Analytica Chimica Acta (v.434, #2)

A novel technique for the determination of hydroxyl radical is described. The method employed the reaction of hydroxyl radical with dimethyl sulfoxide quantitatively to generate a methyl radical, which then coupled with nitroxide-linked naphthalene and led to a large increase in fluorescence intensity. The fluorescence increment was proportional to the concentration of hydroxyl radicals down to 1×10−7  mol l−1. The proposed method was demonstrated to be capable of detecting hydroxyl radicals generated in Fenton systems. It proved to be a simple and sensitive technique for the determination of hydroxyl radicals.
Keywords: Hydroxyl radical; Nitroxide; Naphthalene; Fluorimetry;

Multivariate evaluation of the fluorescence of aquatic organic matter by Tomas Persson; Margareta Wedborg (179-192).
Analysis of three-dimensional fluorescence spectra has proved to be a valuable tool in the characterisation of fluorescent dissolved organic matter (FDOM). Application of multivariate methods is a development that further enhances the capabilities of fluorescence spectroscopy for qualitative as well as quantitative analysis. In this work, principal component analysis (PCA) and partial least squares regression (PLS) are applied on fully corrected excitation–emission matrices (EEMs). The relation between sampling site and fluorescence properties in a coastal environment is analysed with PCA and PLS. The applicability of fluorescence spectroscopy and multivariate analysis for the evaluation of properties of humic substances (HS) is discussed.
Keywords: Baltic Sea; Coastal environment; Dissolved organic matter; FDOM; Estuaries; Excitation–emission matrix; EEM; Fluorescence spectroscopy; Humic substances; Kattegatt; Chemometrics; Multivariate analysis; PLS; PCA; Partial least squares regression; Principal component analysis; Skagerrak;

Synchronous fluorescence possibilities for the determination of three water-soluble vitamins, thiamine, pyridoxine and riboflavin, in presence of bis-2-ethylhexysulfosuccinate sodium salt (AOT) micelles, were studied. Synchronous fluorescence was chosen due to the good analytical characteristics found. Repeatability, expressed as %R.S.D. was smaller than 1.4, sensitivity calculated as the slope of the straight line was of the order of 1010  l mol−1  cm−1 in all the cases and detection limits were 12 μg l−1 for thiamine, 10 μg l−1 for pyridoxine and 9 μg l−1 for riboflavin. Synchronous fluorescence was applied to carry out the analysis of the three vitamins in pharmaceuticals.
Keywords: Room temperature fluorescence; Synchronous fluorescence; Water-soluble vitamins; AOT;

A simple, rapid, cost-effective, accurate and precise differential laser-induced fluorimetric method for the direct determination of uranium in mineralised rocks at the percentage level is presented, based on the comparison of the fluorescence of the standard with a sample of similar but unknown concentration on the same sample weight basis, by the use of H3PO4–NH4H2PO4 as a fluorescence enhancing reagent. The relative standard deviation of the method was 0.3–0.5% (n=9) for 0.04 and 3.4% U3O8. The proposed technique is suitable for the determination of uranium in samples containing >0.01% uranium. The method has been evaluated using standards, SY-2, SY-3, low grade uranium ore-IAEA reference samples and core samples of diverse matrices. The results are in good agreement with the published data and those obtained by conventional fluorimetry and other methods and are of comparable precision to those obtained by titrimetric assay.
Keywords: Differential technique; Laser-induced fluorimetry; Fluorescence enhancing reagent; Uranium determination; Mineralised rock samples;

Magnesium ions in brackish water or sea water complex with the citrate reagent (CIT) and thus interfere with the indophenol blue spectrophotometric method for ammonia measurement by altering the final pH of the color solution. The complex system, possibly Mg(CIT)+OH⇔Mg(OH)(CIT)2−, shows buffer characteristics and can be identified by acid–base titration. A maximum capacity of ca. 40 μeq pH−1  ml−1 was found at pH 10.2 when [Mg2+] was 0.045 M. To compensate for this capacity, extra alkali reagent should be added to sea water samples for the optimal IPB color reaction. For samples with a wide salinity range, the results still need to be corrected empirically either for pH, [Mg2+] or salinity.
Keywords: Ammonia; Indophenol blue; pH; Buffering capacity;

Tunable ultraviolet visible photoacoustic detection by Tom Autrey; Nancy Foster; Derek Hopkins; John Price (217-222).
A fundamental understanding of the physical and chemical properties that dictate the absorption of pulsed light and subsequent release of heat to generate a transient pressure wave was used to test the concept of a multi-wavelength photoacoustic detector. Analysis of the wave equation predicts that with long-pulse excitation, i.e. microseconds FWHM, absorption by the solvent, and not electrostriction determines the limit of detection. Calculation of the mechanical to electrical conversion efficiencies of piezoelectric transducers shows that absorbed pulse energies of microjoules, typical of pulsed flash lamps, are sufficient to provide measurable photoacoustic signals. A pulsed xenon flash lamp (2 μs FWHM) which emits a broad spectrum over the UV and visible region is used as an excitation source to detect trace quantities of metals in aqueous solution using pulsed photoacoustic detection. In this work, dielectric mirrors were used to detect two analytes, CrO4 2− (monitored at 355 nm) and Co2+ (monitored at 532 nm), simultaneously. This approach increases both the flexibility and selectivity of pulsed photoacoustic methods. We obtained a detection limit of 2.6×10−4 absorbance units per centimeter in aqueous samples. This work shows that pulsed lasers are not a necessity for ultra-sensitive photoacoustic spectroscopy.
Keywords: Photoacoustic; Xenon flashlamp; Chromium; Cobalt; Piezoelectric transducer; Spectroscopy;

Simple and rapid detection method using surface plasmon resonance for dioxins, polychlorinated biphenylx and atrazine by Mifumi Shimomura; Yoko Nomura; Wei Zhang; Masato Sakino; Kyong-Hoon Lee; Kazunori Ikebukuro; Isao Karube (223-230).
Endocrine disrupting chemicals (EDCs) such as dioxins, PCBs and pesticides are very harmful to human. In this work, a rapid detection method for 2,3,7,8-TCDD, PCB, and atrazine is described, which is based upon a competitive immunoassay using surface plasmon resonance (SPR). With this method, it was possible to detect 2,3,7,8-TCDD, PCB, and atrazine with respective detection limits of 0.1, 2.5, and 5 ng ml−1, requiring 15 min for a single sample measurement. To achieve better sensitivities, a sandwich assay was additionally performed. For dioxins and atrazine, response amplification could be observed, enabling better differentiation of small concentration differences among analytes.
Keywords: Endocrine disrupting chemicals; TCDD; PCB; Atrazine; SPR; Immunoassay;

Homogeneously aligned nematic liquid crystals have been tested for the sensing of volatile organic compounds using surface plasmon resonance (SPR). Oriented nematic liquid crystals are birefringent materials. A loss of birefringence of these nematic liquid crystals can be achieved by raising the temperature or by sorption of analyte. At a certain temperature, the so-called clearing point, or at a certain analyte concentration, a phase transition from the nematic to the isotropic state takes place, and the birefringence of the liquid crystal is completely lost. Within this transition, the sensor signal becomes extremely sensitive to small changes in analyte’s concentration. Therefore the set-up can be used for threshold sensing.
Keywords: Surface plasmon resonance; Volatile organic compounds; Nematic liquid crystals; Phase transition; Threshold sensing;

A rapid approach for preparing sol–gel based fiber-optic biosensor with encapsulated pH-sensitive fluorophore-acetylcholinesterase conjugates is developed for the determination of acetylcholine. The sensor consists of a pH-sensitive fluorescent indicator encapsulated with enzyme in a sol–gel network on a glass cap that can be fixed on an optical fiber and then integrated with a flow-through reactor for continuous monitoring. The sol–gel preparation procedure has been modified for maintaining a stable pH value and enzyme activity. Among the nine fluorescent indicators tested, FITC-dextran was the most suitable candidate for biosensor. The optimized ratio of TMOS:HCl:H2O and enzyme-dye:sol solution for the rapid preparation of sol–gel film were 1:3.6×10−5:2 and 3:5, respectively. The 1 mM pH 8.5 Tris–buffer had sufficient buffer capacity for acetylcholine preparation and the substrate concentration must be below 50 mM to obtain a stable pH value. The response of the developed biosensor to acetylcholine was highly reproducible (R.S.D.=3.5%, n=8). A good linearity of acetylcholine in the range from 0.5 to 20 mM was also obtained (R 2=0.98). Furthermore, a 30% inhibition can be achieved within 30 min when 0.54 μM (152 ppb) paraoxon was added into the system. This indicates that the developed biosensor is suitable for the analysis of acetylcholine and organophosphorus neurotoxins.
Keywords: Acetylcholinesterase; Sol–gel; Fiber-optic biosensor; pH-sensitive fluorescent indicator; Paraoxon;

Novel potentiometric membrane sensor for selective determination of aluminum(III) ions by Mohamed B Saleh; Saad S.M Hassan; Ahmed A.Abdel Gaber; Nahed A.Abdel Kream (247-253).
A poly(vinyl chloride) matrix membrane sensor incorporating 7-ethylthio-4-oxa-3-phenyl-2-thioxa-1,2-dihydropyrimido[4,5-d]pyrimidine (ETPTP) ionophore exhibits good potentiometric response for Al3+ over a wide concentration range (10−5 to 10−1  M) with a slope of 19.5 mV per decade. The sensor response is stable for at least 1 month. Good selectivity for Al3+ in comparison with alkali, alkaline earth, transition and heavy metal ions and minimal interference are caused by Hg2+ and Pb2+ which are known to interfere with other aluminum membrane sensors. The ETPTP-based sensor is suitable for use in aqueous solutions of pH 2.25–3.25 and in partially non-aqueous medium with up to 15% (v/v) organic content. The proposed sensor is used for potentiometric titration of HPO4 2− with Al3+ and for direct potentiometry of Al3+ content of some rocks.
Keywords: Aluminum sensor; Potentiometry; PVC membrane; Aluminum rocks assay;

Hydrogen peroxide biosensor based on sol–gel-derived glasses doped with Eastman AQ polymer by Xu Chen; Jingzhong Zhang; Bingquan Wang; Guangjin Cheng; Shaojun Dong (255-260).
A hydrogen peroxide biosensor based on sol–gel-derived glasses doped with poly(ester sulfonic acid) Eastman AQ 55D was constructed. Thionine (TH), as a mediator, was incorporated in this matrix by electrostatic force between TH+ and the negatively charged sulfonic acid group in Eastman AQ polymer. Performance and characteristics of the sensor were evaluated with respect to response time, sensitivity and storage stability. The enzyme electrode has a sensitivity of 11.36 μA mM−1 with a detection limit of 5.0×10−7  M H2O2, and the sensor achieved 95% of the steady state current within 20 s.
Keywords: Sol–gel; Eastman AQ; Amperometric biosensor; Thionine; Horseradish peroxidase;

Flow-injection biamperometry is used for the direct determination of hydroxylamine based on coupling two independent and irreversible electrode processes, the oxidation of hydroxylamine and the reduction of platinum oxide. In operation a potential difference of 0 V is imposed between two platinum wire electrodes pretreated by an anodization step, and the resulting current is measured. Hydroxylamine can be determined in the range of 6×10−7 to 4×10−5  mol l−1 with the detection limit of 1×10−7  mol l−1. Nitrogen-containing compounds such as nitrate, nitrite and ammonia that often accompany hydroxylamine in the process of nitrogen cycle do not cause significant interference. The stability of the detector is shown by a relative standard deviation (R.S.D.) of 1.4% for 50 replicate determinations of 1×10−5  mol l−1 hydroxylamine.
Keywords: Flow-injection analysis; Hydroxylamine; Biamperometry; Pretreated platinum electrode;

Ion mobility spectrometers (IMS) are routinely used by airport personnel to screen for explosive residues on luggage, and law enforcement officials use ion mobility spectrometers to screen samples at crime scenes for drugs of abuse. IMS has several advantages when screening which include high sensitivity, low detection limits, and short analysis times. The Barringer Ionscan® 350, coupled to LabVIEW™ for data acquisition, can collect 50 spectra per second. These features are very important because of the high throughput demanded for screening luggage.Carry-on luggage in airports undergoes screening for explosive residues by swiping a small filter over the surface. The filter is then inserted into the spectrometer and analyzed. Results of the analysis are available in a matter of seconds. The detection method used must be accurate and rapid because of the large number of samples that must be analyzed in short periods. Unfortunately, IMS spectra can contain interferents because of the sample collection method. Rapid temperature programming coupled with chemometrics has been shown as a useful tool for the separation of analytes from interferents.Pentaerythritol tetranitrate (PETN) and cyclotetramethylene tetranitrate (HMX) are nonvolatile explosives. They decompose rapidly upon heating, making traditional analyses by gas chromatography difficult. In this work, PETN and HMX with interfering compounds were analyzed using rapid temperature programming. The Barringer Ionscan® 350, was modified so that the desorber heater could be programmed and the spectra acquired in real-time. Simple-to-use interative self-modeling mixture analysis (SIMPLISMA) was applied to the data to resolve spectral features that vary with respect to temperature.
Keywords: Ion mobility spectrometry; SIMPLISMA; Thermal desorption; PETN; HMX;

Pressurized fluid extraction (PFE) is becoming popular for solid extraction due to its advantages of considerable time and solvent saving. The use of PFE was evaluated for the recovery of six agrochemicals from a Hawaiian soil. Six acid herbicides spiked at a 0.5 mg/kg level in soils were extracted with pressurized and heated acetone (1500 psi, 100°C) and analyzed by gas chromatography–mass spectrometry (GC–MS) in negative chemical ionization (NCI) mode after ex situ derivatization of the analytes. In situ derivatization of the analytes with pentafluorobenzyl bromide (PFBBr) in presence of ethylenediaminetetraacetic acid tetrasodium salt (Na4EDTA) was accomplished as one step in the PFE extraction cell, followed by GC–MS quantification. The effect of the amount of derivatizing reagents, static extraction time and roles of PFBBr and Na4EDTA on the extraction were investigated. Use of Na4EDTA significantly improved the recoveries of the analytes, with results from 69 to 117% (an overall average of 92±16%). The analysis time reduced from several hours (up to 24 h) for the conventional extraction to less than 1 h, and the good and consistent recoveries make this method very attractive for the analysis of acidic herbicides traditionally presenting difficulties for their test at low levels.
Keywords: Pressurized solvent extraction; Na4EDTA; Derivatization; Herbicides;

A combined electrochemical-inductively coupled plasma-mass spectrometry (EC-ICP-MS) method for the determination of trace mercury in water samples is presented. It takes advantage of the electrochemical preconcentration of mercury onto a gold spiral electrode followed by ICP-MS analysis after the electrochemical reoxidation of deposited mercury in pure supporting electrolyte. The advantages of the EC-ICP-MS approach with respect to conventional ICP-MS, are the increased sensitivity and the elimination of the effect of interfering substances eventually present in the sample.EC-ICP-MS is applied to the determination of nanomolar and subnanomolar concentrations of mercury(II) ions in real samples such as process waters (from a chlor-alkali plant) and lagoon waters (from Venice channels). Analytical performances obtained by EC-ICP-MS are discussed and compared with those obtained by anodic stripping voltammetry at gold and at Tosflex-coated glassy carbon electrodes.
Keywords: Mercury; Electrochemical preconcentration; ICP-MS; Process water; Venice lagoon;

Separation of niobium(V) and tantalum(V) as ternary complexes with citrate and metallochromic ligands by capillary electrophoresis by Narumol Vachirapatama; Philip Doble; Zongshou Yu; Miroslav Macka; Paul R Haddad (301-307).
A method was developed for the determination of Nb(V) and Ta(V) as anionic ternary complexes of a metallochromic ligand and citrate as the auxiliary ligand by capillary electrophoresis. Three metallochromic ligands were evaluated, namely 4-(2-pyridylazo) resorcinol (PAR), 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) and 2-(5-bromo-2-pyridylazo)-5-[N-propyl-N-(3-sulfopropyl)amino]-phenol (Br-PAPS). Each complex was detected spectrophotometrically using a light-emitting diode (LED) at 568 nm. The separation of Nb(V) and Ta(V) was optimised for all three ligands, of which PAR displayed the best analytical method performance parameters, with detection limits of 9.1 and 25.2 μg/l for Nb(V) and Ta(V), respectively, using a 20 s injection time and linear ranges of 25–1000 and 50–4000 μg/l for Nb(V) and Ta(V), respectively. The method was applied to the analysis of representative rock samples. Cleanup of the digested rock samples with a C18 pre-column coated with tetrabutylammonium hydroxide prior to analysis was used to remove excess salts which otherwise caused electrophoretic de-stacking. It was found that Nb(V) could be determined satisfactorily at natural levels in the tested reference rock materials (granite and basalt), but Ta(V) was below the detection limit for these samples. The values of Nb(V) found by the proposed method agreed well with the certified values and with results obtained by inductively coupled plasma–mass spectrometry (ICP–MS).
Keywords: Capillary electrophoresis; Metallochromic ligands; Niobium(V); Tantalum(V); Ternary complexes;

Analysis of metal complexes by capillary electrophoresis by Bi-Feng Liu; Liang-Bin Liu; Hong Chen; Jie-Ke Cheng (309-313).
In this paper it was demonstrated that Ru(III), Rh(III) Os(IV) and Co(II) were separated as their 4-(2-thiazolyazo)resorcinol (TAR) complexes by capillary electrophoresis coupled with pre-column derivatization scheme. Some separation parameters such as buffer acidity, buffer concentration, injection time and applied voltage were investigated and optimized. Emphasis was placed on the effect of organic additives introduced into running buffer on the electrophoretic separation. Under the chosen separation conditions, a separation of the four metal complexes was accomplished in 20 min together with the ligand TAR. The detection limits (S/N=2) of Co(II), Ru(III), Rh(III) and Os(IV) were 92, 294, 843 and 510 ng/ml, respectively.
Keywords: Capillary electrophoresis; Metal complexes; Organic additives;

Chiral separation of ephedrines by liquid chromatography using β-cyclodextrins by Rosa Herráez-Hernández; Pilar Campı́ns-Falcó (315-324).
β-cyclodextrins (β-CDs) have been evaluated as chiral recognition agents for the enantioseparation of ephedrines by liquid chromatography (LC). The compounds tested were norephedrine, ephedrine, pseudoephedrine, N-methylephedrine and N-methylpseuoephedrine. Two possibilities have been investigated, the employment of a chiral stationary phase consisting of immobilized β-cyclodextrin, and the addition of the chiral selector to the mobile phase. In the latter approach the following chiral agents have been assayed, native β-cyclodextrin (β-CD), methyl-β-cyclodextrin (M-β-CD), carboxyethyl-β-cyclodextrin (CE-β-CD), carboxymethyl-β-cyclodextrin (CM-β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD). The experimental factors affecting the chiral separations have been examined, including the percentage of organic solvent, the ionic strength and pH of the buffer, the type and concentration of chiral selector, and the mobile-phase flow rate. Differences in enantioselectivity displayed by the different methods are discussed.
Keywords: Liquid chromatography; β-cyclodextrins; Solvent; Chiral analysis; Ephedrines;

Cloud point extraction has been used for the preconcentration of cobalt and nickel, after the formation of a complex with 1-(2-thiazolylazo)-2-naphthol (TAN), and later analysis by flame atomic absorption spectrometry using octylphenoxypolyethoxyethanol (Triton X-114) as surfactant. The chemical variables affecting the separation phase and the viscosity affecting the detection process were optimized. Under the optimum conditions, preconcentration of only 50 ml of sample in the presence of 0.05% Triton X-114 permitted the detection of down to 0.24 μg l−1 cobalt and 0.44 μg l−1 nickel. The enhancement factors were 57 for cobalt and 65 for nickel. The proposed method has been applied to the determination of cobalt and nickel in water samples.
Keywords: Cobalt; Nickel; Cloud point extraction; Flame atomic absorption spectrometry;

The use of visible spectrometry, applied to the chromatic characterization of Roman wall paintings, allows easy and trustworthy grouping of the samples studied. The use of other spectroscopic techniques as Fourier transform infrared (FTIR) and energy dispersive X-ray fluorescence (EDS) in conjunction with X-ray diffraction (XRD) allows good identification of the substances present in the pictorial layers that define and differentiate each chromatic group. In this paper, a study of 21 Roman wall painting samples, from Cripta del Museo and Alcazaba (Merida, Badajoz, Spain), is described.
Keywords: Pigments; Roman paintings; Chromatic measurements (CIE64); Energy dispersive X-ray fluorescence (EDS); Fourier transform infrared (FTIR); X-ray diffraction (XRD);

AUTHOR INDEX (347-349).