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Analytica Chimica Acta (v.636, #2)

Editorial Board (pp. i).

Mapping of RNA–protein interactions by Subash Chandra Bose Gopinath (pp. 117-128).

RNA–protein interactions are important biological events that perform multiple functions in all living organisms. The wide range of RNA interactions demands diverse conformations to provide contacts for the selective recognition of proteins. Various analytical procedures are presently available for quantitative analyses of RNA–protein complexes, but analytical-based mapping of these complexes is essential to probe specific interactions. In this overview, interactions of functional RNAs and RNA-aptamers with target proteins are discussed by means of mapping strategies.

Keywords: Ribonucleic acid; Ribonucleic acid-aptamer; Ribonucleic acid–protein; Mapping

A review of non-chromatographic methods for speciation analysis by A. Gonzalvez; M.L. Cervera; S. Armenta; M. de la Guardia (pp. 129-157).

This manuscript overviews relevant scientific literature concerning speciation of trace elements by using non-chromatographic methods. The main principles of the different strategies proposed in the published works and their advantages and drawbacks are discussed in order to provide to the readers an appropriate picture of the state-of the-art of fast and cheap methodologies available to obtain information about the presence of different chemical forms of trace elements in environmental, clinical and food samples. A selection of the methods proposed for the speciation of the different elements studied was also provided together with their main features.

Keywords: Non-chromatographic; Speciation; Trace elements; Sample preparation

Determination of beryllium by electrothermal atomic absorption spectrometry using tungsten surfaces and zirconium modifier by M.A. Castro; L.C. Robles; J.M. Lumbreras; B. de Celis; A.J. Aller; D. Littlejohn (pp. 158-162).

Electrothermal atomization of beryllium from graphite and tungsten surfaces was compared with and without the use of various chemical modifiers. Tungsten proved to be the best substrate, giving the more sensitive integrated atomic absorption signals of beryllium. Tungsten platform atomization with zirconium as a chemical modifier was used for the determination of beryllium in several NIST SRM certified reference samples, with good agreement obtained between the results found and the certified values. The precision of the measurements (at 10μgL⁻¹), the limit of detection (3 σ), and the characteristic mass of beryllium were 2.50%, 0.009μgL⁻¹ and 0.42pg, respectively.

Keywords: Beryllium; Electrothermal atomic absorption spectrometry; Chemical modifiers; Atomizers; Zirconium modifier; Tungsten platform atomization; Graphite platform atomization

Characterization of the measurement error structure in 1D¹H NMR data for metabolomics studies by Tobias K. Karakach; Peter D. Wentzell; John A. Walter (pp. 163-174).

NMR-based metabolomics is characterized by high throughput measurements of the signal intensities of complex mixtures of metabolites in biological samples by assaying, typically, bio-fluids or tissue homogenates. The ultimate goal is to obtain relevant biological information regarding the dissimilarity in patho-physiological conditions that the samples experience. For a long time now, this information has been obtained through the analysis of measured NMR signals via multivariate statistics.NMR data are quite complex and the use of such multivariate statistical methods as principal components analysis (PCA) for their analysis assumes that the data are multivariate normal with errors that are identical, independent and normally distributed ( i.e. iid normal). There is a consensus that these assumptions are not always true for these data and, thus, several methods have been devised to transform the data or weight them prior to analysis by PCA. The structure of NMR measurement noise, or the extent to which violations of error homoscedasticity affect PCA results have neither been characterized nor investigated.A comprehensive characterization of measurement uncertainties in NMR based metabolomics was achieved in this work using an experiment designed to capture contributions of several sources of error to the total variance in the measurements. The noise structure was found to be heteroscedastic and highly correlated with spectral characteristics that are similar to the mean of the spectra and their standard deviation. A model was subsequently developed that potentially allows errors in NMR measurements to be accurately estimated without the need for extensive replication.

Keywords: Metabolomics; ¹; H NMR; Measurement error; Error covariance; Maximum likelihood principal components analysis (MLPCA); Principal components analysis (PCA)

Application of soft- and hard-modelling approaches to resolution of kinetics of electron donor–acceptor complex formation of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone with imipramine in different solutions by Masoumeh Hasani; Masoud Shariati-Rad; Hamid Abdollahi (pp. 175-182).

Kinetics of electron donor–acceptor (EDA) complex formation of imipramine and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) was investigated spectrophotometrically in acetonitrile, 1,2-dichloroethane, and chloroform solutions using soft- and hard-modelling approaches. From the results of exploratory analysis of kinetic data and the spectral changes by soft-modelling approaches, evolving factor analysis (EFA) and orthogonal projection approach (OPA), a consecutive two-steps reaction with two intermediates was proposed for the process in acetonitrile and 1,2-dichloroethane media and one with a single intermediate in chloroform solution. Secondly, by applying, multivariate nonlinear least squares hard-modelling approach on the collected experimental kinetic data matrix, the nonlinear parameters (rate constants) as well as the linear parameters (spectral profiles) were obtained by fitting the collected experimental kinetic data matrix to the proposed model. Small values of standard deviation in the resulting parameters and sum of squares of the residuals ( ssq) obtained showed the proper selection of the model. Furthermore, the values of lack of fit and percent of explained variance confirmed the correct identified models. Identification of the model with the aid of soft-modelling approaches followed by application of the hard-modelling approaches decreases significantly the rotational ambiguity associated with the obtained concentration and spectral profiles. Variations in the kinetic constants were in complete agreement with the model proposed and the solvent polarities.

Keywords: Electron donor–acceptor complex; Evolving factor analysis; Soft-modelling; Orthogonal projection approach; Hard-modelling

Feasibility study on the use of infrared spectroscopy for the direct authentication of Iberian pig fattening diet by Lourdes Arce; Ana Domínguez-Vidal; Vicente Rodríguez-Estévez; Silvia López-Vidal; María José Ayora-Cañada; Miguel Valcárcel (pp. 183-189).

The feasibility of using both middle- and near-infrared spectroscopy for discrimination between subcutaneous fat of Iberian pigs reared on different fattening diets has been evaluated. The sample set was formed by subcutaneous fat of pigs fattened outdoors (extensively) with natural resources ( montanera) and pigs fattened on commercial feeds, either with standard feed or with especial formulations with higher content in oleic acid (HO-formulated feed). Linear discriminant analysis was used to classify the samples according to the fattening diet using the scores obtained from principal component analysis of near- and middle-infrared spectra as variables to construct the discriminant functions. The most influential variables were identified using a stepwise procedure. The discriminant potential of each spectral region was investigated. Best results were obtained with the combination of both regions with 91.7% of the standard feed and 100% of montanera and HO-formulated feed samples correctly classified. Chemical explanations are provided based on the correlation of these variables with fatty acid content in the samples.

Keywords: Fourier transform infrared; Near infrared; Classification; Linear discriminant analysis; Iberian pig; Fat analysis

Comprehensive investigation and optimisation of the main experimental variables in stir-bar sorptive extraction (SBSE)-thermal desorption-capillary gas chromatography (TD-CGC) by Kevin MacNamara; Riccardo Leardi; Frank McGuigan (pp. 190-197).

A chemometric study has been completed in order to investigate the relative contributions and interactions between the many experimental variables involved in SBSE-TD-CGC. The study was centered on data after extraction and analysis of important organophosphorous pesticides from water under different controlled conditions. An enhanced flame photometric detector was used for target compound area response. The seemingly independent operations of extraction with the stir bar followed by thermal transfer of the absorbed compounds to the chromatographic system are usually studied by independent designs for the corresponding blocks of variables. In this work all variables are treated at the same time in a single design to study the interactions and give a more robust model while requiring a lower number of experiments. The relative importance of contributing variables was clearly established and an optimum set of conditions was established for more uniform enrichment in a single analysis for a test mix of compounds with wide ranging polarities.

Keywords: Stir-bar sorptive extraction; Organophosphorous pesticides; Chemometrics; Experimental design; Capillary gas chromatography; Flame photometric detection

Peat as a natural solid-phase for copper preconcentration and determination in a multicommuted flow system coupled to flame atomic absorption spectrometry by A.P.S. Gonzáles; M.A. Firmino; C.S. Nomura; F.R.P. Rocha; P.V. Oliveira; I. Gaubeur (pp. 198-204).

The physical and chemical characteristics of peat were assessed through measurement of pH, percentage of organic matter, cationic exchange capacity (CEC), elemental analysis, infrared spectroscopy and quantitative analysis of metals by ICP OES. Despite the material showed to be very acid in view of the percentage of organic matter, its CEC was significant, showing potential for retention of metal ions. This characteristic was exploited by coupling a peat mini-column to a flow system based on the multicommutation approach for the in-line copper concentration prior to flame atomic absorption spectrometric determination. Cu(II) ions were adsorbed at pH 4.5 and eluted with 0.50molL⁻¹ HNO₃. The influence of chemical and hydrodynamic parameters, such as sample pH, buffer concentration, eluent type and concentration, sample flow-rate and preconcentration time were investigated. Under the optimized conditions, a linear response was observed between 16 and 100μgL⁻¹, with a detection limit estimated as 3μgL⁻¹ at the 99.7% confidence level and an enrichment factor of 16. The relative standard deviation was estimated as 3.3% ( n=20). The mini-column was used for at least 100 sampling cycles without significant variation in the analytical response. Recoveries from copper spiked to lake water or groundwater as well as concentrates used in hemodialysis were in the 97.3–111% range. The results obtained for copper determination in these samples agreed with those achieved by graphite furnace atomic absorption spectrometry (GFAAS) at the 95% confidence level.

Keywords: Peat; Preconcentration; Flow analysis; Multicommutation; Copper

A novel fluorescence sensor based on covalent immobilization of 3-amino-9-ethylcarbazole by using silver nanoparticles as bridges and carriers by Shu-Zhen Tan; Yan-Jun Hu; Fu-Chun Gong; Zhong Cao; Jiao-Yun Xia; Ling Zhang (pp. 205-209).

A novel technique of covalent immobilization of indicator dyes in the preparation of fluorescence sensors is developed. Silver nanoparticles are used as bridges and carriers for anchoring indicator dyes. 3-amino-9-ethylcarbazole (AEC) was employed as an example of indicator dyes with terminal amino groups and covalently immobilized onto the outmost surface of a quartz glass slide. First, the glass slide was functionalized by (3-mercaptopropyl) trimethoxysilane (MPS) to form a thiol-terminated self-assembled monolayer, where silver nanoparticles were strongly bound to the surface through covalent bonding. Then, 16-mercaptohexadecanoic acid (MHDA) was self-assembled to bring carboxylic groups onto the surface of silver nanoparticles. A further activation by using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) converted the carboxylic groups into succinimide esters. Finally, the active succinimide esters on the surface of silver nanoparticles were reacted with AEC. Thus, AEC was covalently bound to the glass slide and an AEC-immobilized sensor was obtained. The sensor exhibited very satisfactory reproducibility and reversibility, rapid response and no dye-leaching. Rutin can quench the fluorescence intensity of the sensor and be measured by using the sensor. The linear response of the sensor to rutin covers the range from 2.0×10⁻⁶ to 1.5×10⁻⁴molL⁻¹ with a detection limit of 8.0×10⁻⁷molL⁻¹. The proposed technique may be feasible to the covalent immobilization of other dyes with primary amino groups.

Keywords: 3-amino-9-ethylcarbazole; Silver nanoparticles; Fluorescence sensor; Covalent immobilization

Multianalyte imaging in one-shot format sensors for natural waters by A. Lapresta-Fernández; Rafael Huertas; Manuel Melgosa; L.F. Capitán-Vallvey (pp. 210-217).

A one-shot multisensor based on ionophore–chromoionophore chemistry for optical monitoring of potassium, magnesium and hardness in water is presented. The analytical procedure uses a black and white non-cooled CCD camera for image acquisition of the one-shot multisensor after reaction, followed by data treatment for quantitation using the grey value pixel average from a defined region of interest from each sensing area to build the analytical parameter 1− α. In optimised experimental conditions, the procedure shows a large linear range, up to 6 orders using the linearised model and good detection limits: 9.92×10⁻⁵mM, 1.86×10⁻³mM and 1.30×10⁻²mgL⁻¹ of CaCO₃ for potassium, magnesium and hardness, respectively. This analysis system exhibits good precision in terms of relative standard deviation (RSD%) from 2.3 to 3.8 for potassium, from 5.0 to 6.8 for magnesium and from 5.4 to 5.9 for hardness. The trueness of this multisensor procedure was demonstrated comparing it with results obtained by a DAD spectrophotometer used as a reference. Finally, it was satisfactorily applied to the analysis of these analytes in miscellaneous samples, such as water and beverage samples from different origins, validating the results against atomic absorption spectrometry (AAS) as the reference procedure.

Keywords: Multianalyte sensor; Imaging; Charge Coupled Device camera; One-shot format sensor; Natural waters

Rapid chemiluminescence biosensing of microcystin-LR by Petra Lindner; Ramona Molz; Erwin Yacoub-George; Hans Wolf (pp. 218-223).

A rapid immunoassay for sensitive detection of microcystin-LR using a portable chemiluminescence multichannel immunosensor (CL-MADAG) was developed. The sensor device is based on a capillary ELISA technique in combination with a miniaturized fluidics system and uses chemiluminescence as the detection principle. Minimum concentrations of at least 0.2μgL⁻¹ microcystin-LR could be unambiguously measured in a spiked buffer system as well as in spiked real water samples. A single sample analysis for detection of microcystin-LR could be accomplished in just 13min on the CL-MADAG. Besides providing a highly reproducible, fast and easy to perform test format, one major advantage of the newly established capillary immunoassay is represented by the feasibility of an internal retrospective quality control mechanism. Finally, simultaneous CL-MADAG measurements employing our inhibition immunoassay and a sandwich ELISA could be successfully demonstrated.

Keywords: Microcystin; Biosensor; ELISA; FSC; Cyanobacteria

Capillary electrophoresis with contactless conductivity detection for uric acid determination in biological fluids by Worapan Pormsila; Stephan Krähenbühl; Peter C. Hauser (pp. 224-228).

The suitability of capillary electrophoresis (CE) with capacitively coupled contactless conductivity detection (C⁴D) for the direct determination of uric acid in human plasma and urine was investigated. It was found that a careful optimization of the buffer composition and pH was necessary to achieve selective determination in the complex sample matrices. An electrolyte solution consisting of 10mM 2-morpholinoethanesulfonic acid (MES), 10mM histidine and 0.1mM hexadecyltrimethylammonium bromide (CTAB), pH 6.0, was finally found suitable for use as running buffer for both sample matrices. The limit of detection (3 S/N) was determined as 3.3μM. The linearity of the response was tested for the range between 10 and 500μM and a correlation coefficient of 0.9996 was obtained. Intra- and inter-day variabilities were <10%. Quantitative analysis of urine and plasma samples showed a good correlation with the routine enzymatic method currently used at the University Hospital of Basel.

Keywords: Uric acid; Plasma; Urine; Capillary electrophoresis; Contactless conductivity detection

Quantification of fatty acids as methyl esters and phospholipids in cheese samples after separation of triacylglycerides and phospholipids by Simone Hauff; Walter Vetter (pp. 229-235).

Determination of the individual fatty acid composition of neutral- and phospholipids as well as the phospholipid content of dairy food and other foodstuffs are important tasks in life sciences. For these purposes, a method was developed for the separation of lipids (standards of triolein and diacylphosphatidylcholines as well as three cheese samples) by solid-phase extraction using a self-packed column filled with partly deactivated silica. Non-halogenated solvents were used for the elution of the lipid classes. Cyclohexane/ethyl acetate (1:1, v/v) served for the elution of neutral lipids, while polar lipids were eluted with three solvents (ethyl acetate/methanol, methanol, and methanol/water) into one fraction. The separated lipid fractions were transesterified and the individual fatty acids were quantified by using gas chromatography coupled to electron ionization mass spectrometry (GC/EI-MS) in the selected ion monitoring (SIM) mode. The recovery rate for standard phosphatidylcholines was ∼90% and cross-contamination from neutral lipids was negligible. The method was applied to cheese samples. Quantitative amounts of individual fatty acids in the phospholipid fraction were <0.002–0.29% of total lipids from camembert, <0.002–0.12% of total lipids from mozzarella, and <0.002–0.18% of total lipids in a goat cream cheese. Differences in the fatty acid pattern of neutral and polar lipids were detected. The quantity of the fatty acids determined in the phospholipid fraction was divided by the factor 0.7 in order to convert the fatty acid content into the phospholipid content of the cheese samples. This factor is based on the contribution of 16:0 to dipalmitoylphosphatidylcholine (DPPC). The resulting DPPC equivalents (DPPC_eq) were found to be representative for the average contribution of fatty acids to all classes of phospholipids in dairy products. Using this approach, the phospholipid content of lipids from mozzarella, camembert, and goat cream cheese was 0.60%, 1.42% and 0.79%, respectively.

Keywords: Gas chromatography/mass spectrometry; Solid-phase extraction; Fatty acids; Fatty acid methyl esters; Phospholipids; Cheese

Rapid and quantitative determination of critical micelle concentration by automatic continuous mixing and static light scattering by Sabrina Paillet; Bruno Grassl; Jacques Desbrières (pp. 236-241).

We present the rapid and quantitative characterization of ionic, non-ionic and zwitterionic surfactants based upon the combination of an automatic continuous mixing technique and static light scattering. Collection and subsequent analysis of data are both rapid and semiautomatic, which increases precision, sensitivity, and range of applicability while substantially decreasing the amount of manual intervention required by the investigator. By treating the continuous data, the entire data set may be rapidly analyzed in the context of the closed association model to determine the critical micelle concentration cmc and aggregation number Nag of a detergent; these technique are comparable in the scope and resolution currently obtainable from other conductimetric, fluorescence and surface tension techniques.

Keywords: cmc; Surfactant; Static light scattering; Automatic continuous mixing technique

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Analytica Chimica Acta (v.636, #2)

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Analytica Chimica Acta (v.636, #2)