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Analytica Chimica Acta (v.607, #2)

Contents (pp. iii-iv).

Editorial Board (pp. co1).

Paradigms in isotope dilution mass spectrometry for elemental speciation analysis by Juris Meija; Zoltán Mester (pp. 115-125).

Isotope dilution mass spectrometry currently stands out as the method providing results with unchallenged precision and accuracy in elemental speciation. However, recent history of isotope dilution mass spectrometry has shown that the extent to which this primary ratio measurement method can deliver accurate results is still subject of active research. In this review, we will summarize the fundamental prerequisites behind isotope dilution mass spectrometry and discuss their practical limits of validity and effects on the accuracy of the obtained results. This review is not to be viewed as a critique of isotope dilution; rather its purpose is to highlight the lesser studied aspects that will ensure and elevate current supremacy of the results obtained from this method.

Keywords: Mass spectrometry; Isotope dilution; Elemental speciation; Isotopic equilibration

Applications of ionic liquids in electrochemical sensors by Di Wei; Ari Ivaska (pp. 126-135).

Ionic liquids (ILs) are molten salts with the melting point close to or below room temperature. They are composed of two asymmetrical ions of opposite charges that only loosely fit together (usually bulky organic cations and smaller anions). The good solvating properties, high conductivity, non-volatility, low toxicity, large electrochemical window (i.e. the electrochemical potential range over which the electrolyte is neither reduced nor oxidized on electrodes) and good electrochemical stability, make ILs suitable for many applications. Recently, novel ion selective sensors, gas sensors and biosensors based on ILs have been developed. IL gels were found to have good biocompatibility with enzymes, proteins and even living cells. Besides a brief discussion of the properties of ILs and their general applications based on these properties, this review focuses on the application of ILs in electroanalytical sensors.

Keywords: Ionic liquid; Electrochemical analysis; Sensors

Evaluation of a hydride generation-atomic fluorescence system for the determination of arsenic using a dielectric barrier discharge atomizer by Zhenli Zhu; Jixin Liu; Sichun Zhang; Xing Na; Xinrong Zhang (pp. 136-141).

A new atomizer based on atmospheric pressure dielectric barrier discharge (DBD) plasma was specially designed for atomic fluorescence spectrometry (AFS) in order to be applied to the measurement of arsenic. The characteristics of the DBD atomizer and the effects of different parameters (power, discharge gas, gas flow rate, and KBH₄ concentration) were discussed in the paper. The DBD atomizer shows the following features: (1) low operation temperature (between 44 and 70°C, depending on the operation conditions); (2) low power consumption; (3) operation at atmospheric pressure. The detection limit of As(III) using hydride generation (HG) with the proposed DBD-AFS was 0.04μgL⁻¹. The analytical results obtained by the present method for total arsenic in reference materials, orchard leaves (SRM 1571) and water samples GBW(E) 080390, agree well with the certified values. The present HG-DBD-AFS is more sensitive and reliable for the determination of arsenic. It is a very promising technique allowing for field arsenic analysis based on atomic spectrometry.

Keywords: Atomic fluorescence spectrometry; Dielectric barrier discharge; Atomizer; Microplasma

Spectrophotometric determination of acidity constants by two-rank annihilation factor analysis by Bahram Hemmateenejad; Abdolkarim Abbaspour; Homeyra Maghami; Alireza Foroumadi (pp. 142-152).

Annihilation of the contribution of one chemical component from the original data matrix is a general method in rank annihilation factor analysis (RAFA). However, RAFA is not applicable for studying the protonation equilibria of multiprotic acids. In this work, a two-rank annihilation factor analysis (TRAFA) method was proposed for determination of the acidity constants of diprotic acids. After recording the electronic absorbance spectra of the acids at different pH, the contributions of both H₂A and A²⁻ were annihilated from the absorbance data, which made feasible the determination of two successive acidity constants. The method was validated by analysis of simulated data and its application to the determination of the acidity constants of calmagite, as a reference compound. A close agreement was obtained between the resulted values by TRAFA and the declared values. Indeed, the method was used for determination of the acidity constants of two new chromenone derivatives in binary solvents mixtures of methanol and water. The effects of changing solvent composition on acidity constant data were explained by linear solvation free energy relationships (LSFER) utilizing solvatochromic parameters.

Keywords: Rank annihilation factor analysis; Acidity constant; Spectrophotometry; Methanol–water; Chromenone

Multivariate near infrared spectroscopy models for predicting the methyl esters content in biodiesel by Patrícia Baptista; Pedro Felizardo; José C. Menezes; M. Joana Neiva Correia (pp. 153-159).

Biodiesel is the main alternative to fossil diesel. The key advantages of its use are the fact that it is a non-toxic renewable resource, which leads to lower emissions of polluting gases. European governments are targeting the incorporation of 20% of biofuels in the general fuels until 2020.Chemically, biodiesel is a mixture of fatty acid methyl esters, derived from vegetable oils or animal fats, which is usually produced by a transesterification reaction, where the oils/fats react with an alcohol, in the presence of a catalyst. The European Standard (EN 14214) establishes 25 parameters that have to be analysed to certify biodiesel quality and the analytical methods that should be used to determine those properties.This work reports the use of near infrared (NIR) spectroscopy to determine the esters content in biodiesel as well as the content in linolenic acid methyl esters (C18:3) in industrial and laboratory-scale biodiesel samples. Furthermore, calibration models for myristic (C14:0), palmitic (C16:0), stearic (C18:0), oleic (C18:1), linoleic (C18:2) acid methyl esters were also obtained. Principal component analysis was used for the qualitative analysis of the spectra, while partial least squares regression was used to develop the calibration models between analytical and spectral data. The results confirm that NIR spectroscopy, in combination with multivariate calibration, is a promising technique to assess the biodiesel quality control in both laboratory-scale and industrial scale samples.

Keywords: Biodiesel; Near infrared; Calibration models; Methyl esters

Determination of Sudan I in chilli powder from solvent components gradual change–visible spectra data using second order calibration algorithms by Jintao Yuan; Lifu Liao; Yingwu Lin; Changai Deng; Bo He (pp. 160-167).

It was discovered that a second order spectra data matrix of Sudan I produced from the solvent components gradual change–visible absorption spectra can be expressed as the combination of two bilinear data matrices. Based on this discovery, a new method for the determination of Sudan I in gray systems using second order calibration algorithms has been developed. The second order calibration algorithms were based on the popular parallel factor analysis (PARAFAC) and rank annihilation factor analysis (RAFA), respectively. In the method described here, the components of the solvent were changed gradually by adding ethanol into cyclohexane, the absorption spectra of Sudan I and chilli samples in a series of cyclohexane–ethanol mixed solvents with various ethanol volume fractions were recorded, and then the second order data were obtained from the solvent components gradual change–visible absorption spectra. Thus, the concentration of Sudan I in a gray system could be determined from the spectra matrices using second order calibration algorithms. This method is simple, convenient and dependable. The method has been used to determine Sudan I in chilli powder with satisfactory results.

Keywords: Sudan I; Second order calibration; Parallel factor analysis; Rank annihilation factor analysis; Visible absorption spectra; Solvent components gradual change

Super-resolution and Raman chemical imaging: From multiple low resolution images to a high resolution image by Ludovic Duponchel; Peyman Milanfar; Cyril Ruckebusch; Jean-Pierre Huvenne (pp. 168-175).

Imaging in Raman spectroscopy is a valuable tool for analytical chemistry. Although molecular characterization at micron level is achieved for many applications, it usually fails producing chemical images of micron size samples as expected in chemical, environmental and biological analysis. The aim of the work is to introduce the potential of super-resolution in vibrational spectroscopic imaging. This original chemometrics approach uses several low resolution images of the same sample in order to retrieve a higher resolution chemical image. It is thus possible to overcome in a certain way some physical and instrumentals limitations. To illustrate the methodology, sub-micronic details of a Si/Au sample are retrieved from low resolution images with different super-resolution algorithms. The better results are obtained with Iterative L2/Bilateral Total Variation regularization method. The use of a regularization procedure gives also better results since its first property is to preserve edges during the reconstruction of the super-resolved image. This concept of chemical image data processing should open new analytical opportunities.

Keywords: Super-resolution; Chemometrics; Raman spectroscopy; Imaging

Composite planar electrode for sensing electrochemical oxygen demand by Jahir Orozco; César Fernández-Sánchez; Ernest Mendoza; Mireia Baeza; Francisco Céspedes; Cecilia Jiménez-Jorquera (pp. 176-182).

This work reports on the development of a graphite–polystyrene composite electrode of planar configuration, containing silver(II) oxide and copper(II) oxide catalysts (AgO–CuO), for the measurement of electrochemical oxygen demand (EOD). Optimisation studies of the composite composition as well as conditions for its processing on planar substrates and generation of an appropriate electrochemical active area resulted in the scalable fabrication of robust composite electrodes. These were evaluated with glucose as target analyte. They showed competitive low limits of detection in a linear concentration range from 5mgL⁻¹ to 1400mgL⁻¹ of O₂. Besides, they were stable for at least one year. The determination of EOD in wastewater samples coming from production lines of parenteral food and winemaking was successfully carried out.

Keywords: Electrochemical oxygen demand; Planar electrode; AgO/CuO catalysts; Carbon–polystyrene composite

Separation and preconcentration of ultra trace amounts of beryllium in water samples using mixed micelle-mediated extraction and determination by inductively coupled plasma-atomic emission spectrometry by Assadollah Beiraghi; Saeed Babaee (pp. 183-190).

In the present study a cloud point extraction process using mixed micelle of the cationic surfactant cetyl-pyridinium chloride (CPC) and non-ionic surfactant Triton X-114 for extraction of beryllium from aqueous solutions is developed. The extraction of analyte from aqueous samples was performed in the presence of 1,8-dihydroxyanthrone as chelating agent in buffer media of pH 9.5. After phase separation, the surfactant-rich phase was diluted with 0.4mL of a 60:40 methanol–water mixture containing 0.03mL HNO₃. Then, the enriched analyte in the surfactant-rich phase was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The different variables affecting the complexation and extraction conditions were optimized. Under the optimum conditions (i.e. 1.6×10⁻⁴molL⁻¹ 1,8-dihydroxyanthrone, 1.2×10⁻⁴molL⁻¹ CPC, 0.15% (v/v) Triton X-114, 50°C equilibrium temperature) the calibration graph was linear in the range of 0.006–80ngmL⁻¹ with detection limit of 0.001ngmL⁻¹ and the precision (R.S.D.%) for five replicate determinations at 18ngmL⁻¹ of Be(II) was better than 2.9%. In this manner the preconcentration and enrichment factors were 16.7 and 24.8, respectively. Under the presence of foreign ions no significant interference was observed. Finally, the proposed method was successfully utilized for the determination of this cation in water samples.

Keywords: Cloud point extraction; Beryllium; Mixed micelle; Triton X-114; Inductively coupled plasma-atomic emission spectrometry; 1,8-Dihydroxyanthrone; Cetyl-pyridinium chloride

Multiresidue analytical methods for the ultra-trace quantification of 33 priority substances present in the list of REACH in real water samples by Jean-Baptiste Baugros; Barbara Giroud; Guy Dessalces; Marie-Florence Grenier-Loustalot; Cécile Cren-Olivé (pp. 191-203).

Innovative and simultaneous multiresidue analytical methods of 33 multi-class pollutants in wastewaters, surface and ground waters, using solid phase extraction (SPE) followed by gas chromatography–mass spectrometry (GC–MS) and liquid chromatography–tandem mass spectrometry (LC–MS/MS) are presented. Target compounds include several groups of emerging and persistent contaminants derived from the European priority list of the registration evaluation and authorisation of chemicals system (REACH): organochlorine (8) and organophosphorus (2) pesticides, carbamates (2), fungicides (8), phthalates (2), alkylphenols (10) and bisphenol A.The recovery rates of the SPE gave levels ranged from 84 to 118% with exception of some compounds that yielded lower (methamidophos (50%), p, p′-DDT (60%) and o, p′-DDT (72%)) but all recoveries were acceptable. Low limits of detection (LOD) varied between 0.2 and 88.9ngL⁻¹ (except for HPTE and 4 n-octylphenol giving 161 and 220ngL⁻¹, respectively). A study of matrix effects was performed in order to evaluate the best and reliable calibration approach.The developed analytical method was successfully applied to the analysis of the 33 substances in wastewater effluents as well as surface and ground waters. The most frequently detected families were alkylphenols and industrial endocrine disrupting compounds (phthalates and bisphenol A).

Keywords: Multiresidue analysis; Registration evaluation and authorisation of chemicals; Liquid and gas chromatographies; Mass spectrometry; Wastewaters

Reusable optical bioassay platform with permeability-controlled hydrogel pads for selective saccharide detection by Kwan Yee Cheung; Wing Cheung Mak; Dieter Trau (pp. 204-210).

A reusable optical bioassay platform using permeability-controlled hydrogel pads for selective saccharide detection has been developed. An optical glucose detection assay based on fluorescence resonance energy transfer (FRET) between dye-labeled dextran and Concanavalin A (ConA) was incorporated into hydrogel pads by entrapment. The hydrogel pads are constructed from hemispherical hydrogel attached onto hydrophobic surfaces of a microtiter plate. The resulted hemispherical hydrogel pads entrapping the sensing biological materials were further surface coated with polyelectrolyte multilayers through a Layer-by-Layer (LbL) self-assembly process to create a permeability-controlled membrane with nanometer thickness. The selective permeable LbL film deposited on the hydrogel surface allows small molecular weight analytes to diffuse into the hydrogel pads while the large molecular weight sensing biological molecules are immobilized. An encapsulation efficiency of 75% for the ConA/Dextran complex within the coated hydrogel pads was achieved and no significant leakage of the complex was observed. Glucose calibration curve with linear range from 0 to 10mM glucose was obtained. Selective permeability of the hydrogel pads has been demonstrated by measurement of saccharides with various molecular weights. The LbL hydrogel pads could selectively detect monosaccharides (glucose, MW=180) and disaccharides (sucrose, MW=342) while polysaccharides (dextran, MW∼70kDa) cannot diffuse through the LbL layer and are excluded. LbL hydrogel pads allow regeneration of the FRET system with good signal reproducibility of more than 90% to construct a reusable and reagentless optical bioassay platform.

Keywords: Layer-by-Layer; Hydrogel pad; Permeability control; Saccharide detection; Optical bioassay

Uric acid determination using uricase and the autotransducer molecular absorption properties of peroxidase by Vanesa Sanz; Susana de Marcos; Javier Galbán (pp. 211-218).

Uric acid (UA) is determined using the UV–vis molecular absorption properties of peroxidase (HRP). The method as a whole involves UA oxidation in the presence of uricase (UOx), giving H₂O_2. The H₂O₂ then reacts with HRP forming the compound I species which returns to its initial form by reaction with UA and intramolecular reduction. The molecular absorption changes of HRP at 420nm during the reaction enable the UA to be determined. A mathematical model relating the analytical signal to UA, UOx and HRP has been developed and experimentally validated. The possibility of carrying out both enzymatic reactions sequentially or simultaneously is discussed, the latter option producing better analytical performances. The method permits UA determination in the range 1.5×10⁻⁶–4.0×10⁻⁵M, with an R.S.D. of about 3% ( n=5, 1.5×10⁻⁶M UA). It has been applied to analyte determination in synthetic serum samples.

Keywords: Autotransducer protein; Horseradish peroxidase; Uricase; Uric acid; Mathematical model; Serum

¹H NMR determination of hypericin and pseudohypericin in complex natural mixtures by the use of strongly deshielded OH groups by Evangelos C. Tatsis; Vassiliki Exarchou; Anastassios N. Troganis; Ioannis P. Gerothanassis (pp. 219-226).

The¹H NMR spectra of the commercially available compounds hypericin and its derivative pseudohypericin in CD₃OH solutions indicate significantly deshielded signals in the region of 14–15ppm. These resonances are attributed to the peri hydroxyl protons OH(6), OH(8) and OH(1), OH(13) of hypericins which participate in a strong six-membered ring intramolecular hydrogen bond with CO(7) and CO(14), respectively, and therefore, they are strongly deshielded. In the present work, we demonstrate that one-dimensional¹H NMR spectra of hypericin and pseudohypericin, in Hypericum perforatum extracts show important differences in the chemical shifts of the hydroxyl groups with excellent resolution in the region of 14–15ppm. The facile identification and quantification of hypericin and its derivative compound pseudohypericin was achieved, without prior HPLC separation, for two H. perforatum extracts from Greek cultivars and two commercial extracts: a dietary supplement, and an antidepressant medicine. The results were compared with those obtained from UV–vis and LC/MS measurements.

Keywords: Abbreviations; NMR; nuclear magnetic resonance spectroscopy; MS; mass spectrometry; HPLC; high performance liquid chromatography¹; H nuclear magnetic resonance; Hypericin; Pseudohypericin; Hypericum perforatum; Intramolecular hydrogen bond; peri; Hydroxyl groups

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Analytica Chimica Acta (v.607, #2)

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Analytica Chimica Acta (v.607, #2)