving ethanol with acetone or chloroform, are important in the Mikania extraction process.
Keywords: Extraction optimization; Simplex–centroid design; Coumarin; Mikania laevigata Schultz. Bip. leaves; Secondary metabolites;

Display OmittedNew kinds of amino acid ionic liquids (AAILs) with pyridinium as cations and l-lysine (l-Lys) as anion have been developed as the available chiral ligands coordinated with Zn(II) in chiral ligand-exchange capillary electrophoresis (CLE-CE). Four kinds of AAILs, including [1-ethylpyridinium][l-lysine], 1-butylpyridinium][l-lysine], [1-hexylpyridinium][l-lysine] and 1-[octylpyridinium][l-lysine], were successfully synthesized and characterized by nuclear magnetic resonance and mass spectrometry. Compared with other AAILs, the best chiral separation of Dns-d, l-amino acids could be achieved when [1-ethylpyridinium][l-lysine] was chosen as the chiral ligand. It has been found that after investigating the influence of key factors on the separation efficiency, such as pH of buffer solution, the ratio of Zn(II) to ligand and complex concentration, eight pairs of Dns-d, l-AAs enantiomers could be baseline separated and three pairs were partly separated under the optimum conditions. The proposed CLE-CE method also exhibited favorable quantitative analysis property of Dns-d, l-Met with good linearity (r 2  = 0.998) and favorable repeatability (RSD ≤ 1.5%). Furthermore, the CLE-CE system was applied in investigating the kinetic contents of l-amino acid oxidase, which implied that the proposed system has the potential in studying the enzymatic reaction mechanism.
Keywords: Amino acid ionic liquid; Chiral ligand exchange capillary electrophoresis; l-Amino acid oxidase; Enzyme kinetics;