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Analytica Chimica Acta (v.546, #1)

Outside front cover - Editorial Board (pp. co1).

Contents (pp. v-vi).

Calendar of forthcoming meetings (pp. n1-n2).

The potential of flow-through microdialysis for probing low-molecular weight organic anions in rhizosphere soil solution by Michael Sulyok; Manuel Mir�; Gerhard Stingeder; Gunda Koellensperger (pp. 1-10).

In this paper, flow-through microdialysis is presented as a novel analytical tool for automatic sampling of low molecular weight organic anions (LMWOA), such as oxalate and citrate, in solid samples of environmental concern. The microsampling methodology involves the implantation of dedicated capillary-type probes offering unrivalled spatial resolution (ca. 200μm) in definite soil sites. These passive samplers are aimed at monitoring local processes, such as the release of organic acids occurring in the rhizosphere environment, in nearly real-time.The influence of chemical and physical variables (composition and flow rate of the perfusion liquid, ionic strength and pH of the outer medium and presence of metal ions therein) was assessed in vitro using liquid-phase assays. On the other hand, the resistance of the external solid medium to mass transfer, and the actual applicability of in vivo calibration methods were investigated using quartz sand as an inert model soil. Microdialysers furnished with 3cm long semipermeable tubular membranes were perfused with 0.01M NaNO₃ at a flow rate of 2.0μl/min, yielding dialysis recoveries ≥45% for both assayed LMWOAs in simulated background soil electrolyte solutions, and ≥24% in the interstitial liquid of complex solid samples.Full knowledge of the fate of LMWOAs in soils was obtained through the application of stimulus-response approaches that mimic the discrete exudation pulses of roots. Highly time-resolved microdialysates were used to discern readily available species such as free carboxylic anions and LMW metal–organic acid complexes from adsorbed, precipitated or mineralised analyte species in a variety of soil samples containing variable amounts of organic matter, exchangeable cations and different levels of metal pollution.

Keywords: Microdialysis; Aliphatic organic acids; Soil solution; In situ analysis

Characterization of the aroma profile of Madeira wine by sorptive extraction techniques by R.F. Alves; A.M.D. Nascimento; J.M.F. Nogueira (pp. 11-21).

The characterization of the aroma profile of 33 samples of Madeira wine from five monovarieties ( Sercial, Verdelho, Boal, Malvasia and Tinta Negra Mole) having different type and categories is presented, using solid phase microextraction and stir bar sorptive extraction techniques (SPME and SBSE) followed by capillary gas chromatography and mass spectrometry detection (GC–MS).Headspace SPME/GC–MS provided effectiveness to identify the major constituents of the aroma profile of Madeira wine, where no remarkable differences occur among the samples studied. The volatile compounds are mainly constituted by ethyl octanoate (11.3–256.9μgL⁻¹), ethyl decanoate (21.5–210.5μgL⁻¹), ethyl decenoate (0.1–112.8μgL⁻¹), diethyl succinate (0.9–65.6μgL⁻¹), ethyl dodecanoate (1.2–6.5μgL⁻¹), ethyl nonanoate (0.6–5.2μgL⁻¹), ethyl hexanoate (0.2–3.7μgL⁻¹) and isoamyl octanoate (0–2.2μgL⁻¹). C₁₃ norisoprenoids such as vitispirane (0.9–7.0μgL⁻¹) and 1,1,6-trimethyl 1,2-dihydro naphthalene (0.7–12.5μgL⁻¹), as well as phenyl ethanol (0–8.1μgL⁻¹), were also found in Madeira wine samples.The powerful capabilities of SBSE followed thermal desorption and GC–MS analysis allowed higher ability for profiling traces and ultra traces of compounds in Madeira wine samples, including esters (80.7–89.7%), carboxylic acids (1.6–4.2%), alcohols (3.5–8.2%), aldehydes (0.9–3.7%), pyrans (0.2–1.7%), lactones (<3%), monoterpenes (0.1–1.4%), sesquiterpenes (0.1–0.8%) and C₁₃ norisoprenoids (1.7–6.5%), which some of them play a remarkable impact on the aroma complexity. C₁₃ norisoprenoids in particular, seem to play an important role on Madeira wine bouquet since presenting very low sensorial threshold limits. Excellent correlation between Madeira wine ageing and the abundance of cis-oak lactone was attained showing to be an important chemical descriptor to characterize reserves and Vintages as well as a contributor to wine flavour.The differentiation between reserves, dry/medium dry and sweet/medium sweet young wines could be well established by means of chemometric analysis, using particular aroma compounds such as diethyl succinate, cis-oak lactone and ethyl octanoate as discriminating variables.

Keywords: Madeira wine; Aroma; C; ₁₃; norisoprenoids; Oak lactones; Ultra traces; Solid phase microextraction; Stir bar sorptive extraction; Capillary gas chromatography–mass spectrometry; Chemometric analysis

A sol–gel immobilization of nano and micron size sorbents in poly(dimethylsiloxane) (PDMS) microchannels for microscale solid phase extraction (SPE) by Mahesh Karwa; Dittmar Hahn; Somenath Mitra (pp. 22-29).

Sorbent particles consisting of nano and micro silica, and micron size octadecylsilica (ODS) were immobilized using sol–gel chemistry onto poly(dimethylsiloxane) (PDMS) microfluidic channels to serve as μ-chip solid phase extraction (SPE) devices. Extraction, preconcentration and purification of biological and chemical analytes were carried out using these. Micro and nano scale silica-immobilized μ-SPE were used for the extraction/purification of DNA from recombinant Escherichia coli crude lysate. The average DNA recovery was 77±9% ( X±R.S.D.) for the micron size silica particles and 70±5% ( X±R.S.D.) for the nano silica particles. The extracted DNA could be amplified by polymerase chain reaction (PCR) whereas the DNA from the crude lysate solution could not be. This was a testimony to the purification capability of the μ-SPE device. ODS immobilized μ-SPE were used to study the extraction efficiency (EE) and enhancement factor (EF) for three groups of organic compounds, aromatics, phenols and carboxylic acids. They showed poor recovery and low enrichment because the analytes sorbed into the PDMS and were not quantitatively extracted.

Keywords: Sol–gel; Poly(dimethylsiloxane) (PDMS); Solid phase extraction (SPE)

Molecularly imprinted polyethersulfone microspheres for the binding and recognition of bisphenol A by Kaiguang Yang; Zongbin Liu; Mao Mao; Xiaohua Zhang; Changsheng Zhao; Norio Nishi (pp. 30-36).

BPA-imprinted polyethersulfone (PES) microspheres for the binding and recognition of bisphenol A (BPA) were fabricated by means of a liquid–liquid phase separation technique. The imprinted novel PES microspheres had a porous structure with a skin layer, under which was followed by a finger-like structure. The recognition experiments with the BPA-imprinted microspheres were carried out by applying the microspheres to various BPA solutions. In water, high binding amounts of BPA were observed in the range of 19–42μmol/g capacity, but the recognition was low in the BPA water solution. With the increase of the concentration in BPA solution, the binding amounts and the recognition coefficient increased. However, 1,4-butylene glycol/water media showed high recognition of the imprinted microspheres with a low binding capacity of BPA. In addtion, with the increase of the BPA amounts in the PES solution used to prepare the imprinted microspheres, the specific recognition sites increased, and the recognition ability increased. Evidence revealed that microsphere recognition was effective for BPA due to the binding to specific recognition sites [ S]_(sites). The imprinted microspheres showed the selectivity for BPA in the wine including BPA and other organic compounds. Charge transfer and special cavities could be employed to explain the mechanism.

Keywords: Microspheres; Molecular imprinting; Bisphenol A; Polyethersulfone (PES); Recognition coefficient

Spectral filtering of light-emitting diodes for fluorescence detection by Ebbing P. de Jong; Charles A. Lucy (pp. 37-45).

The use of light-emitting diodes (LEDs) for fluorescence detection has recently gained much interest. The broad wavelength emission of LEDs requires spectral filtering that is not necessary when using a laser. For instance, filtering the LED light using a bandpass filter improves the signal-to-background ratio for riboflavin by a factor of 70. The bandwidth of the necessary bandpass filters affects both the signal and the background in these measurements. Fluorescence signal can be maximized with wider-bandpass high-transmittance filters. Background is governed by scattering of the LED emission light transmitted by two bandpass filters. When there is large crosstalk between the filters, the LED intensity is linearly related to the background. By estimating and optimizing the crosstalk between excitation and emission filters with a method presented here, the signal-to-background can be optimized. Bandpass filters should be selected with sharp on–off transition, strong blocking outside their transmitting region and the widest bandwidth with minimal crosstalk. Using optimized spectral filtering and capillary electrophoresis analysis, LODs of 50, 3 and 20nM are obtained for riboflavin, fluorescein and eosin Y, respectively. These results are superior to those reported in the literature for 5mW LEDs.

Keywords: Light-emitting diode; Spectral filtering; Fluorescence detection; Bandpass filters

Sizes of water-soluble luminescent quantum dots measured by fluorescence correlation spectroscopy by Pudun Zhang; Liang Li; Chaoqing Dong; Huifeng Qian; Jicun Ren (pp. 46-51).

In this paper, fluorescence correlation spectroscopy (FCS) was applied to measure the size of water-soluble quantum dots (QDs). The measurements were performed on a home-built FCS system based on the Stokes–Einstein equation. The obtained results showed that for bare CdTe QDs the sizes from FCS were larger than the ones from transmission electron microscopy (TEM). The brightness of QDs was also evaluated using FCS technique. It was found that the stability of the surface chemistry of QDs would be significantly improved by capping it with hard-core shell. Our data demonstrated that FCS is a simple, fast, and effective method for characterizing the fluorescent quantum dots, and is especially suitable for determining the fluorescent nanoparticles less than 10nm in water solution.

Keywords: Fluorescence correlation spectroscopy; Water-soluble quantum dot; Size; Diffusion

Multivariate curve resolution analysis excitation-emission matrices of fluorescence of humic substances by Maria Cristina G. Antunes; Joaquim C.G. Esteves da Silva (pp. 52-59).

Excitation–emission matrices (EEM) of fluorescence of aqueous solutions of humic substances (HS), and sets of EEM acquired as function of the HS concentration, were analysed by multivariate curve resolution alternating least squares (MCR-ALS). Three types of HS samples were studied: one commercial humic acid; two samples of fulvic acid (FA) extracted from a pinewood soil; two samples of FA extracted from recycled wastes. The fluorescence measurements were carried out at HS concentration between 5 and 100mg/L and at pH 6. The application of MCR-ALS algorithm on each individual EEM, as well as on column-wise augmented matrices, allows the identification of three major fluorophores in all HS samples analysed. The emission and excitation spectra of these fluorophores were recovered and are characteristic of each sample. Moreover, the variation of the fluorescence intensities of each fluorophore with HS concentration shows deviations from linearity at HS concentration higher than 30mg/L, depending on the fluorophore and/or sample. This behaviour reveals the existence of inner filter effects that affect the proportionally between the fluorescent signal and concentration but do not provoke measurable distortions on the fluorescence spectra of the detected fluorophores.

Keywords: Humic substances; Molecular fluorescence; EEM; Concentration effect; Multivariate curve resolution (MCR-ALS)

Sensitive determination of captopril by time-resolved chemiluminescence using the stopped-flow analysis based on potassium permanganate oxidation by Jos� A. Murillo Pulgar�n; Luisa F. Garc�a Bermejo; Pablo Fern�ndez L�pez (pp. 60-67).

The chemiluminescent behaviour of captopril when reacted with a common oxidant, potassium permanganate in different acidic media is described, using the stopped-flow technique in a continuous-flow system. A 22 bit analogue-to-digital converter that acquires analogue signals at −10 and +10V and allows the power supply to the peristaltic pump to be interrupted is used in the time-resolved chemiluminescence manifold to ensure rapid, efficient mixing of chemiluminescent reagent and sample immediately before reaching the detector; this results in high precision and detectability, particularly with fast, short-lived emissions.The optimum chemical conditions for the chemiluminescence emission were investigated. It was found that a weak CL emission was emitted during the oxidation of this drug with potassium permanganate in acidic solution. The effect of common emission enhancers such as formic acid, formaldehyde, glutaraldehyde, acetaldehyde, quinine, fluorescein, rhodamine B and rhodamine 6G was studied. The parameters selected were 4.0molL⁻¹ sulphuric acid, 0.25mmolL⁻¹ permanganate and 0.75molL⁻¹ formaldehyde.Four quantitative parameters adjustable via software settings, two of them typically kinetic parameters, such as rate of the light-development reaction and rate of the light-decay reaction, and the other conventional parameters, such as maximum emission intensity and total emission area, were used to obtain linear calibration graphs with each measurement parameter. The detection limits ranged from 0.011 to 0.026μgmL⁻¹ and R.S.D. values ( n=10) of 1.21–3.93 at a 0.50μgmL⁻¹ and 2.01–3.41 at a 1.60μgmL⁻¹ concentration levels were obtained. The method was satisfactorily applied to the determination of captopril in pharmaceutical preparations.

Keywords: Captopril; Chemiluminescence; Time-resolved chemiluminescence; Stopped-flow analysis

The use of Nafion-coated thin mercury film electrodes for the determination of the dissolved copper speciation in estuarine water by Matthew P. Hurst; Kenneth W. Bruland (pp. 68-78).

Differential pulse anodic stripping voltammetry (DPASV) using a Nafion-coated thin mercury film electrode (NCTMFE) was implemented to determine the dissolved copper speciation in saline estuarine waters containing high concentrations of dissolved organic matter (DOM). The study used model ligands and estuarine water from San Francisco Bay, California, USA to demonstrate that the NCTMFE is more effective at distinguishing between electrochemically inert and labile copper species when compared to the conventional thin mercury film electrode (TMFE). Copper titration results verify that the NCTMFE better deals with high concentrations of DOM by creating a size-exclusion barrier that prevents DOM from interacting with the mercury electrode when performing copper speciation measurements. Pseudovoltammograms were used to illustrate that copper complexes found in natural waters were more apt to be electrochemically inert at the NCTMFE relative to the TMFE when subjected to high negative overpotentials. Copper speciation results using the NCTMFE from samples collected in San Francisco Bay estimated that >99.9% of all copper was bound to strong copper-binding ligands. These L₁-class ligands exceeded the concentration of total dissolved copper in all samples tested and control the equilibrium of ambient [Cu²⁺] in the San Francisco Bay estuary.

Keywords: Nafion-coated thin mercury electrode; Rotating disk electrode; Anodic stripping voltammetry; Copper speciation; San Francisco Bay

Determination of aluminum as contaminant in dialysis concentrates by adsorptive cathodic stripping voltammetry by Leandro M. de Carvalho; Paulo C. do Nascimento; Denise Bohrer; Raquel Stefanello; Denise Bertagnolli (pp. 79-84).

This paper describes a stripping voltammetric method for the determination of aluminum as contaminant in dialysis concentrates. It is based on the adsorptive deposition of the complex Al-1,2-dihydroxyanthraquinone-3-sulfonic acid (DASA) at the hanging mercury drop electrode (HMDE) at −0.9V (versus Ag/AgCl) and its cathodic stripping during the potential scan. The method was optimized concerning the main electrochemical parameters that influence the voltammetric determination in highly saline solutions (sample dilution factor, deposition time and DASA concentration). The linear range for the measurements was from −1 with a detection limit calculated as 0.8μgl⁻¹ for a deposition time of 15s. Aluminum recoveries between 91.0 and 105.5% for spiked samples of dialysis concentrates proof the accuracy of the proposed voltammetric method. The method allows the rapid aluminum determination at μgl⁻¹ levels in the presence of Zn(II) as interfering ion, and it was applied to its determination in commercial dialysis concentrates used routinely by hemodialysis centers in Brazil. Aluminum concentrations between 9.0 and 78.7μgl⁻¹ were determined in five different samples of three kinds of dialysis concentrates.

Keywords: Aluminum; Dialysis concentrates; Contamination; Adsorptive stripping voltammetry

Electrochemical detection of the herbicide paraquat in natural water and citric fruit juices using microelectrodes by D. De Souza; S.A.S. Machado (pp. 85-91).

A novel electroanalytical procedure for detecting the paraquat herbicide in natural water and citric fruit juice samples using gold microelectrodes and square wave voltammetry at high frequencies is proposed. The results obtained showed two reversible peaks for the reduction of paraquat, the first peak associated with the reduction of the paraquat molecule in solution, with subsequent adsorption of the intermediate on the electrode surface. This adsorbed species was shown to undergo electroreduction in a reaction associated to the second voltammetric peak. The variation in pH and square wave parameters indicated that the best conditions under which paraquat could be reduced were a pH of 5.0, a frequency of 1000s⁻¹, a scan increment of 2mV and a square wave amplitude of 50mV. Under these conditions, the variation of the concentrations of paraquat from 1.00×10⁻⁶ to 1.66×10⁻⁴molL⁻¹ presented, for peak 1, detection and quantification limits of 4.51 and 15.05μgL⁻¹ respectively in pure electrolyte with a recovery factor of 99.50%. The proposed analytical procedure was also applied to natural water samples giving recovery factors of 95.00, 89.50 and 92.50% in three water samples collected from an urban stream. The recovery factor was observed to depend on the content of organic matter which was determined by the biochemical and chemical oxygen demand. In lemon and orange juice samples that were spiked with 5.70×10⁻⁵molL⁻¹ of paraquat, the recovery factors obtained were 94.30 and 92.70% respectively.

Keywords: Square wave voltammetry; Paraquat; Microelectrodes

Development and complete validation of a method for the determination of dimethyl mercury in air and other media by Nicolas S. Bloom; Angela K. Grout; Eric M. Prestbo (pp. 92-101).

The rigorous validation of a previously developed research method for the determination of dimethyl mercury ((CH₃)₂Hg) in environmental samples is presented. During atmospheric analysis, (CH₃)₂Hg was trapped on Carbotrap™ and analyzed by thermal desorption, isothermal GC separation, and cold vapor atomic fluorescence spectrometry (CVAFS). Water samples were analyzed after direct purging of 100mL aliquots onto Carbotrap™, while sediment and tissue samples were digested with 10mL of 25% KOH in methanol at 60°C and diluted to 40mL with methanol. An ambient air-spiking manifold, which allowed simultaneous replicate sampling, was constructed in a room controlled for temperature and humidity. (CH₃)₂Hg was introduced into the feed airflow (0.4m³min⁻¹) from a well-calibrated diffusion cell, to obtain a concentration of approximately 5.5ngm⁻³ as Hg. Samples were collected onto Carbotrap™ columns, and the total volumes quantified by integrating mass flow meters. Trapping efficiency was investigated over a range of sampler flow rates (0.05–0.25Lmin⁻¹), volumes (2–200L), collection temperatures (15–42°C) and relative humidity levels (10–70%). Method detection limits (MDLs), analytical precision and accuracy were quantified for all media. Carbotrap™ was found to be the best choice as a sampling media, whereas Tenax™ was found to be inadequate due to high breakthrough (>70%). This study verified that the method is sufficiently precise, accurate and robust for field sampling at mercury contaminated sites. No interferences were observed from elevated levels of potential co-contaminants, Hg⁰ (125ngm⁻³) and H₂S (1.27ppmv).

Keywords: Dimethyl mercury; Mercury; Environmental chemistry; Landfill gas

Loss of polychlorinated dibenzo- p-dioxins, polychlorinated dibenzofurans and coplanar polychlorinated biphenyls during nitrogen gas blowdown process for ultra-trace analysis by Osamu Kiguchi; Takashi Kobayashi; Katsumi Saitoh; Nobuaki Ogawa (pp. 102-111).

This study examined standard solutions to assess the influence of the gas flow rate and organic solvent type on losses caused by gas blowdown of polychlorinated dibenzo- p-dioxins and dibenzofurans (PCDD/DFs) and coplanar polychlorinated biphenyls (Co-PCBs). Results obtained here will contribute to maintaining analytical method performance and system quality for PCDD/DFs and Co-PCBs analyses. An organic solvent (with 0.5ml each of acetone, dichloromethane, n-hexane, and toluene), PCDD/DFs or Co-PCBs, and their¹³C₁₂-labeled compounds were put separately into 10ml pear-shaped flasks. The samples were blown to dryness at room temperature until the last trace of solvent disappeared. They were subsequently reconstituted in those flasks. Analyte recoveries were calculated by comparing blown samples to those that had not been blown. Recoveries of Co-PCBs were more affected than those of PCDD/DFs when the gas flow rates were set at 203, 261, 332, and 456ml/min. Losses of Co-PCBs were least at 203–332ml/min. Regarding losses of PCDD/DFs and Co-PCBs, the toluene solution showed the least variation in recovery. An actual soil sample extract was also examined using optimized conditions for the gas flow rate and solvent types obtained by experiments in standard solutions. Thereby, the blowdown conditions gave quantitative recoveries of¹³C₁₂-labeled compounds in the sample extract.

Keywords: Polychlorinated dibenzo-; p; -dioxins and dibenzofurans (PCDD/DFs); Coplanar polychlorinated biphenyls (Co-PCBs); Analyte loss; Nitrogen gas; Blowdown process

An assisted approach of the global optimization for the experimental conditions in capillary electrophoresis by Hong Lin Zhai; Hong Gao; Xing Guo Chen; Zhi De Hu (pp. 112-118).

Experimental conditions have effect on the separation of capillary electrophoresis (CE) directly. In this work, a set of index to describe the separation in CE was established properly. Based on a combination of genetic algorithm and least square support vector machine, an assisted approach of global optimization for experimental conditions was proposed for the first time, and it was applied to the separation of four synthetic compounds by CE in nonaqueous system. Under the optimum conditions obtained by this approach, the result of the experiment was satisfactory and proved that this novel approach was effective. Furthermore, we investigated the most important conditions that mainly affect the separation effectiveness of CE by partial least squares regression analysis. Because of the generalization of this new approach proposed, it can be applied to the optimization of other experimental processes.

Keywords: Experimental condition; Optimization; Least square support vector machine; Genetic algorithm; Partial least squares regression

Enantioseparation of dansyl amino acids by a novel permanently positively charged single-isomer cyclodextrin: Mono-6- N-allylammonium-6-deoxy-β-cyclodextrin chloride by capillary electrophoresis by Wei-Hua Tang; I. Wayan Muderawan; Teng-Teng Ong; Siu-Choon Ng (pp. 119-125).

A permanently positively charged single-isomer β-CD, mono-6- N-allylammonium-6-deoxy-β-CD chloride, ALAM-β-CD, has been synthesized and successfully used as chiral selector in capillary electrophoresis for enantioseparation of amino acids and dansyl amino acids. The effects of pH, CD's concentration and capillary length on enantioseparation were studied in order to investigate its potential as chiral selector for acidic racemates. By increasing buffer's pH and CD's concentration, fairly robust separations were achieved. Good separations were also obtained with a shorter capillary. In addition, baseline enantiomeric separations of a mixture of six pairs of dansyldl-amino acids were achieved under different pH conditions.

Keywords: Abbreviations; CD; cyclodextrin; BGE; background electrolyte; Dns-; dansyl-; ALAM-β-CD; mono-6-; N; -allylammonium-6-deoxy-β-CD chlorideEnantioseparation; Permanently positively charged single-isomer β-CD; Dansyl amino acids; ALAM-β-CD; Capillary electrophoresis

Electrochemical hydride generation atomic absorption spectrometry for determination of cadmium by M.H. Arbab-Zavar; M. Chamsaz; A. Youssefi; M. Aliakbari (pp. 126-132).

An electrolytic hydride generation system for determination of another hydride forming element, cadmium, by catholyte variation electrochemical hydride generation (EcHG) atomic absorption spectrometry is described. A laboratory-made electrolytic cell with lead–tin alloy as cathode material is designed as electrolytic generator of molecular hydride. The influences of several parameters on the analytical signal have been evaluated using a Plackett–Burman experimental design. The significant parameters such as cathode surface area, electrolytic current, carrier gas flow rate and catholyte concentration have been optimized using univariate method. The analytical figures of merit of procedure developed were determined. The calibration curve was linear up to 20ngml⁻¹of cadmium. The concentration detection limit (3 σ, n=8) of 0.2ngml⁻¹ and repeatability (relative standard deviation, n=7) of 3.1% were achieved at 10.0ngml⁻¹. It was shown that interferences from major constituents at high concentrations were significant. The accuracy of method was verified using a real sample (spiked tap water) by standard addition calibration technique. Recovery of 104% was achieved for Cd in the spiked tap water sample.

Keywords: Cadmium; Electrochemical hydride generation; Atomic absorption spectrometry

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Analytica Chimica Acta (v.546, #1)