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Analytica Chimica Acta (v.664, #2)

Editorial Board (pp. i).

Multi-dimensional liquid chromatography in proteomics—A review by Xiang Zhang; Aiqin Fang; Catherine P. Riley; Mu Wang; Fred E. Regnier; Charles Buck (pp. 101-113).

Proteomics is the large-scale study of proteins, particularly their expression, structures and functions. This still-emerging combination of technologies aims to describe and characterize all expressed proteins in a biological system. Because of upper limits on mass detection of mass spectrometers, proteins are usually digested into peptides and the peptides are then separated, identified and quantified from this complex enzymatic digest. The problem in digesting proteins first and then analyzing the peptide cleavage fragments by mass spectrometry is that huge numbers of peptides are generated that overwhelm direct mass spectral analyses. The objective in the liquid chromatography approach to proteomics is to fractionate peptide mixtures to enable and maximize identification and quantification of the component peptides by mass spectrometry. This review will focus on existing multidimensional liquid chromatographic (MDLC) platforms developed for proteomics and their application in combination with other techniques such as stable isotope labeling. We also provide some perspectives on likely future developments.

Keywords: Multi-dimensional liquid chromatography; Stable isotope labeling; Label-free; Proteomics

Multi-dimensional liquid chromatography in proteomics—A review by Xiang Zhang; Aiqin Fang; Catherine P. Riley; Mu Wang; Fred E. Regnier; Charles Buck (pp. 101-113).

Keywords: Multi-dimensional liquid chromatography; Stable isotope labeling; Label-free; Proteomics

Determination of iron in seawater by electrothermal atomic absorption spectrometry and atomic fluorescence spectrometry: A comparative study by J.Y. Cabon; P. Giamarchi; A. Le Bihan (pp. 114-120).

Two methods available for direct determination of total Fe in seawater at low concentration level have been examined: electrothermal atomization atomic absorption spectrometry (ETAAS) and electrothermal atomization laser excited atomic fluorescence spectrometry (ETA-LEAFS). In a first part, we have optimized experimental conditions of ETAAS (electrothermal program, matrix chemical modification) for the determination of Fe in seawater by minimizing the chemical interference effects and the magnitude of the simultaneous background absorption signal. By using the best experimental conditions, a detection limit of 80ngL⁻¹ (20μL, 3 σ) for total Fe concentration was obtained by ETAAS. Using similar experimental conditions (electrothermal program, chemical modification), we have optimized experimental conditions for the determination of Fe by LEAFS. The selected experimental conditions for ETA-LEAFS: excitation wavelength (296.69nm), noise attenuation and adequate background correction led to a detection limit (3 σ) of 3ngL⁻¹ (i.e. 54pM) for total Fe concentration with the use a 20μL seawater sample. For the two methods, concentration values obtained for the analysis of Fe in a NASS-5 (0.2μgL⁻¹) seawater sample were in good agreement with the certified values.

Keywords: Atomic absorption; Atomic fluorescence; Spectrometry; Iron; Seawater

Determination of iron in seawater by electrothermal atomic absorption spectrometry and atomic fluorescence spectrometry: A comparative study by J.Y. Cabon; P. Giamarchi; A. Le Bihan (pp. 114-120).

Keywords: Atomic absorption; Atomic fluorescence; Spectrometry; Iron; Seawater

Classification of cultivated mussels from Galicia (Northwest Spain) with European Protected Designation of Origin using trace element fingerprint and chemometric analysis by M. Costas-Rodríguez; I. Lavilla; C. Bendicho (pp. 121-128).

Inductively coupled plasma-mass spectrometry (ICP-MS) in combination with different supervised chemometric approaches has been used to classify cultivated mussels in Galicia (Northwest of Spain) under the European Protected Designation of Origin (PDO). 158 mussel samples, collected in the five rías on the basis of the production, along with minor and trace elements, including high field strength elements (HFSEs) and rare earth elements (REEs), were used with this aim. The classification of samples was achieved according to their origin: Galician vs. other regions (from Tarragona, Spain, and Ethang de Thau, France) and between the Galician Rías. The ability of linear discriminant analysis (LDA), soft independent modelling of class analogy (SIMCA) and artificial neural network (ANN) to classify the samples was investigated. Correct assignations for Galician and non-Galician samples were obtained when LDA and SIMCA were used. ANNs were more effective when a classification according to the ría of origin was to be applied.

Keywords: Galician mussel classification; Linear discriminant analysis; Soft independent modelling of class analogy; Artificial neural networks; Trace elements; ICP-MS

Evaluation of the residual liquid junction potential contribution to the uncertainty in pH measurement: A case study on low ionic strength natural waters by Rouvim Kadis; Ivo Leito (pp. 129-135).

The residual liquid junction potential (RLJP) needs to be accounted for in pH uncertainty estimation. Attempts to do this and the currently available methods for evaluating the RLJP are critically discussed and their weak sides are pointed out. In this work an empirical approach to the problem is proposed. It is based on the use of the RLJP bias estimated on a variety of measurement conditions for a specific class of analytical objects essentially differing in ionic strength from the pH calibration buffers. The data from five independent studies, including interlaboratory comparisons, on pH measurement in low ionic strength waters were used to find the overall bias observed in the 10⁻⁴moldm⁻³ strong acid solution. The procedure includes quantifying the uncertainty of bias values from separate studies by combination of the relevant uncertainty components and testing the consistency of the data. The weighted mean bias in pH was found to be 0.043±0.007 ( k=2). With this estimate, the pH measurement uncertainties calculated according to the previously suggested procedure (I. Leito, L. Strauss, E. Koort, V. Pihl, Accredit. Qual. Assur. 7 (2002) 242–249.) can be enlarged to take the uncorrected bias into account. The resulting uncertainties on the level of 0.10–0.14 ( k=2) are obtained in this way for pH measurement in water and poorly buffered aqueous solutions in the range of pH 7.5–3.5.

Keywords: pH measurement; Residual liquid junction potential; Measurement uncertainty; Measurement bias; Low ionic strength waters

Evaluation of the residual liquid junction potential contribution to the uncertainty in pH measurement: A case study on low ionic strength natural waters by Rouvim Kadis; Ivo Leito (pp. 129-135).

Keywords: pH measurement; Residual liquid junction potential; Measurement uncertainty; Measurement bias; Low ionic strength waters

Significance of data treatment and experimental setup on the determination of copper complexing parameters by anodic stripping voltammetry by Dario Omanović; Cédric Garnier; Yoann Louis; Véronique Lenoble; Stéphane Mounier; Ivanka Pižeta (pp. 136-143).

Different procedures of voltammetric peak intensities determination, as well as various experimental setups were systematically tested on simulated and real experimental data in order to identify critical points in the determination of copper complexation parameters (ligand concentration and conditional stability constant) by anodic stripping voltammetry (ASV). Varieties of titration data sets (Cu_measured vs. Cu_total) were fitted by models encompassing discrete sites distribution of one-class and two-class of binding ligands (by PROSECE software). Examination of different procedures for peak intensities determination applied on voltammograms with known preset values revealed that tangent fit (TF) routine should be avoided, as for both simulated and experimental titration data it produced an additional class of strong ligand (actually not present). Peak intensities determination by fitting of the whole voltammogram was found to be the most appropriate, as it provided most reliable complexation parameters.Tests performed on real seawater samples under different experimental conditions revealed that in addition to importance of proper peak intensities determination, an accumulation time (control of the sensitivity) and an equilibration time needed for complete complexation of added copper during titration (control of complexation kinetics) are the keypoints to obtain reliable results free of artefacts.The consequence of overestimation and underestimation of complexing parameters is supported and illustrated by the example of free copper concentrations (the most bioavailable/toxic specie) calculated for all studied cases. Errors up to 80% of underestimation of free copper concentration and almost two orders of magnitude overestimation of conditional stability constant were registered for the simulated case with two ligands.

Keywords: Anodic stripping voltammetry; Metal speciation; Copper complexing capacity; Data treatment; Fitting

Dissolved oxygen amperometric sensor based on layer-by-layer assembly using host–guest supramolecular interactions by Flavio S. Damos; Rita C.S. Luz; Auro A. Tanaka; Lauro T. Kubota (pp. 144-150).

The development of a simple, efficient and sensitive sensor for dissolved oxygen is proposed using the host–guest binding of a supramolecular complex at a host surface by combining a self-assembled monolayer (SAM) of mono-(6-deoxy-6-mercapto)-β-cyclodextrin (βCDSH), iron (III) tetra-(N-methyl-4-pyridyl)-porphyrin (FeTMPyP) and cyclodextrin-functionalized gold nanoparticles (CDAuNP). The supramolecular modified electrode showed excellent catalytic activity for oxygen reduction. The reduction potential of oxygen was shifted about 200mV toward less negative values with this modified electrode, presenting a peak current much higher than those observed on a bare gold electrode. Cyclic voltammetry and rotating disk electrode (RDE) experiments indicated that the oxygen reduction reaction involves probably 4-electrons with a rate constant ( k_obs) of 7×10⁴mol⁻¹Ls⁻¹. A linear response range from 0.2 up to 6.5mgL⁻¹, with a sensitivity of 5.5μALmg⁻¹ (or 77.5μAcm⁻²Lmg⁻¹) and a detection limit of 0.02mgL⁻¹ was obtained with this sensor. The repeatability of the proposed sensor, evaluated in terms of relative standard deviation was 3.0% for 10 measurements of a solution of 6.5mgL⁻¹ oxygen.

Keywords: Oxygen; Electrocatalysis; Layer-by-layer assembly; Host–guest interface; Supramolecular interaction; Amperometric sensor

Application of a modified enzyme-linked immunosorbent assay for 3-amino-2-oxazolidinone residue in aquatic animals by Yu Liu; Dapeng Peng; Lingli Huang; Yulian Wang; Chao Chang; Awais Ihsan; Yanfei Tao; Bo Yang; Zonghui Yuan (pp. 151-157).

Furazolidone has been banned from use in food animals because of its carcinogenicity and mutagenicity, but its continued misuse is widespread in aquacultures. Therefore, there is an urgent need for a simple, reliable, and rapid method for the detection of its marker residue, 3-amino-2-oxazolidinone (AOZ), in aquatic products. In this regard, we modified a simplified indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) to address this need. A good linearity was achieved over a concentration range of 0.05–12.15μgL⁻¹, and the IC₅₀ value was 0.96μgL⁻¹. The sample preparation was simple and effective included water bath treatments, acid hydrolysis combined with overnight derivatization of AOZ by benzaldehyde. The limit of detection and the limit of quantification were 0.15 and 0.3μgkg⁻¹. The recoveries of AOZ in all tissues were between 78.0–95.3% at the levels of 0.3, 1.0, and 2.0μgkg⁻¹. The inter-assay variability was less than 19.1%. The modified ic-ELISA was applied in quantification of AOZ elimination in carp. The results showed that AOZ was quite difficult to eliminate. Good correlations of the results obtained by ELISA and LC–MS/MS were observed in incurred carp muscle ( r=0.9923) and carp plasma ( r=0.9915) at the levels of 2.5–571.8μgkg⁻¹ (μgL⁻¹). Better results were obtained by modified ic-ELISA when compared with commercial ELISA kit. Therefore, the present assay is considered a rapid, accurate, reliable, and inexpensive method for the detection of furazolidone-residues in the edible tissues of aquatic animals.

Keywords: Enzyme-linked immunosorbent assay; Furazolidone; 3-Amino-2-oxazolidinone; Residue; Aquatic animals

Modeling of electrokinetic transport in silica nanofluidic channels by Moran Wang; Qinjun Kang; Eli Ben-Naim (pp. 158-164).

We present a theoretical and numerical modeling study of the multiphysicochemical process in electrokinetic transport in silica nanochannels. The electrochemical boundary condition is solved by considering both the chemical equilibrium on solid–liquid interfaces and the salt concentration enrichment caused by the double layer interaction. The transport behavior is modeled numerically by solving the governing equations using the lattice Poisson–Boltzmann method. The framework is validated by good agreements with the experimental data for all range of ionic concentrations. The modeling results suggest that when the double layers interact, the bulk salt concentration enrichment results in the saturation of conductances for low ionic concentrations. Both the streaming conductance and the electrical conductance are enhanced by the double layer interaction, and such enhancements diminish when the channel size is larger than 10 times of the Debye length. The streaming conductance increases with pH almost linearly when pH<8, while the electrical conductance increases with pH exponentially.

Keywords: Electrokinetic transport; Nanofluidics; Surface dissociation; Double layer interaction

Modeling of electrokinetic transport in silica nanofluidic channels by Moran Wang; Qinjun Kang; Eli Ben-Naim (pp. 158-164).

Keywords: Electrokinetic transport; Nanofluidics; Surface dissociation; Double layer interaction

An ultra-high-performance liquid chromatography-tandem mass spectrometry method for simultaneous determination of aflatoxins B1, B2, G1, G2, M1 and M2 in traditional Chinese medicines by Zheng Han; Yunliang Zheng; Lianjun Luan; Zengxuan Cai; Yiping Ren; Yongjiang Wu (pp. 165-171).

An ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC–MS/MS) method for simultaneous determination of aflatoxins B1, B2, G1, G2, M1 and M2 in traditional Chinese medicines (TCMs) was developed. The approach was characterized in details and a special focus was placed on the recovery rates of isolation procedure in different TCM matrices, i.e. rhizomes and roots, seeds, flowers, grasses and leaves. For this purpose, [¹³C₁₇]-aflatoxinB1 was employed as the internal standard and a reliable solid phase extraction-based clean-up method was developed. The observed recovery rates of the six aflatoxins ranged from 85.6% to 117.6% in different matrices. Then, the established method was successfully applied to the determination of the six aflatoxins in various TCMs. For 30 commercial samples analyzed, 16 were contaminated with aflatoxins. The mean levels (incidence) of aflatoxins B1, B2, G1 and G2 in positive samples were 1.40 (68.8%), 1.27 (50.0%), 0.50 (43.8%) and 0.94 (43.8%) μgkg⁻¹, respectively. Interestingly, aflatoxin M1 was detected in two samples with the maximal content of 0.70μgkg⁻¹. No sample was contaminated with aflatoxin M2. Meanwhile, a possible association between the contamination levels and the selected herbs was clarified in the present study.

Keywords: UHPLC–MS/MS; Aflatoxin; Internal standard; Traditional Chinese medicine

Determination of terbium in phosphate rock by Tb³⁺-selective fluorimetric optode based on dansyl derivative as a neutral fluorogenic ionophore by Morteza Hosseini; Mohammad Reza Ganjali; Bahareh Veismohammadi; Farnoush Faridbod; Shiva Dehghan Abkenar; Parviz Norouzi (pp. 172-177).

For the first time a highly sensitive and selective fluorimetric optode membrane was prepared for determination of trace amounts of Tb(III) ions in phosphate rock samples. The Tb(III) sensing system was constructed by incorporating 5-(dimethylamino)-N′-(2-hydroxy-1-naphthoyl) naphthalene-1-sulfonohydrazine (L) as a neutral Tb(III)-selective fluoroionophore, in the plasticized PVC membrane containing sodium tetraphenyl borate as a liphophilic anionic additive. The response of the optode is based on the strong fluorescence quenching of L by Tb³⁺ ions. At a pH value of 5.0, the optode displays a wide concentration range of 1.0×10⁻⁷ to 1.0×10⁻²M, with a relatively fast response time of less than 45s. In addition, to high stability and reproducibility, the sensor shows a unique selectivity towards Tb³⁺ ion with respect to common cations. The optode was applied successfully to the trace determination of terbium ion in binary mixture and water samples and the determination of Tb³⁺ in phosphate rock samples.

Keywords: Sensor; Optode; Terbium; Fluorescence

Development of a fully automated sequential injection solid-phase extraction procedure coupled to liquid chromatography to determine free 2-hydroxy-4-methoxybenzophenone and 2-hydroxy-4-methoxybenzophenone-5-sulphonic acid in human urine by Zacarías León; Alberto Chisvert; Ángel Balaguer; Amparo Salvador (pp. 178-184).

2-Hydroxy-4-methoxybenzophenone and 2-hydroxy-4-methoxybenzophenone-5-sulphonic acid, commonly known as benzophenone-3 (BZ3) and benzophenone-4 (BZ4), respectively, are substances widely used as UV filters in cosmetic products in order to absorb UV radiation and protect human skin from direct exposure to the deleterious wavelengths of sunlight. As with other UV filters, there is evidence of their percutaneous absorption.This work describes an analytical method developed to determine trace levels of free BZ3 and BZ4 in human urine. The methodology is based on a solid-phase extraction (SPE) procedure for clean-up and pre-concentration, followed by the monitoring of the UV filters by liquid chromatography–ultraviolet spectrophotometry detection (LC–UV). In order to improve not only the sensitivity and selectivity, but also the precision of the method, the principle of sequential injection analysis was used to automate the SPE process and to transfer the eluates from the SPE to the LC system. The application of a six-channel valve as an interface for the switching arrangements successfully allowed the on-line connection of SPE sample processing with LC analysis.The SPE process for BZ3 and BZ4 was performed using octadecyl (C18) and diethylaminopropyl (DEA) modified silica microcolumns, respectively, in which the analytes were retained and eluted selectively. Due to the matrix effects, the determination was based on standard addition quantification and was fully validated. The relative standard deviations of the results were 13% and 6% for BZ3 and BZ4, respectively, whereas the limits of detection were 60 and 30ngmL⁻¹, respectively. The method was satisfactorily applied to determine BZ3 and BZ4 in urine from volunteers that had applied a sunscreen cosmetic containing both UV filters.

Keywords: 2-Hydroxy-4-methoxybenzophenone; 2-Hydroxy-4-methoxybenzophenone-5-sulphonic acid; Sequential injection analysis; Solid-phase extraction; Urine; Ultraviolet filter

Open tubular capillary electrochromatography: A useful microreactor for collagen I glycation and interaction studies with low-density lipoprotein particles by Lucia D’Ulivo; Joanna Witos; Katariina Öörni; Petri T. Kovanen; Marja-Liisa Riekkola (pp. 185-189).

Diabetes, a multifunctional disease and a major cause of morbidity and mortality in the industrialized countries, strongly associates with the development and progression of atherosclerosis. One of the consequences of high level of glucose in the blood circulation is glycation of long-lived proteins, such as collagen I, the most abundant component of the extracellular matrix (ECM) in the arterial wall. Glycation is a long-lasting process that involves the reaction between a carbonyl group of the sugar and an amino group of the protein, usually a lysine residue. This reaction generates an Amadori product that may evolve in advanced glycation end products (AGEs). AGEs, as reactive molecules, can provoke cross-linking of collagen I fibrils. Since binding of low-density lipoproteins (LDLs) to the ECM of the inner layer of the arterial wall, the intima, has been implicated to be involved in the onset of the development of an atherosclerotic plaque, collagen modifications, which can affect the affinity of native and oxidized LDL for collagen I, can promote the entrapment of LDLs in the intima and accelerate the progression of atherosclerosis.In this study, open tubular capillary electrochromatography is proposed as a new microreactor to study in situ glycation of collagen I. The kinetics of glycation was first investigated in a fused silica collagen I-coated capillary. Dimethyl sulphoxide, injected as an electroosmotic flow marker, gave information about the charge of coating. Native and oxidized LDL, and selected peptide fragments from apolipoprotein B-100, the protein covering LDL particles, were injected as marker compounds to clarify the interactions between LDLs and the glycated collagen I coating. The method proposed is simple and inexpensive, since only small amounts of collagen and LDL are required. Atomic force microscopy images complemented our studies, highlighting the difference between unmodified and glycated collagen I surfaces.

Keywords: Abbreviations; AFM; atomic force microscopy; AGEs; advanced glycation end products; apoB-100; apolipoprotein B-100; BGE; background electrolyte; BSA; bovine serum albumin; CEC; capillary electrochromatography; CuSO; ₄; copper sulphate; DIEA; N,N-diisopropylethylamine; DMF; N,N-dimethylformamide; DMSO; dimethyl sulphoxide; ECM; extracellular matrix; EDTA; ethylenediaminetetraacetic acid; EOF; electroosmotic flow; Fmoc; 9-fluorenylmethoxycarbonyl; HCTU; O-(6-chlorobenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate; HDL; high-density lipoprotein; i.d.; inner diameter; L; _det; detection length; LDL; low-density lipoproteins; L; _tot; total length; MWCO; molecular weight cut-off; NaN; ₃; sodium azide; NeuP; ₂; neutral peptide 2; nLDL; native low-density lipoprotein; NMM; 4-methylmorpholine; o.d.; outer diameter; OT-CEC; open tubular capillary electrochromatography; oxLDL; oxidized low-density lipoprotein; PP; positive peptide; RSD; relative standard deviation; TFA; trifluoroacetic acid; TIS; triisopropylsilaneOpen tubular capillary electrochromatography; Atherosclerosis; Collagen I; Diabetes; Glycation; Low-density lipoprotein

Gas chromatography–mass spectrometric determination of polybrominated diphenyl ethers in complex fatty matrices from aquaculture activities by Jaime Nácher-Mestre; Roque Serrano; Félix Hernández; Laura Benedito-Palos; Jaume Pérez-Sánchez (pp. 190-198).

Gas chromatography coupled to mass spectrometry in negative chemical ionization mode (GC–(NCI)MS) has been applied to the quantification and reliable identification of polybrominated diphenyl ethers (PBDEs) in animal and vegetable samples from aquaculture activities. Matrices analyzed included fish fillet, fish feed, fish oil and linseed oil, their fat content ranged from 5% to 100%. Solid-phase extraction (SPE) (using Florisil and silica cartridges) and normal-phase high performance liquid chromatography were tested for an efficient clean-up in order to obtain sample extracts free of interfering compounds. Combining sulphuric acid digestion and SPE with Florisil led to the highest efficiency in the elimination of interferences from the extracts. The sample procedure developed, together with the application of GC–(NCI)MS for measurement, led to the satisfactory determination of PBDEs at μgkg⁻¹ levels in complex aquaculture matrices with high lipid content. The use of a short and thin film-thickness fused-silica capillary column allowed to determine the problematic BDE 209 with satisfactory results. Three m/ z ions were acquired for each analyte, which ensured a reliable identification of compounds detected in samples.

Keywords: Polybrominated diphenyl ethers; Fatty samples; Gas chromatography; Mass spectrometry; Clean-up; Aquaculture matrices

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Analytica Chimica Acta (v.664, #2)