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Analytica Chimica Acta (v.689, #2)

Editorial Board (pp. i).

Controlled porosity monolithic material as permselective ion exchange membranes by Xiaojia Huang; Purnendu K. Dasgupta (pp. 155-159).

Ion exchange membranes (IEMs) are used in a variety of analytical devices, including suppressors, eluent generators and other components used in ion chromatography. Such membranes are flexible and undergo substantial dimensional changes on hydration. Presently the push to miniaturization continues; a resurgent interest in open tubular ion chromatography requires microscale adaptation of these components. Incorporating IEMs in microscale devices is difficult. Although both macroporous and microporous ion exchange materials have been made for use as chromatographic packing, ion exchange material used as membranes are porous only on a molecular scale. Because such pores have vicinal ion exchange sites, ions of the same charge sign as those of the fixed sites are excluded from the IEMs. Monolithic polymers, including ion exchangers derived therefrom, are presently extensively used. When used in a separation column, such a monolithic structure contains an extensively connected porous network. We show here that by controlling the amount of porogen added during the synthesis of monolithic polymers derived from ethylene dimethacrylate – glycidyl methacrylate, which are converted to an anion exchanger by treatment with trimethylamine, it is possible to obtain rigid ion exchange polymers that behave like IEMs and allow only one charge type of ions to pass through, i.e., are permselective. We demonstrate successful open tubular cation chromatography suppressor performance.

Keywords: Monolithic material; Ion exchange membrane; Suppressor; Capillary ion chromatography

Enzyme based assays in a sequential injection format: A review by Cristina I.C. Silvestre; Paula C.A.G. Pinto; Marcela A. Segundo; M. Lúcia M.F.S. Saraiva; José L.F.C. Lima (pp. 160-177).

Sequential injection analysis systems have been extensively exploited in the last decades for the implementation of enzyme based assays aiming the evaluation of enzyme activity or the determination of specific analytes. The most prominent aspects of the automation of enzymatic assays in these systems are discussed in this review. Special attention is devoted to the mode of enzyme manipulation in homogeneous or heterogeneous media and to the comparison with batch and flow injection enzyme methodologies. The possibility of implementing strategies for the enhancement of selectivity in specific determinations is also addressed. The more recent trends in this field are discussed focusing mainly on the miniaturization resorting to the lab-on valve platform as well as on the bead injection concept.

Keywords: Enzyme; Sequential injection analysis; Enzyme activity; Immobilized enzyme; Enzymatic reactor

The determination of antimony and arsenic concentrations in fly ash by hydride generation inductively coupled plasma optical emission spectrometry by Aki Ilander; Ari Väisänen (pp. 178-183).

Hydride generation inductively coupled plasma optical emission spectrometry (HG-ICP-OES) was used in the determination of As and Sb concentrations in fly ash samples. The effect of sample pre-treatment reagents and measurement parameters used for hydride generation was evaluated. Due to memory effects observed, the appropriate read delay time was adjusted to 60s resulting in RSDs 0.6% and 2.3% for As and Sb, respectively. The most suitable volumes of pre-reduction reagents for 10mL of sample were 4mL of KI/ascorbic acid (5%) and 6mL of HCl (conc.). The determination of Sb was significantly interfered by HF, but the interference could be eliminated by adding 2mL of saturated boric acid and heating the samples to 60°C at least 45min. The accuracy of the method was studied by analyses of SRM 1633b and two fly ash samples with the recovery test of added As and Sb. As high a recovery as 96% for SRM 1633b was reached for As using 193.696nm with two-step ultrasound-assisted digestion. A recovery rate of 103% was obtained for Sb using 217.582nm and the pre-reduction method with the addition of 2mL of saturated boric acid and heating. The quantification limits for the determination of As and Sb in the fly ash samples using two-step ultrasound-assisted digestion followed with HG-ICP-OES were 0.89 and 1.37mgkg⁻¹, respectively.

Keywords: Antimony; Arsenic; Fly ash; Hydride generation; ICP-OES

Determination of rare earth elements in seawater by inductively coupled plasma mass spectrometry with off-line column preconcentration using 2,6-diacetylpyridine functionalized Amberlite XAD-4 by Cennet Karadaş; Derya Kara; Andrew Fisher (pp. 184-189).

An off-line column preconcentration technique using a micro-column of 2,6 diacetylpyridine functionalized Amberlite XAD-4 with inductively coupled plasma mass spectrometry (ICP-MS) as a means of detection has been developed. The aim of the method was to determine rare earth elements (REEs) (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in seawater. Sample solutions (2–10mL) were passed through the column which was then washed with ultra-pure water to remove residual matrix. The adsorbed cations on the resin were eluted by using 2mL of 0.1molL⁻¹ HNO₃ containing 10ngmL⁻¹ indium as an internal standard. The eluent was analyzed for the metal concentrations using ICP-MS. Sample pH as well as the sample and eluent flow rates were optimized. The sorption capacity of resin was determined by the batch process, by equilibrating 0.05g of the resin with solutions of 50mL of 25mgL⁻¹ of individual metal ions for 4h at pH 6.0 at 26°C. The sorption capacities for the resin were found to range between 47.3μmolg⁻¹ (for Lu) and 136.7μmolg⁻¹ (for Gd). Limits of detection (3σ), without any preconcentration, ranged from 2ngL⁻¹ to 10.3ngL⁻¹ (for Tm and Lu respectively). The proposed method was applied to the determination of REEs in seawater and tap water samples.

Keywords: Rare earth elements; Seawater; 2,6-Diacetylpyridine; Amberlite XAD-4; Preconcentration

Biodiesel classification by base stock type (vegetable oil) using near infrared spectroscopy data by Roman M. Balabin; Ravilya Z. Safieva (pp. 190-197).

The use of biofuels, such as bioethanol or biodiesel, has rapidly increased in the last few years. Near infrared (near-IR, NIR, or NIRS) spectroscopy (>4000cm⁻¹) has previously been reported as a cheap and fast alternative for biodiesel quality control when compared with infrared, Raman, or nuclear magnetic resonance (NMR) methods; in addition, NIR can easily be done in real time (on-line). In this proof-of-principle paper, we attempt to find a correlation between the near infrared spectrum of a biodiesel sample and its base stock. This correlation is used to classify fuel samples into 10 groups according to their origin (vegetable oil): sunflower, coconut, palm, soy/soya, cottonseed, castor, Jatropha, etc. Principal component analysis (PCA) is used for outlier detection and dimensionality reduction of the NIR spectral data. Four different multivariate data analysis techniques are used to solve the classification problem, including regularized discriminant analysis (RDA), partial least squares method/projection on latent structures (PLS-DA), K-nearest neighbors (KNN) technique, and support vector machines (SVMs). Classifying biodiesel by feedstock (base stock) type can be successfully solved with modern machine learning techniques and NIR spectroscopy data. KNN and SVM methods were found to be highly effective for biodiesel classification by feedstock oil type. A classification error ( E) of less than 5% can be reached using an SVM-based approach. If computational time is an important consideration, the KNN technique ( E=6.2%) can be recommended for practical (industrial) implementation. Comparison with gasoline and motor oil data shows the relative simplicity of this methodology for biodiesel classification.

Keywords: Petroleum (fossil) fuel; Vegetable (plant) oil; Biofuel (biodiesel, bioethanol, ethanol–gasoline fuel); Vibrational spectroscopy (infrared, near-infrared, and Raman); Artificial neural networks; Support vector machines

Parametric signal fitting by gaussian peak adjustment: A new multivariate curve resolution method for non-bilinear voltammetric measurements by Santiago Cavanillas; José Manuel Díaz-Cruz; Cristina Ariño; Miquel Esteban (pp. 198-205).

A new methodology based on the fitting of signals to parametric functions is proposed for the multivariate curve resolution (MCR) analysis of overlapping and peak-shaped voltammetric signals which progressively get broader or narrower and move along the potential axis, thus causing a dramatic loss of linearity. The method is based on the least squares fitting of gaussian functions at both sides of the peaks by using adjustable parameters for the peak height, position and symmetry. It consists of several home-made programs written in Matlab environment, which are freely available as supplementary material of the present work. The application to the systems Zn(II)–oxalate, and to the phytochelatin PC₅ in a wide pH range provides excellent results as compared to these of more conventional linear methods, which raises good expectations about future application to electrochemical and even non-electrochemical data.

Keywords: Voltammetry; Multivariate curve resolution (MCR); Non-linearity; Gaussian peak adjustment (GPA); Potential shift; Peak broadening

A new application of scanning electrochemical microscopy for the label-free interrogation of antibody–antigen interactions by Joanne L. Holmes; Frank Davis; Stuart D. Collyer; Séamus P.J. Higson (pp. 206-211).

Within this work we present a ‘proof of principle’ study for the use of scanning electrochemical microscopy (SECM) to detect and image biomolecular interactions in a label-free assay as a potential alternative to current fluorescence techniques. Screen-printed carbon electrodes were used as the substrate for the deposition of a dotted array, where the dots consist of biotinylated polyethyleneimine. These were then further derivatised, first with neutravidin and then with a biotinylated antibody to the protein neuron specific enolase (NSE). SECM using a ferrocene carboxylic acid mediator showed clear differences between the array and the surrounding unmodified carbon. Imaging of the arrays before and following exposure to various concentrations of the antigen showed clear evidence for specific binding of the NSE antigen to the antibody derivatised dots. Non-specific binding was quantified. Control experiments with other proteins showed only non-specific binding across the whole of the substrate, thereby confirming that specific binding does occur between the antibody and antigen at the surface of the dots. Binding of the antigen was accompanied by a measured increase in current response, which may be explained in terms of protein electrostatic interaction and hydrophobic interactions to the mediator, thereby increasing the localised mediator flux. A calibration curve was obtained between 500fgmL⁻¹ to 200pgmL⁻¹ NSE which demonstrated a logarithmic relationship between the current change upon binding and antigen concentration without the need for any labelling of the substrate.

Keywords: Scanning electrochemical microscopy; Neuron specific enolase; Label-free; Antibody

Electrocatalytic detection of phenolic estrogenic compounds at NiTPPS|carbon nanotube composite electrodes by Xiaoqiang Liu; Heqing Feng; Xiuhua Liu; Danny K.Y. Wong (pp. 212-218).

A Ni(II)tetrakis(4-sulfonatophenyl) porphyrin (NiTPPS)|carbon nanotube composite electrode that shows strong catalytic and antifouling capability was developed to detect a series of phenolic endocrine compounds including bisphenol A, nonylphenol and ethynylestradiol. This electrode was fabricated by electropolymerizing NiTPPS complexes on a carbon nanotube-modified glassy carbon electrode. Optimized experimental parameters including a hydrodynamic potential of 0.7V for flow injection analysis (FIA) and a NiTPPS surface coverage of 2.2nmolcm⁻² (standard deviation 0.2nmolcm⁻²; n=6) were obtained for detection of the endocrine disrupting compounds. The sensor responded well to all the tested compounds with limits of detection ranging from 15nmolL⁻¹ to 260nmolL⁻¹ (based on three times S/N ratio) under FIA conditions. Both carbon nanotubes and NiTPPS account for the excellent performance of the composite modified electrode.

Keywords: Electrochemical detection; Endocrine disrupting compounds; Carbon nanotubes; Ni(II)tetrakis(4-sulfonatophenyl) porphyrin; Electrochemical impedance spectroscopy; Flow injection analysis

Orthogonal array design for the optimization of hollow fiber protected liquid-phase microextraction of salicylates from environmental waters by Cong Zhang; Lei Ye; Li Xu (pp. 219-225).

In the present study, a three phase-based hollow fiber protected liquid-phase microextraction (HF-LPME) method combined with high-performance liquid chromatography (HPLC) for the determination of salicylates in environmental waters was developed. The HF-LPME procedure was optimized by an L₁₆(4⁵) orthogonal array experimental design (OAD) with five factors at four levels. Under the optimal extraction condition (pHs of donor and receiving phases of 3.0 and 6.2, respectively, extraction time of 45min, stirring speed of 1000rpm, and salt addition of 20% (w/v)), salicylates could be determined in a linear range from 0.025 to 1.0μgmL⁻¹ with a good correlation ( r²>0.9930). The limits of detection (LODs) ranged between 0.6ngmL⁻¹ and 1.2ngmL⁻¹ for the target analytes. The relative standard deviations (RSDs) of intra-day and inter-day were in the range of 0.64–14.58% and 0.16–15.45%, respectively. This procedure afforded a convenient, sensitive, accurate and cost-saving operation with high extraction efficiency for the model analytes. The method was applied satisfactorily to the determination of salicylates in two environmental waters.

Keywords: Orthogonal array design; Hollow fiber protected liquid-phase microextraction; High-performance liquid chromatography; Salicylates; Environmental analysis

An automatic optosensing device for the simultaneous determination of resveratrol and piceid in wines by Lucía Molina-García; Antonio Ruiz-Medina; María Luisa Fernández-de Córdova (pp. 226-233).

For the first time, a spectrofluorimetric method is reported for the simultaneous determination of resveratrol (RVT) and piceid (PCD), two stilbenes showing diverse interesting physiological and biochemical attributes, as well as a wide range of health benefits ranging from cardioprotection to chemoprevention. The method makes use of a multicommutated flow-through optosensor in which the resolution of RVT and PCD is accomplished by means the sequential arrival of their photoproducts, on-line generated by UV-irradiation, to the detection area. This is possible due to the different kinetic behaviour of these latter on a solid support (C₁₈ silica gel) filling a minicolumn placed before the detector. The measurement in solid-phase of the photochemically induced fluorescence of the photoproducts ( λ_ex: 257nm/ λ_em: 382nm) is used as analytical signal for monitoring both compounds. The method has been applied to the analysis of RVT and PCD in wines and requires a previous solid-phase extraction (SPE) using Bakerbond C₁₈ cartridges. This pretreatment and the use of a solid-support in both the minicolumn and the flow-cell of the detector allow the determination of RVT and PCD by external calibration. Detection limits (DLs) are 9.3 and 12.6ngmL⁻¹ for RVT and PCD, respectively. Commercial red and white wine samples have been analysed and the results obtained have been satisfactorily validated by high-performance liquid chromatography (HPLC).

Keywords: Resveratrol; Piceid; Multicommutation; Optosensor; Photochemically induced fluorescence; Wine

Regenerable immuno-biochip for screening ochratoxin A in green coffee extract using an automated microarray chip reader with chemiluminescence detection by Jimena C. Sauceda-Friebe; Xaver Y.Z. Karsunke; Susanna Vazac; Scarlett Biselli; Reinhard Niessner; Dietmar Knopp (pp. 234-242).

Ochratoxin A (OTA) can contaminate foodstuffs in the ppb to ppm range and once formed, it is difficult to remove. Because of its toxicity and potential risks to human health, the need exists for rapid, efficient detection methods that comply with legal maximum residual limits. In this work we have synthesized an OTA conjugate functionalized with a water-soluble peptide for covalent immobilization on a glass biochip by means of contact spotting. The chip was used for OTA determination with an indirect competitive immunoassay format with flow-through reagent addition and chemiluminescence detection, carried out with the stand-alone automated Munich Chip Reader 3 (MCR 3) platform. A buffer model and real green coffee extracts were used for this purpose. At the present, covalent conjugate immobilization allowed for at least 20 assay-regeneration cycles of the biochip surface. The total analysis time for a single sample, including measurement and surface regeneration, was 12min and the LOQ of OTA in green coffee extract was 0.3μgL⁻¹ which corresponds to 7μgkg⁻¹.

Keywords: Ochratoxin A (OTA); Peptide–OTA conjugate; Automated immunoassay; Green coffee; Chemiluminescence detection; Regenerable biochip

Magnetic relaxation switch and colorimetric detection of thrombin using aptamer-functionalized gold-coated iron oxide nanoparticles by Guohai Liang; Shaoyu Cai; Peng Zhang; Youyuan Peng; Hui Chen; Song Zhang; Jilie Kong (pp. 243-249).

We describe a sensitive biosensing system combining magnetic relaxation switch diagnosis and colorimetric detection of human α-thrombin, based on the aptamer–protein interaction induced aggregation of Fe₃O₄@Au nanoparticles. To demonstrate the concept, gold-coated iron oxide nanoparticle was synthesized by iterative reduction of HAuCl₄ onto the dextran-coated Fe₃O₄ nanoparticles. The resulting core–shell structure had a flowerlike shape with pretty narrow size distribution (referred to as “nanorose”). The two aptamers corresponding to human α-thrombin were conjugated separately to two distinct nanorose populations. Once a solution containing human α-thrombin was introduced, the nanoroses switched from a well dispersed state to an aggregated one, leading to a change in the spin–spin relaxation time ( T₂) as well as the UV–Vis absorption spectra of the solution. Thus the qualitative and quantitative detection method for human α-thrombin was established. The dual-mode detection is clearly advantageous in obtaining a more reliable result; the detection range is widened as well. By using the dual-mode detection method, a detectable T₂ change is observed with 1.0nM human α-thrombin, and the detection range is from 1.6nM to 30.4nM.

Keywords: Magnetic relaxation switch; Colorimetric sensor; Fe; ₃; O; ₄; @Au; Human α-thrombin

Antifungal evaluation of cholic acid and its derivatives on Candida albicans by microcalorimetry and chemometrics by Weijun Kong; Jiabo Wang; Xiaoyan Xing; Xiaohe Xiao; Yanling Zhao; Qingce Zang; Ping Zhang; Cheng Jin; Zulun Li; Wei Liu (pp. 250-256).

In the last few years, several fungus infections caused by multidrug-resistant pathogenic agents have got tremendous emergence and prevalence. Screening for novel antifungal agents is in great demand, but traditional microbiological techniques are far from sufficient to meet that requirement. In this study, a non-invasive and non-destructive microcalorimetric method was performed to investigate the antifungal activities of cholic acid (CA) and its derivatives, glycocholic acid (GCA) and taurocholic acid (TCA) on the multiplying and non-multiplying metabolism of Candida albicans. Then, the heat-flow power-time curves of C. albicans growth affected by different concentrations of CA, GCA and TCA were studied by similarity analysis (SA), the quantitative thermokinetic parameters from these curves were analyzed by multivariate analysis of variance (MANOVA) and principal component analysis (PCA). By comparing the values of two main parameters, P₂ (the heat-flow output power of the highest peak) and Q₂ (the heat output of the second exponential growth phase) of C. albicans, it could be found that CA had the strongest antifungal activity among the three steroid compounds, which might be used as a potential antifungal agent in the future. This study provided a useful method and idea of microcalorimetry with chemometrics to efficiently evaluate the antifungal activities of bile acid derivatives, giving some references for screening out new antifungal agents.However, it has to be stressed that all these experiments are carried out in vitro and they still require clinical validation.

Keywords: C. albicans; Cholic acid; Antifungal activity; Microcalorimetry; Chemometrics

GC–MS analysis of water-soluble organics in atmospheric aerosol: Response surface methodology for optimizing silyl-derivatization for simultaneous analysis of carboxylic acids and sugars by Maria Chiara Pietrogrande; Dimitri Bacco (pp. 257-264).

This paper describes the development of a derivatization procedure — silylation using N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA) — for the simultaneous GC–MS analysis of a wide range of water-soluble organics in atmospheric aerosols. The reaction operating conditions were optimized using the response surface methodology (RSM) including central composite design (CCD) in order to achieve the highest response for a large number of dicarboxylic acids and sugars. The factors considered were: (i) reaction temperature (50–90°C), (ii) the reaction duration (60–120min), (iii) reagent concentrations (10–100% of the total solution volume) and (iv) pyridine concentration (0–50% of the derivatization reagent). On the basis of RSM and experimental evidence, the optimum derivatization conditions were defined as reaction temperature of 75°C, reaction duration of 70min, BSTFA reagent concentration of 55% and pyridine concentration of 35%. The optimized protocol was extended to a broader range of 22 target analytes that are relevant chemical markers, i.e., 15 carboxylic acids and 7 sugars. In addition, the applicability of the optimized procedure was verified in environmental matrices from PM filters collected under different conditions, i.e., different seasons (summer vs. winter), different sampling sites (urban vs. rural), different particle size dimensions (PM_2.5 vs. PM₁).

Keywords: Dicarboxylic acids and sugars; Atmospheric aerosol; Silylation derivatization procedure; Experimental design; Response surface methodology

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Analytica Chimica Acta (v.689, #2)