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Analytica Chimica Acta (v.567, #2)

Editorial Board (pp. co1).

CONTENTS (pp. iii-iv).

Calendar of forthcoming meetings (pp. n3).

Directly light scattering imaging of the aggregations of biopolymer bound chromium(III) hydrolytic oligomers in aqueous phase and liquid/liquid interface by Jian Ling; Cheng Zhi Huang; Yuan Fang Li (pp. 143-151).

Investigations of inorganic oligomers are important in both chemistry and physiology. In this contribution, we propose a laser induced light scattering imaging (LSI) and a total internal reflected light scattering imaging (TIR-LSI) technique, and apply them to characterize the interactions of inorganic oligomers with biopolymer in aqueous phase and at liquid/liquid interface, respectively. In aqueous medium, synthetic chromium(III) hydrolytic oligomers (CrHO) react with DNA, and the resultant binary could be extracted into the H₂O/CCl₄ interface in the presence of triocyctyl phosphine oxide (TOPO), forming a DNA–CrHO–TOPO ternary amphipathic complex at the interface with the associate constant of 1.32×10³mol⁻¹dm⁴ for a given 1.0×10⁻⁴moll⁻¹ TOPO. Under the excitation of a 441-nm He–Cd laser light beam, the resultant light scattering and total internal reflected light scattering (TIR-LS) signals of the formed binary in aqueous phase and ternary at liquid/liquid interface could be easily captured using a common microscope coupled with a CCD camera. By digitally analyzing the CCD captures, we demonstrate that aggregations of the CrHO–DNA binary in aqueous phase and DNA–CrHO–TOPO ternary at liquid/liquid interface have occurred, respectively.

Keywords: Light scattering image (LSI) technique; Total internal reflection (TIR); Chromium(III) hydrolytic oligomers (CrHO); DNA

Development of a new analytical approach based on cellulose membrane and chelator for differentiation of labile and inert metal species in aquatic systems by André Henrique Rosa; Iramaia C. Bellin; Danielle Goveia; Luciana C. Oliveira; Roberto W. Lourenço; Newton L. Dias Filho; Peter Burba (pp. 152-159).

A new procedure was developed in this study, based on a system equipped with a cellulose membrane and a tetraethylenepentamine hexaacetate chelator (MD–TEPHA) for in situ characterization of the lability of metal species in aquatic systems. To this end, the DM–TEPHA system was prepared by adding TEPHA chelator to cellulose bags pre-purified with 1.0molL⁻¹ of HCl and NaOH solutions. After the MD–TEPHA system was sealed, it was examined in the laboratory to evaluate the influence of complexation time (0–24h), pH (3.0, 4.0, 5.0, 6.0 and 7.0), metal ions (Cu, Cd, Fe, Mn and Ni) and concentration of organic matter (15, 30 and 60mgL⁻¹) on the relative lability of metal species by TEPHA chelator. The results showed that Fe and Cu metals were complexed more slowly by TEPHA chelator in the MD–TEPHA system than were Cd, Ni and Mn in all pH used. It was also found that the pH strongly influences the process of metal complexation by the MD–TEPHA system. At all the pH levels, Cd, Mn and Ni showed greater complexation with TEPHA chelator (recovery of about 95–75%) than did Cu and Fe metals. Time also affects the lability of metal species complexed by aquatic humic substances (AHS); while Cd, Ni and Mn showed a faster kinetics, reaching equilibrium after about 100min, and Cu and Fe approached equilibrium after 400min. Increasing the AHS concentration decreases the lability of metal species by shifting the equilibrium to AHS–metal complexes. Our results indicate that the system under study offers an interesting alternative that can be applied to in situ experiments for differentiation of labile and inert metal species in aquatic systems.

Keywords: Water; Natural organic matter; Metals; Lability; Humic substances

Determination of methylmercury and estimation of total mercury in seafood using high performance liquid chromatography (HPLC) and inductively coupled plasma-mass spectrometry (ICP-MS): Method development and validation by Susan C. Hight; John Cheng (pp. 160-172).

A method was developed for determination of methylmercury and estimation of total mercury in seafood. Mercury (Hg) compounds were extracted from 0.5g edible seafood or 0.2g lyophilized reference material by adding 50ml aqueous 1% w/vl-cysteine·HCl·H₂O and heating 120min at 60°C in glass vials. Hg compounds in 50μl of filtered extract were separated by reversed-phase high performance liquid chromatography using a C-18 column and aqueous 0.1% w/vl-cysteine·HCl·H₂O+0.1% w/vl-cysteine mobile phase at room temperature and were detected by inductively coupled plasma-mass spectrometry at mass-to-charge ratio 202. Total Hg was calculated as the mathematical sum of methyl and inorganic Hg determined in extracts. For seafoods containing 0.055–2.78mgkg⁻¹ methylmercury and 0.014–0.137mgkg⁻¹ inorganic Hg, precision of analyses was ≤5% relative standard deviation (R.S.D.) for methylmercury and ≤9% R.S.D. for inorganic Hg. Recovery of added analyte was 94% for methylmercury and 98% for inorganic Hg. Methyl and total Hg results for reference materials agreed with certified values. Limits of quantitation were 0.007mgkg⁻¹ methylmercury and 0.005mgkg⁻¹ inorganic Hg in edible seafood and 0.017mgkg⁻¹ methylmercury and 0.012mgkg⁻¹ inorganic Hg in lyophilized reference materials. Evaluation of analyte stability demonstrated thatl-cysteine both stabilized and de-alkylated methylmercury, depending on holding time and cysteine concentration. Polypropylene adversely affected methylmercury stability. Total Hg results determined by this method were equivalent to results determined independently by cold vapour-atomic absorption spectrometry. Methylmercury was the predominant form of Hg in finfish. Ratios of methylmercury/total Hg determined by this method were 93–98% for finfish and 38–48% for mollusks.

Keywords: Methylmercury; Total mercury; Seafood; Inductively coupled plasma-mass spectrometry; High performance liquid chromatography

Improving the performance of immobilized molecular beacons through cleavage by Xinbing Zuo; Xiaohai Yang; Kemin Wang; Weihong Tan; Huimin Li; Leiji Zhou; Jianhui Wen; He Zhang (pp. 173-178).

Surface-immobilized molecular beacons (MBs) usually have relatively low fluorescence enhancement after hybridization, which causes significant limitations to their applications. Here we designed a molecular beacon with a restriction endonuclease recognition site at its loop structure, to study the behavior of the MB on solid matrix and to improve its performance. The maximum fluorescence enhancement of the MB caused by restricting cleavage after hybridization was 5.2, close to its corresponding enhancement 5.9 in solution, higher than its corresponding enhancement 2.7 that just caused by hybridization. This result showed that some of the MB molecules were still closed even though hybridization with target cDNA, this maybe due in part to the surface effects or the MB's conformation on solid matrix. Moreover, the cleavage induce fluorescence enhancement have been used to decrease the limit of detection from 10 to 2nM cDNA.

Keywords: Molecular beacon; Cleavage; Immobilization

A surface plasmon resonance biosensor assay for the simultaneous determination of thiamphenicol, florefenicol, florefenicol amine and chloramphenicol residues in shrimps by V. Dumont; A.-C. Huet; I. Traynor; C. Elliott; P. Delahaut (pp. 179-183).

A rapid and sensitive screening qualitative method using a surface plasmon resonance (SPR) biosensor was developed which can detect of all fenicol antibiotic residues in shrimps from a single sample extract. This method requires ethyl acetate extraction followed by a single wash with isooctane/chloroform. Each sample extract is injected over the surfaces of two biosensor chip flow cells, one surface having the capability to detect florefenicol amine (FF amine), florefenicol (FF), and thiamphenicol (TAP) and the second surface for chloramphenicol (CAP) detection. The estimated detection capabilities (CCβ) were 0.1, 0.2, 250, and 0.5ppb for CAP, FF, FF amine, and TAP, respectively. This quick, simple test allowed the detection of CAP residues in shrimps at the minimum required performance limit (MRPL) of 0.1μgkg⁻¹ for this compound and of FF, FF amine, and TAP below their maximum residue limits (MRLs).

Keywords: Antibiotic; Chloramphenicol; Fenicol; SPR biosensor; Food analysis; Shrimp

Design and evaluation of a thorium (IV) selective optode by Afsaneh Safavi; Marzieh Sadeghi (pp. 184-188).

A novel optical sensor has been proposed for sensitive determination of thorium (IV) ion in aqueous solutions. The thorium sensing membrane was prepared by incorporating 4-( p-nitrophenyl azo)–pyrocatechol (NAP) as ionophore in the plasticized PVC membrane containing tributyl phosphate (TBP) as plasticizer. The membrane responds to thorium ion by changing color reversibly from yellow to red-brown in glycine buffer solution at pH 3.5. The proposed sensor displays a linear range of 8.66×10⁻⁶–2.00×10⁻⁴M with a limit of detection of 6×10⁻⁶M. The response time of the optode was about 8.8–12.5min, depending on the concentration of Th (IV) ions. The selectivity of optode to Th (IV) ions in glycine buffer is good. The sensor can readily be regenerated by exposure to a solution mixture of sodium fluoride and 5-sulfosalicylic acid (dihydrate) (0.01M each). The optode is fully reversible. The proposed optode was applied to the determination of thorium (IV) in environmental water samples.

Keywords: Optode; Thorium; 4-(; p; -Nitrophenyl azo)–pyrocatechol

A highly selective fluorescent sensor for Cu²⁺ based on 2-(2′-hydroxyphenyl)benzoxazole in a poly(vinyl chloride) matrix by Xiao-Bing Zhang; Jing Peng; Chun-Lian He; Guo-Li Shen; Ru-Qin Yu (pp. 189-195).

This paper describes a copper selective optical chemical sensor based on static quenching of the fluorescence of 2-(2′-hydroxyphenyl)benzoxazole entrapped in a poly(vinyl chloride) (PVC) membrane. The effect of the composition of the sensing membrane was studied, and experimental conditions were optimized. The sensors exhibit stable response over the concentration range from 4.0×10⁻⁸M to 5.0×10⁻⁵M Cu²⁺ at pH 4.0–6.5, and a high selectivity. The response time for Cu²⁺ with concentration ≤5×10⁻⁶M is less than 7min. The optode can be regenerated using 0.1M HCl and acetate buffer solution. The sensor has been used for direct measurement of copper content in river water samples with a relative error less than 4% with reference to that obtained by atomic absorption spectrometry.

Keywords: Intramolecular proton transfer; Fluorescent chemical sensor; Cu(II) ion; Benzoxazole derivative

Highly selective and sensitive monohydrogen phosphate membrane sensor based on molybdenum acetylacetonate by Mohammad Reza Ganjali; Parviz Norouzi; Mahnaz Ghomi; Masoud Salavati-Niasari (pp. 196-201).

In this work, a highly selective and sensitive monohydrogen phosphate membrane sensor based on a molybdenum bis(2-hydroxyanil) acetylacetonate complex (MAA) is reported. The sensor shows a linear dynamic range between 1.0×10⁻¹ and 1.0×10⁻⁷M, with a nice Nernstian behavior (−29.5±0.3mVdecade⁻¹) in pH of 8.2. The detection limit of the electrode is 6.0×10⁻⁸M (∼6ppb). The best performance was obtained with a membrane composition of 32% poly(vinyl chloride), 58% benzyl acetate, 2% hexadecyltrimethylammonium bromide and 8% MAA. The sensor possesses the advantages of short response time, low detection limit and especially, very good selectivity towards a large number of organic and inorganic anions including salicylate, citrate, tartarate, acetate, oxalate, fluoride, chloride, bromide, iodide, sulfite, sulfate, nitrate, nitrite, cyanide, thiocyanate, perchlorate, metavanadate, and bicarbonate ions. The electrode can be used for at least 10 weeks without any considerable divergence in its slope and detection limit. It was used as an indicator electrode in potentiometric titration of monohydrogenphosphate ion with barium chloride. The proposed sensor was successfully applied to direct determination of monohydrogenphosphate in two fertilizer samples (NPK).

Keywords: Monohydrogenphosphate; Potentiometry; HPO; ₄; ²⁻; Ion-selective electrode; Molybdenum acetylacetonate

Geographical origin and authentication of extra virgin olive oils by an electronic nose in combination with artificial neural networks by Maria S. Cosio; Davide Ballabio; Simona Benedetti; Carmelina Gigliotti (pp. 202-210).

An electronic nose and an electronic tongue, in combination with multivariate analysis, have been used to verify the geographical origin and the uniqueness of specific extra virgin olive oils. The olive oil samples belong to a small production, located in the lake of Garda (north of Italy) and distinguished with a European Protected Denomination of Origin trademark since 1998. In order to obtain a complete description of oil samples, free acidity, peroxide value, ultraviolet indices, and phenol content have been also determined. The dataset includes 36 Garda oils and 17 oils from other regions. Two classification models have been built by means of Counterpropagation Artificial Neural Networks in order to separate Garda and not-Garda oils, as follows: first, by using all the chemical variables and sensor signals; second, by using electronic tongue sensors; finally, by using four selected electronic nose sensors. All the models have been also tested with 19 commercial olive oil samples. Neural networks have provided very satisfactory results and have indicated the electronic nose as the most appropriate tool for the characterization of the analyzed oils. These results have suggested how electronic nose, in combination with neural networks, could represent a fast, cheap and functional method to classify and describe extra virgin olive oils from a circumscribed geographical area.

Keywords: Extra virgin olive oil; Electronic nose; Uniqueness; Neural networks

Preparation of anilinemethyltriethoxysilane/polydimethylsiloxane sol–gel coatings for solid-phase microextraction of aromatic compounds by Yu-ling Hu; Yue-li Fu; Gong-Ke Li (pp. 211-217).

Anilinemethyltriethoxysilane (AMTEOS) was first used as precursor as well as selective stationary phase to prepare the sol–gel derived anilinemethyltriethoxysilane/polydimethylsiloxane (AMTEOS/PDMS) solid-phase microextraction (SPME) fibers. The novel SPME fiber exhibits high extraction efficiency, good thermal stability and long lifetime compared with commercial SPME coatings. In addition, the phenyl groups in the porous layer can exhibit π–π interactions with aromatic compounds, such as monocyclic aromatic hydrocarbons (MAHs) and polycyclic aromatic hydrocarbons (PAHs). Therefore, SPME using the AMTEOS/PDMS sol–gel fiber coupled with GC-FID was recommended as a sensitive and selective method towards the analysis of these compounds in environmental water samples. The optimal extraction conditions were investigated by adjusting extraction time, salt addition, extraction temperature, and desorption time. The method showed linearity between 2 and 4000μgl⁻¹ for MAHs and 1 and 1000μgl⁻¹ for PAHs. The limit of detection (LOD) was 0.6–3.8μgl⁻¹for MAHs and 0.2–1.5μgl⁻¹ for PAHs. The novel AMTEOS/PDMS fiber was applied to extract small amount of aromatic compounds in wastewater and river water respectively. The recovery of the method was acceptable for quantitative analysis.

Keywords: Anilinemethyltriethoxysilane; Solid-phase microextraction; Sol–gel; Aromatic compounds

Qualitative and quantitative determination of nucleosides, bases and their analogues in natural and cultured Cordyceps by pressurized liquid extraction and high performance liquid chromatography–electrospray ionization tandem mass spectrometry (HPLC–ESI–MS/MS) by H. Fan; S.P. Li; J.J. Xiang; C.M. Lai; F.Q. Yang; J.L. Gao; Y.T. Wang (pp. 218-228).

A simple and rapid pressurized liquid extraction (PLE) and high performance liquid chromatography–electrospray ionization tandem mass spectrometry (HPLC–ESI–MS/MS) method was developed for qualitative and quantitative determination of nucleosides, bases and their analogues in natural and cultured Cordyceps. The samples were extracted using PLE. The separation was achieved on a ZORBAX Eclipse XDB-C₁₈ column with gradient elution of methanol and 5mM aqueous ammonium acetate as mobile phase. Target compounds were identified by characterizing their product ions, precursor ions and retention times. Quantitative analysis of investigated compounds were performed using time programmed selective ion monitoring (SIM) or selective reaction monitoring (SRM) with 10 segments in positive (negative for uridine) ion mode. The results showed that 43 bases, nucleosides and their analogues were detected in Cordyceps, of these 16 compounds were identified. The simultaneous determination of seven nucleosides and six bases in Cordyceps was achieved using PLE and HPLC–ESI–MS/MS method described above, which afforded good linearity, selectivity, precision, recovery, short analysis time as well as LOD and LOQ in the ng/ml range.

Keywords: Cordyceps; Nucleosides; Bases; Pressurized liquid extraction; HPLC–ESI–MS/MS; Qualitative and quantitative determination

Determination of flumequine and oxolinic acid in sediments and soils by microwave-assisted extraction and liquid chromatography-fluorescence by M.D. Prat; D. Ramil; R. Compañó; J.A. Hernández-Arteseros; M. Granados (pp. 229-235).

A method is reported for the determination of the quinolones oxolinic acid and flumequine in aquatic sediments and agricultural soils. The analytes are extracted by liquid–liquid partitioning between a sample homogenated in an aqueous buffer solution and dichloromethane. Microwave-assisted extraction (MAE) was tested to improve the speed and efficiency of the extraction process. The parameters affecting the efficiency of MAE, such as irradiation time and temperature, were studied. The clean-up consists of back-extraction in 1M sodium hydroxide. The determination is carried out by reversed phase liquid chromatography on an octyl silica-based column and fluorimetric detection.The optimised method was applied to the analysis of two sediments and one agricultural soil spiked with the analytes. The absolute recovery rates for the whole process range from 79% to 94% (RSD 3–7%), and detection limits are in the low μgkg⁻¹ level.

Keywords: Quinolones; Sediment analysis; Soil analysis; Microwave-assisted extraction; Antibacterial agents; Environment

A mixed hard- and soft-modelling approach for the quantitative determination of oxipurines and uric acid in human urine by José Manuel Amigo; Anna de Juan; Jordi Coello; Santiago Maspoch (pp. 236-244).

A new treatment of kinetic-enzymatic data, joining the benefits of hard- and soft-modelling methods, is proposed for the kinetic resolution and simultaneous quantification of xanthine oxidase induced oxipurines (hypoxanthine and xanthine) and uric acid in human urine without any further separation.These three analytes show highly overlapped spectral bands in the range of 232–320nm and urine contains absorbing species in this wavelength range that behave as a constant interference throughout the monitoring of the kinetic experiment. The proposed method allows the determination of the analytes in human urine by simultaneous analysis of the urine sample and a few (even only one) standards of pure analyte mixtures of hypoxanthine and xanthine in aqueous solution.An iterative mixed hard- and soft-modelling multivariate curve resolution (HS-MCR) algorithm, which includes a hard-modelling constraint based on the enzymatic model in the parent multivariate curve resolution-alternating least squares method (MCR-ALS), is applied to a data set formed by the standards and the urine samples. Hard-modelling is applied to the concentration profiles of the three analytes in the standards, whereas soft-modelling on the analytes and the urine in the samples gives the quantitative information. As a result, quantitative information similar in quality to that obtained by separation techniques (HPLC and HPCE) and other chemometric approaches (PLS and N-PLS) is obtained with less experimental effort and a much smaller number of standards in aqueous solution that do not need to contain the interferences present in the samples.

Keywords: Enzymatic data analysis; Oxipurines; Human urine; Hard-modelling; Soft-modelling; Multivariate curve resolution

A mixed hard- and soft-modelling approach to study and monitor enzymatic systems in biological fluids by José Manuel Amigo; Anna de Juan; Jordi Coello; Santiago Maspoch (pp. 245-254).

Mixed hard- and soft-modelling multivariate curve resolution (HS-MCR) is applied to study and to monitor complex enzymatic systems. Working under the basis of the soft-modelling technique multivariate curve resolution-alternating least squares (MCR-ALS), a hard constraint is introduced to force some or all concentration profiles to fulfil an enzymatic model. In this way, improvements to the application of pure hard- or pure soft-modelling are achieved.The enzymatic reactions of different mixtures of hypoxanthine, xanthine and uric acid with xanthine oxidase are studied. This is a complex enzymatic process, where uric acid acts as a linear competitive inhibitor. The reactions were monitored with UV–vis spectrophotometry coupled to a stopped-flow module.This work has two aims, both of them focusing on different aspects linked to modelling enzymatic systems using HS-MCR. The first goal is related to the elucidation of the real enzymatic mechanism when one of the chemical substances involved in the process apparently deviates from the mechanism found in the literature. The second one focuses on modelling the enzymatic reaction in the presence of a biological interference, such as human urine. The elucidation of the real mechanism of this enzymatic process and of the behaviour of the involved chemical species in a natural absorbing medium are good examples of situations that can benefit from mixed modelling approaches involving the best of hard- and soft-modelling methodologies.

Keywords: Enzymatic analysis; Xanthine oxidase; Hard-modelling; Soft-modelling; Curve resolution

Direct determination of Mancozeb by photoacoustic spectrometry by Sergio Armenta; Javier Moros; Salvador Garrigues; Miguel de la Guardia (pp. 255-261).

A solvent free, fast and environmentally friendly photoacoustic-infrared-based methodology (PAS-FTIR) was developed for the determination of Mancozeb in agrochemicals. This methodology was based on the direct measurement of the transmittance spectra of solid samples and a multivariate calibration model to determine the active ingredient concentration. The proposed partial least squares (PLS) model was made using nine standards prepared by mixing different amounts of kaolin and Mancozeb, with concentrations between 5.43 and 88.10% (w/w).A hierarchical cluster analysis was made in order to classify the samples in terms of similarity in the PAS-FTIR spectra. From their spectra different commercially available fungicide samples were classified in four groups, attending to the presence of other active ingredients co-formulated with Mancozeb. Different PLS models were applied for the analysis of each group of samples.So, for samples containing copper oxychloride (group 1), the information in the spectral range from 1543 to 1474 and 1390 to 1269cm⁻¹ was employed. For samples co-formulated with Fosetyl-Al (group 2) the range between 3334 and 3211cm⁻¹, corrected with a single point baseline located at 3055cm⁻¹, was used. For samples containing Metalaxyl (group 3) it was used the information in the spectral range from 1543 to 1474cm⁻¹ was used to determine Mancozeb. Finally, the range between 1456 and 1306cm⁻¹ was used for Mancozeb determination in samples containing Cymoxanil (group 4).The PLS factors used for Mancozeb determination depends on the PLS model employed. 3, 2, 2 and 3 factors were used for Mancozeb determination in commercially available pesticides for groups 1, 2, 3 and 4, respectively. The mean accuracy errors found were 3.1, 2.1, 2.5 and 3.0% for groups 1, 2, 3 and 4, respectively. The developed PAS-FTIR methodology does not consume any solvent, as no sample preparation is necessary it improves the laboratory efficiency without sacrifice the accuracy and avoids the contact of the operator with toxic substances.

Keywords: Mancozeb; Photoacoustic infrared spectrometry; Partial least squares calibration; Pesticide formulations

Near infrared spectroscopy in the study of polymorphic transformations by Marcel Blanco; Manel Alcalá; Josep M. González; Ester Torras (pp. 262-268).

The potential of near infrared (NIR) spectroscopy for the characterization of polymorphs in the active principle of a commercial formulation prior to and after the manufacturing process was assessed. Polymorphism in active principles is extremely significant to the pharmaceutical industry. Polymorphic changes during the production of commercial pharmaceutical formulations can alter some properties of the resulting end-products. Multivariate curve resolution–alternating least squares (MCR–ALS) methodology was used to obtain the “pure�? NIR spectrum for the active principle without the need to pretreat samples. This methodology exposed the polymorphic transformation of Dexketoprofen Trometamol (DKP) in both laboratory and production samples obtained by wet granulation. No polymorphic transformation, however, was observed in samples obtained by direct compaction. These results were confirmed using by X-ray powder diffractometry (XRD) and differential scanning calorimetry (DSC) measurements. Pure crystalline polymorphs of DKP were available in the laboratory but amorphous form was not, nevertheless the developed methodology allows the identification of amorphous and crystal forms in spite of the lack of pure DKP.

Keywords: Near-infrared spectroscopy; X-ray powder diffractometry; Differential scanning calorimetry; Drugs; Polymorphism; Granulation; MCR–ALS

Evaluation of certified reference materials for oxidation–reduction titration by precise coulometric titration and volumetric analysis by Toshiaki Asakai; Yoshihiro Kakihara; Yasuharu Kozuka; Sadao Hossaka; Mariko Murayama; Tatsuhiko Tanaka (pp. 269-276).

The accuracy and uncertainty of coulometric titration of Japanese certified reference materials (CRMs) for oxidation–reduction titration were examined in this study. The results for potassium dichromate, sodium oxalate, and potassium iodate are presented. Potassium dichromate was directly determined by coulometric titration, and sodium oxalate and potassium iodate were determined by volumetric analysis using potassium dichromate assigned by coulometry.The uncertainty of the method was investigated by examining the dependency on the sample size and on the electrolysis current. Changes in the titration parameters did not result in any significant effects on the titration results. The coulometric system used primarily consists of a constant current source, timer, switching circuit, indicator unit, and voltmeter. They were controlled using the coulometry software by a PC/AT compatible computer. A highly automated coulometric system achieves repeatabilities of less than one part in 30,000 ( k=2) and uncertainties of less than one part in 15,000 ( k=2). In addition, using volumetric method, SI units traceable sodium oxalate and potassium iodate (purity standards for redox reaction) CRMs were developed.Reference materials for volumetric analysis are the most basic substances used in analytical chemistry. These materials are analyzed by several analytical methods and are produced globally; however, their purities have not been compared at the international level. Therefore, the relationship between the purity and reliability of these materials has not yet been established. In this paper, we determine the relationship between these parameters by titrating each material obtained from different laboratories.

Keywords: Coulometry (constant-current); Primary standards; Oxidation–reduction; Potassium dichromate; Sodium oxalate; Potassium iodate

Isolation and quantification of⁵⁵Fe and⁶³Ni in reactor effluents using extraction chromatography and liquid scintillation analysis by P.E. Warwick; I.W. Croudace (pp. 277-285).

A novel extraction chromatographic resin based on di isobutyl ketone has been developed for the isolation of⁵⁵Fe. This material has been used in conjunction with a dimethylglyoxime-based resin for the sequential separation of⁵⁵Fe and⁶³Ni and isolation of these radionuclides from associated contaminants. The preparation and characterisation of these resins is described along with an assessment of their performance for the isolation of⁵⁵Fe and⁶³Ni from a mixture of nuclear activation products. In addition, a refined approach for the measurement of these nuclides by liquid scintillation counting is discussed. The findings of these studies have been combined to develop a robust procedure for the analysis of⁵⁵Fe and⁶³Ni in reactor effluents which is now the preferred method for a number of UK analytical laboratories.

Keywords: Iron-55; Nickel-63; Extraction chromatography; Dimethylglyoxime; Di; iso; butylketone; Reactor effluents

Stopped-flow determination of 2-mercaptopyrimidines as inductors of the iodine-azide reaction by Jan Kurzawa; Agnieszka Wiśniewska; Krystyna Janowicz (pp. 286-292).

A stopped-flow method for the determination of 2-mercaptopyrimidine and its 14 derivatives by the iodine-azide has been elaborated. Spectrophotometric detection was applied and the decrease in the absorbance of the iodine–starch complex was monitored at 595nm within 5s. The effect of the concentration of the reagents on the rate of the reaction was investigated and a kinetic method for determination of compounds is proposed. 2-Mercaptopyrimidines can be determined at 10⁻⁶ to 10⁻⁵molL⁻¹ level and the relative standard deviation is below 1%. The elaborated method has been applied to the determination of 6-methyl- and 6-propyl-2-thiouracil in drugs.

Keywords: 2-Mecaptopyrimidines determination; 2-Thiouracils; Stopped-flow; Iodine-azide

Author Index (pp. 293-295).

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Analytica Chimica Acta (v.567, #2)