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Analytica Chimica Acta (v.602, #1)

Contents (pp. v-vi).

Editorial Board (pp. co1).

Toward Bayesian chemometrics—A tutorial on some recent advances by Hongshu Chen; Bhavik R. Bakshi; Prem K. Goel (pp. 1-16).

Chemometrics is increasingly being perceived as a maturing science. While this perception seems to be true with regards to the traditional methods and applications of chemometrics, this article argues that advances in instrumentation, computation, and statistical theory may combine to drive a resurgence in chemometrics research. Previous surges in chemometrics research activity were driven by the development of new ways of making better use of available information. Bayesian statistics can further enhance the ability to use domain specific information to obtain more accurate and useful models, and presents many research opportunities as well as challenges.Although Bayesian statistics is not new, recent advances via sampling-based Monte Carlo methods make these methods practical for large scale applications without making the common assumptions of Gaussian noise and uniform prior distributions, made by most chemometric methods. This article provides an overview of traditional chemometric methods from a Bayesian view and a tutorial of some recently developed techniques in Bayesian chemometrics, such as Bayesian PCA and Bayesian latent variable regression. New challenges and opportunities for future work are also identified.

Keywords: Regression; Latent variables; Bayesian statistics; Partial least squares (PLS); Principal component analysis (PCA); Maximum likelihood

Speciation of vanadium in oilsand coke and bacterial culture by high performance liquid chromatography inductively coupled plasma mass spectrometry by X. Sherry Li; X. Chris Le (pp. 17-22).

A simple and sensitive method for the speciation of vanadium(III), (IV), and (V) was developed by using high performance liquid chromatography and inductively coupled plasma mass spectrometry (HPLC–ICPMS). The EDTA-complexed vanadium species were separated on a strong anion exchange column with an eluent containing 2mM EDTA, 3% acetonitrile, and 80mM ammonium bicarbonate at pH 6. Each analysis was complete in 5min. The detection limits were 0.6, 0.7 and 1.0μgL⁻¹ for V(III), V(IV), and V(V), respectively. The method was applied to coke pore water samples from an oilsand processing/upgrading site in Fort McMurray, Alberta, Canada and to Shewanella putrefaciens CN32 bacterial cultures incubated with V(V). In the coke pore water samples, V(IV) and V(V) were found to be the major species. For the first time, V(III) was detected in the bacterial cultures incubated with V(V).

Keywords: Vanadium speciation; High performance liquid chromatography–inductively coupled plasma mass spectrometry; Vanadium(III); Coke pore water; Shewanella putrefaciens; CN32; Oilsand; Environmental contamination

Determination of trace elements in paints by direct sampling graphite furnace atomic absorption spectrometry by Fabrina R.S. Bentlin; Dirce Pozebon; Paola A. Mello; Érico M.M. Flores (pp. 23-31).

A direct sampling graphite furnace atomic absorption spectrometric (DS-GFAAS) method for the determination of Cd, Pb, Cr, Ni, Co and Cu in paints has been developed. Serigraphy, acrylic and tattoo paints were analysed. Approaches like pyrolysis and atomization temperatures, modifiers and sample mass introduced in the atomizer were studied. Quantification was performed using calibration curves measured with aqueous standard solutions pipetted onto the platform. The sample mass introduced in the graphite tube ranged from 0.02 to 8.0mg. Palladium was used as modifier for Cd, Pb and Cu, while Mg(NO₃)₂ was used for Co. For Ni determination, the graphite platform was covered with carbon powder. The characteristic masses of Cd, Pb, Cr, Ni, Co and Cu were 1.4, 22.5, 7.9, 11.0, 9.6 and 12.5pg, while the limits of detection were 0.0004, 0.001, 0.03, 0.22, 0.11 and 0.05μgg⁻¹ of Cd, Pb, Cr, Ni, Co and Cu, respectively. The accuracy was determined by comparison of the results with those obtained by inductively coupled plasma mass spectrometry (ICP-MS) and graphite furnace atomic absorption spectrometry (GFAAS), using liquid sampling of digests. For matrix characterization, major and minor elements (Al, Mg, Ba, Ca, Cr, Cu, Pb, Sr, Ti and Mg) were determined by inductively coupled plasma optical emission spectrometry (ICP OES).

Keywords: Direct sampling; Graphite furnace atomic absorption spectrometry; Trace elements determination; Paints

Speciation analysis of inorganic arsenic in water samples by immobilized nanometer titanium dioxide separation and graphite furnace atomic absorption spectrometric determination by Pei Liang; Rui Liu (pp. 32-36).

A novel solid phase extraction sorbent, nanometer titanium dioxide immobilized on silica gel (immobilized nanometer TiO₂), was prepared by sol–gel method, and the adsorptive potential of immobilized nanometer TiO₂ for inorganic arsenic species (As(III) and As(V)) was assessed. It was found that both As (III) and As(V) could be adsorbed quantitatively on immobilized TiO₂ within a pH range of 5.0–7.5, and only As(III) could be quantitatively retained on the sorbent within a pH range of 9.5–10.5 while As(V) was passed through the microcolumn without the retention. Based on this fact, a new speciation scheme for inorganic arsenic was established, which involved determining total As at pH 6.0 and As(III) at pH 10, with As(V) obtained by difference. The adsorption capacity of immobilized nanometer TiO₂ for As(III) was found to be 4.22mgg⁻¹. Under the optimized conditions, the detection limits of this method for As(III) were 24ngL⁻¹ with an enrichment factor of 50, and the relative standard deviation (R.S.D.%) was 4.8% ( n=11, c=5ngmL⁻¹). The proposed method has been applied to the speciation of inorganic arsenic in natural water samples with satisfactory results.

Keywords: Immobilized nanometer titanium dioxide; Arsenic; Speciation; Graphite furnace atomic absorption Spectrometry

Modelling the quality of enantiomeric separations using Mutual Information as an alternative variable selection technique by Sónia Caetano; Catherine Krier; Michel Verleysen; Yvan Vander Heyden (pp. 37-46).

This paper uses Mutual Information as an alternative variable selection method for quantitative structure–property relationships data. To evaluate the performance of this criterion, the enantioselectivity of 67 molecules, in three different chiral stationary phases, is modelled. Partial Least Squares together with three commonly used variable selection techniques was evaluated and then compared with the results obtained when using Mutual Information together with Support Vector Machines. The results show not only that variable selection is a necessary step in quantitative structure–property relationship modelling, but also that Mutual Information associated with Support Vector Machines is a valuable alternative to Partial Least Squares together with correlation between the explanatory and the response variables or Genetic Algorithms. This study also demonstrates that by producing models that use a rather small set of variables the interpretation can be also be improved.

Keywords: Enantioseparation; Mutual Information; Variable selection

Problem of mixtures with known compositions and IRONFLEA method for multivariate curve resolution by Yegor Zyrianov (pp. 47-54).

A special case of gray spectral data systems [(a) F.-T. Chau, Y.-Z. Liang, J. Gao, X.-G. Shao (Eds.), Chemometrics: From Basics to Wavelet Transform, Chemical Analysis Series, vol. 164, John Wiley & Sons, Inc., 2004; (b) Y.Z. Liang, O.M. Kvalheim, R. Manne, Chemom. Intell. Lab. Syst. 18 (1993) 235–250] is discussed here and the least-squares method for the multivariate curve resolution (MCR) named IRONFLEA is proposed. The system under consideration is the bilinear spectral data of the samples with known chemical compositions and unknown concentration matrix. If the spectra of samples (A_i) and (Q+A_i) ( i=1, …, n, n≥2) are available, then the spectrum and the concentrations of Q could be found and the solution is unique. A practical chemical model for this problem could be mixtures, polymers, peptides, oligosaccharides, or supramolecular formations made of a limited number of monomeric components. In the cases of polymeric or oligomeric samples the spectral contributions and the concentrations of the particular monomeric units are extracted. The method is capable of extracting chemically meaningful spectra of components. The method is implemented in SAS IML code and tested for the deconvolution of spectra of polymers made of styrene derivatives with known monomeric compositions [(a) H. Fenniri, L. Ding, A.E. Ribbe, Y. Zyrianov, J. Am. Chem. Soc. 123 (2001) 8151–8152; (b) H. Fenniri, S. Chun, L. Ding, Y. Zyrianov, K. Hallenga, J. Am. Chem. Soc. 125 (2003) 10546–10560]. The method performs calculations fast enough to allow the incorporation of leave-one-out outlier removal procedure.

Keywords: Multivariate curve resolution; Multivariate calibration; IRONFLEA

Radial basis function network calibration model for near-infrared spectra in wavelet domain using a genetic algorithm by Yan Wang; Bingren Xiang (pp. 55-65).

Near-infrared (NIR) spectrometry is now widely used in various fields and great attention is paid to the application of it to addressing complex problems, which brings about the need for the calibration of systems that fail to exhibit satisfactional linear relationship between input–output data. In this work we present a novel method to build a multivariate calibration model for NIR spectra, i.e. genetic algorithm–radial basis function network in wavelet domain (WT-GA–RBFN), which combines the advantages of wavelet transform and genetic algorithm. The variable selection is accomplished in two stages in wavelet domain: at the first stage, the variables are pre-selected (compressed) by variance and at the second stage the variables are further reduced by a special designed GA. The proposed method is illustrated through presenting its application to three NIR data sets in different fields and the comparison to PLS model.

Keywords: Near-infrared spectrometry; Radial basis functions networks; Discrete wavelet transform; Variable selection; Genetic algorithms

Multivariate calibration applied to the time-resolved chemilumnescence for the simultaneous determination of morphine and its antagonist naloxone by José A. Murillo Pulgarín; Luisa F. García Bermejo; M. Nieves Sánchez García (pp. 66-74).

A novel alternative for the simultaneous determination of compounds with similar structure is described, using the whole chemiluminescence-time profiles, acquired by the stopped-flow technique, in combination with mathematical treatments of multivariate calibration. The proposed method is based on the chemiluminescent oxidation of morphine and naloxone by their reaction with potassium permanganate in an acidic medium, using formaldehyde as co-factor. The whole chemiluminescence-time profiles, acquired using the stopped-flow technique in a continuous-flow system, allowed the use of the time-resolved chemiluminescence (CL) data in combination with multivariate calibration techniques, as partial least squares (PLS), for the quantitative determination of both opiate narcotics in binary mixtures.In order to achieve overcoat the additivity of the CL profiles and beside to obtain CL profiles for each drug the most separated as possible in the time, the optimum chemical conditions for the CL emission were investigated. The effect of common emission enhancers on the CL emission obtained in the oxidation reaction of these compounds in different acidic media was studied. The parameters selected were sulphuric acid 1.0molL⁻¹, permanganate 0.2mmolL⁻¹ and formaldehyde 0.8molL⁻¹. A calibration set of standard samples was designed by combination of a factorial design, with three levels for each factor and a central composite design. Finally, with the aim of validating the chemometric proposed method, a prediction set of binary samples was prepared. Using the multivariate calibration method proposed, the analytes were determined in synthetic samples, obtaining recoveries of 97–109%.

Keywords: Multivariate calibration; Partial least squares; Time-resolved chemiluminescence; Stopped-flow; Morphine; Naloxone

A model of mass transport near the tube wall in a flow-injection manifold by Milivoj Lovrić; Šebojka Komorsky-Lovrić; Heike Kahlert; Fritz Scholz (pp. 75-81).

The time dependence on analyte concentration at the surface of a flow-by potentiometric sensor in the flow-injection analysis was investigated theoretically. The relationships between the properties of response and experimental parameters, such as the sample volume and the distance between the injection point and the sensor, was calculated by finite difference numerical simulation.

Keywords: Flow-injection analysis; Theory; Flow-by potentiometric sensor; Digital simulation

Simultaneous microwave-assisted solid–liquid extraction of polar and nonpolar compounds from alperujo by J.A. Pérez-Serradilla; R. Japón-Luján; M.D. Luque de Castro (pp. 82-88).

Microwave-assisted extraction (MAE) has been used for the simultaneous isolation of polar and nonpolar compounds from alperujo using methanol–water and n-hexane as extractant system. Multivariate methodology has been used to establish the optimum extraction conditions. The target fractions (phenol compounds and fatty acids) were quantitatively extracted within 14min. Following leaching and separation of the two phases by centrifugation, the polar and nonpolar fractions were analysed by HPLC–MS–MS and GC–MS, respectively. The proposed method was compared with the reference method for the isolation of each fraction (Folch method and stirring-based method for fatty acids and biophenols extraction, respectively) in terms of efficiency and extract composition. The paramount importance of both fractions, the simplicity of the MAE approach and the low costs of the raw material make advisable the implementation of the proposed method at an industrial scale.

Keywords: Microwave-assisted extraction; Fatty acids; Polyphenols; Gas chromatography–mass spectrometry; Liquid chromatography–mass spectrometry; Alperujo

Development of method for the speciation of inorganic iron in wine samples by Sergio L.C. Ferreira; Hadla S. Ferreira; Robson M. de Jesus; Joao V.S. Santos; Geovani C. Brandao; Anderson S. Souza (pp. 89-93).

In this paper, we proposed a procedure for the determination of iron(II) and total iron in wine samples employing molecular absorption spectrophotometry. The ligand used is 2-(5-bromo-2-pyridylazo)-5-(diethylamino)-phenol (Br-PADAP) and the chromogenic reaction in absence or presence of ascorbic acid (reducing agent) allows the determination of iron(II) or total iron, respectively. The optimization step was performed using a multivariate technique (Box Behnken design) involving the factors pH, acid ascorbic concentration and reaction time.The method allows the determination of iron(II) and iron(III) in wine samples, with limits of detection and quantification 0.22 and 0.72μgL⁻¹, respectively. The precision expressed as relative standard deviation (R.S.D.) was 1.43 and 0.56% (both, n=11) for content of iron(II) in wine samples of 1.68 and 4.65mgL⁻¹, and 1.66 and 0.87% (both, n=11) for content of total iron in wine samples of 1.72 and 5.48mgL⁻¹.This method was applied for determination of iron(II) and total iron in six different wine samples. In these, the iron(II) content varied from 0.76 to 4.65mgL⁻¹ and from 1.01 to 5.48mgL⁻¹ for total iron. The results obtained in the determination of total iron by Br-PADAP method were compared with those that were performed after complete acid digestion in open system and determination of total iron employing FAAS. The method of regression linear was used for comparison of these results and demonstrated that there is no significant difference between the results obtained with these two procedures.

Keywords: Speciation analysis; Iron(II) determination; Total iron determination; Wine samples; Box Behnken design

A chemiluminescence biochemical oxygen demand measuring method by Hideaki Nakamura; Yuta Abe; Rui Koizumi; Kyota Suzuki; Yotaro Mogi; Takumi Hirayama; Isao Karube (pp. 94-100).

A new chemiluminescence biochemical oxygen demand (BOD_CL) determining method was studied by employing redox reaction between quinone and Baker's yeast. The measurement was carried out by utilizing luminol chemiluminescence (CL) reaction catalyzed by ferricyanide with oxidized quinone of menadione, and Saccharomyces cerevisiae using a batch-type luminometer. In this study, dimethyl sulfoxide was used as a solvent for menadione. After optimization of the measuring conditions, the CL response to hydrogen peroxide in the incubation mixture had a linear response between 0.1 and 100μM H₂O₂ ( r²=0.9999, 8 points, n=3, average of relative standard deviation; R.S.D._av=4.22%). Next, a practical relationship between the BOD_CL response and the glucose glutamic acid concentration was obtained over a range of 11–220mgO₂L⁻¹ (6 points, n=3, R.S.D._av 3.71%) with a detection limit of 5.5mgO₂L⁻¹ when using a reaction mixture and incubating for only 5min. Subsequently, the characterization of this method was studied. First, the BOD_CL responses to 16 pure organic substances were examined. Second, the influences of chloride ions, artificial seawater, and heavy metal ions on the BOD_CL response were investigated. Real sample measurements using river water were performed. Finally, BOD_CL responses were obtained for at least 8 days when the S. cerevisiae suspension was stored at 4°C (response reduction, 69.9%; R.S.D. for 5 testing days, 18.7%). BOD_CL responses after 8 days and 24 days were decreased to 69.9% and 35.8%, respectively, from their original values (R.S.D. for 8 days involving 5 testing days, 18.7%).

Keywords: Biochemical oxygen demand; Rapid measurement; Luminol-ferricyanide chemiluminescence; Oxidized quinone; Dimethyl sulfoxide; Saccharomyces cerevisiae

Optical organophosphate/phosphonate sensor based upon gold nanoparticle functionalized quartz by Janelle D.S. Newman; John M. Roberts; G.J. Blanchard (pp. 101-107).

Organophosphate/phosphonate (OPP) compounds represent a serious concern to public safety as nerve agents and pesticides. This study discusses the design and use of a class-selective optical sensor which may be used as a presumptive test for OPP presence. This sensor is based upon monitoring of the plasmon resonance (PR) of gold nanoparticles (AuNPs) attached to a planar quartz substrate. Following binding the AuNPs are modified using established zirconium-phosphorous gold modification chemistry. The development of the sensor, including the determination of the appropriate quartz-AuNP linking chemistry will be discussed. These sensors were found to exhibit a blue shift in the PR upon exposure to the OPP compounds methylphosphonic acid (MPA) and diethylchlorophosphate (DECP). It was found that the sensor was able to detect DECP at concentrations greater than 5×10⁻⁵M, a value similar to limits of detection reported for other techniques using ultra-violet visible detection.

Keywords: Organophospate detection; UV–vis spectroscopy; Sensor; Nanoparticles

A novel potentiometric biosensor for selectivel-cysteine determination usingl-cysteine-desulfhydrase producing Trichosporon jirovecii yeast cells coupled with sulfide electrode by Saad S.M. Hassan; Ashraf F. El-Baz; Hisham S.M. Abd-Rabboh (pp. 108-113).

Trichosporon jirovecii yeast cells are used for the first time as a source ofl-cysteine desulfhydrase enzyme (EC 4.4.1.1) and incorporated in a biosensor for determiningl-cysteine. The cells are grown under cadmium stress conditions to increase the expression level of the enzyme. The intact cells are immobilized on the membrane of a solid-state Ag₂S electrode to provide a simplel-cysteine responsive biosensor. Upon immersion of the sensor inl-cysteine containing solutions,l-cysteine undergoes enzymatic hydrolysis into pyruvate, ammonia and sulfide ion. The rate of sulfide ion formation is potentiometrically measured as a function ofl-cysteine concentration. Under optimized conditions (phosphate buffer pH 7, temperature 37±1°C and actual weight of immobilized yeast cells 100mg), a linear relationship betweenl-cysteine concentration and the initial rate of sulfide liberation (d E/d t) is obtained. The sensor response covers the concentration range of 0.2–150mgL⁻¹ (1.7–1250μmolL⁻¹)l-cysteine. Validation of the assay method according to the quality control/quality assurance standards (precision, accuracy, between-day variability, within-day reproducibility, range of measurements and lower limit of detection) reveals remarkable performance characteristics of the proposed biosensor. The sensor is satisfactorily utilized for determination ofl-cysteine in some pharmaceutical formulations. The lower limit of detection is ∼1μmolL⁻¹ and the accuracy and precision of the method are 97.5% and ±1.1%, respectively. Structurally similar sulfur containing compounds such as glutathione, cystine, methionine, andd-cysteine do no interfere.

Keywords: Biosensors; Potentiometry; l; -Cysteine; Initial slope method; Pharmaceutical analysis; Sulfide electrode; l; -Cysteine desulfhydrase enzyme; Trichosporon jirovecii; yeast cells

A comparative uncertainty study of the calibration of macrolide antibiotic reference standards using quantitative nuclear magnetic resonance and mass balance methods by Shu-Yu Liu; Chang-Qin Hu (pp. 114-121).

This study introduces the general method of quantitative nuclear magnetic resonance (qNMR) for the calibration of reference standards of macrolide antibiotics. Several qNMR experimental conditions were optimized including delay, which is an important parameter of quantification. Three kinds of macrolide antibiotics were used to validate the accuracy of the qNMR method by comparison with the results obtained by the high performance liquid chromatography (HPLC) method. The purities of five common reference standards of macrolide antibiotics were measured by the¹H qNMR method and the mass balance method, respectively. The analysis results of the two methods were compared. The qNMR is quick and simple to use. In a new medicine research and development process, qNMR provides a new and reliable method for purity analysis of the reference standard.

Keywords: Quantitative nuclear magnetic resonance; Mass balance method; Reference standard; Uncertainty

Conjugated linoleic acid determination in human milk by fast-gas chromatography by Carolina Moltó-Puigmartí; Ana Isabel Castellote; M. Carmen López-Sabater (pp. 122-130).

An efficient direct method for measuring c9, t11- and t10, c12-conjugated linoleic acid (CLA) isomer content in human and rat milk was developed and validated using an RTX-2330 capillary column (40m×0.18mm×0.1μm). In comparison with the commonly used 100m×0.25mm×0.20μm columns, this new type of fast column allowed the separation of FAMEs with the same resolution but in much less time. An additional advantage for biological samples was that only a small volume of sample was needed. Two different procedures were tested in order to select the best methylation of CLA isomers, and the alkali plus acid-catalyzed procedure was selected. The precision results showed relative standard deviations (R.S.D.) of repeatability and reproducibility ranging between 0.10 and 8.71%. The application of this method to human and rat milk samples showed that it was a rapid, simple and reliable method for the analysis of biological samples.

Keywords: Human milk; Rat milk; Fast-gas chromatography; Conjugated linoleic acid; Rumenic acid; Fatty acids

Assessment of single extractions for the determination of mobile forms of metals in highly polluted soils and sediments—Analytical and thermodynamic approaches by Vojtěch Ettler; Martin Mihaljevič; Ondřej Šebek; Tomáš Grygar (pp. 131-140).

Highly polluted forest/tilled soils and stream sediments from a mining and smelting area were subjected to single-extraction procedures to determine the extractable contents of Cd, Cu, Pb, and Zn. The results obtained from four widely used operationally defined single extraction tests were compared: deionised water, 0.01molL⁻¹ CaCl₂, 1molL⁻¹ NH₄NO₃ and 0.005molL⁻¹ DTPA. The analytical data were coupled with measurement of the pH and Eh in extracts, mineralogical investigations and thermodynamic modelling using the PHREEQC-2 code. The changes in the pH of the equilibrated suspensions significantly influenced the metal extractabilities, with higher values in the lower pH regions. Although the DTPA procedure generally extracted the highest amounts of metals, it was found to be unsuitable for highly organic acidic forest soils, where anionic metal–DTPA complexes are assumed to be re-adsorbed on the positively charged surfaces of soil organic matter and oxides. The NH₄NO₃ extraction was also unsuitable due to the high ionic strength (1molL⁻¹), limiting the use of the thermodynamic speciation model and the formation of the Cu(NH₃)²⁺ complex, leading to acidification of the suspension. 0.01molL⁻¹ CaCl₂ can be proposed as the most appropriate extraction medium, suitable for speciation modelling and analytical determinations using ICP-techniques and having an ionic strength similar to that of the soil solution. The metals are present in free ionic forms or chlorocomplexes in the CaCl₂ extracts, preventing their re-adsorption on the positively charged surfaces of soil solids (organic matter, Fe- and Mn-oxides) in acidic and circum-neutral conditions.

Keywords: Metals; Soil; Stream sediment; Extractability; Single extractions; PHREEQC-2

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Analytica Chimica Acta (v.602, #1)