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Analytica Chimica Acta (v.606, #1)

Editorial Board (pp. co1).

Contents (pp. v-vi).

A speciation methodology to study the contributions of humic-like and fulvic-like acids to the mobilization of metals from compost using size exclusion chromatography–ultraviolet absorption–inductively coupled plasma mass spectrometry and deconvolution analysis by Francisco Laborda; Eduardo Bolea; María P. Górriz; María P. Martín-Ruiz; Sergio Ruiz-Beguería; Juan R. Castillo (pp. 1-8).

High performance size-exclusion chromatography (HP-SEC) with UV absorption for organic matter detection and inductively coupled plasma mass spectrometry (ICP-MS) for elemental detection have been used to study the mobilization of metals from compost as a function of pH and the molecular mass of their complexes with dissolved organic matter (DOM). Due to its heterogeneous nature, organic matter mobilized from compost shows a continuous distribution of molecular masses in the range studied (up to 80kDa). In order to differentiate between the contribution of humic and fulvic acids (FA) to the organic matter mobilized in the pH range 5–10, their UV absorption chromatographic profiles have been deconvoluted with respect to the adjusted gaussian profiles of the humic and fulvic acids isolated from compost. Results show a preponderant contribution of fulvic acids at low pH values and an increasing percentage of humic acids (HA) mobilized at basic pH (up to 49% of total DOM at pH 10). A similar deconvolution procedure has been applied to the ICP-MS chromatograms of selected metals (Co, Cu, Pb and Bi). In general, both fulvic and humic acids contribute to the mobilization of divalent transition metals, such as copper or cobalt, whereas bismuth or lead are preferably associated to humic acids. Non-humic substances (HS) also contribute to the mobilization of cations, especially at acidic pHs. These conclusions have been extended to different elements based on deconvolution analysis results at pH 7.

Keywords: Inductively coupled plasma mass spectrometry; Size exclusion chromatography; Deconvolution analysis; Humic-like acid; Fulvic-like acid; Compost

Optimization and comparison of chemical and electrochemical hydride generation for optical emission spectrometric determination of arsenic and antimony using a novel miniaturized microwave induced argon plasma exiting the microstrip wafer by Pawel Pohl; Israel Jiménez Zapata; Nicolas H. Bings (pp. 9-18).

Continuous flow (CF) chemical hydride generation (CHG) and electrochemical hydride generation (ECHG) directly coupled to a novel 40W, atmospheric pressure, 2.45GHz microwave microstrip Ar plasma exiting a microstrip wafer has been developed for the emission spectrometric determination of As and Sb using a miniaturized optical fiber spectrometer and a CCD-array detector. The experimental conditions for both procedures were optimized with respect to the relative net intensities of the As I 228.8nm and Sb I 252.8nm lines and their signal-to-background intensity ratios. Additionally, the susceptibility to interferences from Cd, Co, Cr, Cu, Fe, Ni, Pb and Zn and other hydride-forming elements in the determination of As and Sb using the CHG and ECHG techniques was investigated in detail. Under the optimized conditions, it was found that ECHG is more prone to interferences compared to CHG. The detection limits (3 σ) of As (6ngmL⁻¹) and Sb (7ngmL⁻¹) obtained for the ECHG–MSP–OES method are about three times lower than in the case of the CHG–MSP–OES method due to a two-fold lower amount of H₂ introduced into the MSP in case of the ECHG, resulting in a better plasma stability and reduced background level. The linearity ranges for both calibration curves to a concentration of up to 5μgmL⁻¹ and a precision between 2% and 7% (2μgmL⁻¹ and 0.050μgmL⁻¹ of As and Sb, respectively) were found for both methods. The developed ECHG–MSP–OES method was validated for As through the analysis of a certified coal fly ash standard reference material (NIST SRM 1633a) after sample dissolution. The derived concentration (140±8μgg⁻¹) was found to agree well with the certified data (145±15μgg⁻¹). The method was also successfully applied to the analysis of both a galvanic bath sample, which contained Sb and was spiked with As, and a tap water sample spiked with both analytes. Recovery rates of 99–101% and a Sb concentration of 6.6μgmL⁻¹ in the galvanic bath sample were revealed. The latter value showed a good agreement with the data obtained from ICP-OES analysis, which was also used for validation purpose.

Keywords: Arsenic; Antimony; Microstrip microwave plasma; Chemical hydride generation; Electrochemical hydride generation; Optical emission spectrometry

Synchronous fluorescence and UV–vis spectroscopic studies of interactions between the tetracycline antibiotic, aluminium ions and DNA with the aid of the Methylene Blue dye probe by Yongnian Ni; Daiqin Lin; Serge Kokot (pp. 19-25).

Synchronous fluorescence spectroscopy (SFS) was applied for the investigation of interactions of the antibiotic, tetracycline (TC), with DNA in the presence of aluminium ions (Al³⁺). The study was facilitated by the use of the Methylene Blue (MB) dye probe, and the interpretation of the spectral data with the aid of the chemometrics method, parallel factor analysis (PARAFAC). Three-way synchronous fluorescence analysis extracted the important optimum constant wavelength differences, Δ λ, and showed that for the TC–Al³⁺–DNA, TC–Al³⁺ and MB dye systems, the associated Δ λ values were different (Δ λ=80, 75 and 30nm, respectively). Subsequent PARAFAC analysis demonstrated the extraction of the equilibrium concentration profiles for the TC–Al³⁺, TC–Al³⁺–DNA and MB probe systems. This information is unobtainable by conventional means of data interpretation. The results indicated that the MB dye interacted with the TC–Al³⁺–DNA surface complex, presumably via a reaction intermediate, TC–Al³⁺–DNA–MB, leading to the displacement of the TC–Al³⁺ by the incoming MB dye probe.

Keywords: Tetracycline antibiotic; Fluorophoric probe; DNA; UV–vis and fluorescence spectroscopy; Chemometrics; Parallel factor analysis

Modelling of chemical fractionation patterns of metals in soils by two-way and three-way principal component analysis by R. Pardo; M. Vega; L. Debán; C. Cazurro; C. Carretero (pp. 26-36).

The ‘pseudo-total’ contents and the chemical fractionation pattern of eight toxic elements (As, Cd, Co, Cu, Cr, Ni, Pb and Zn) have been determined in 12 soil samples collected around a coal-fuelled power plant (Velilla del Río Carrión, Spain) by using, respectively, the US-EPA 3051 norm and the modified BCR (SM&T) chemical fractionation procedure. The ‘pseudo-total’ dataset has been analyzed by classical two-way principal component analysis (PCA) finding a PC accounting for the metal ‘pollution’ of the area. On the other hand, the three-dimensional (samples×metals×fractions)X array obtained after application of the modified BCR SM&T procedure, has been studied by matrix augmentation (MA-PCA) and three-way principal component analysis (3-PCA) using PARAFAC and TUCKER3 models. Whereas both MA-PCA and PARAFAC originated two-factor models biased towards the different chemical fractionation of the samples, the best TUCKER3 model [1, 2, 2] takes into account simultaneously both the ‘pseudo-total’ contents and the chemical fractionation of the soil samples. Therefore, the TUCKER3 originated a better representation of the global environmental impact caused by the power plant, and the plotting of the soil samples loadings, A1, in the physical space allowed to locate the most potentially hazardous areas.

Keywords: Toxic metals; Soils; Sequential extraction; BCR; Chemical fractionation; Two-way PCA; Matrix augmentation-PCA; Three-way PCA; PARAFAC; TUCKER3

Speciation of trace metals in natural waters: The influence of an adsorbed layer of natural organic matter (NOM) on voltammetric behaviour of copper by Yoann Louis; Petra Cmuk; Dario Omanović; Cédric Garnier; Véronique Lenoble; Stéphane Mounier; Ivanka Pižeta (pp. 37-44).

The influence of an adsorbed layer of the natural organic matter (NOM) on voltammetric behaviour of copper on a mercury drop electrode in natural water samples was studied. The adsorption of NOM strongly affects the differential pulse anodic stripping voltammogram (DPASV) of copper, leading to its distortion. Phase sensitive ac voltammetry confirmed that desorption of adsorbed NOM occurs in general at accumulation potentials more negative than −1.4V. Accordingly, an application of negative potential (−1.6V) for a very short time at the end of the accumulation time (1% of total accumulation time) to remove the adsorbed NOM was introduced in the measuring procedure. Using this protocol, a well-resolved peak without interferences was obtained. It was shown that stripping chronopotentiogram of copper (SCP) in the depletive mode is influenced by the adsorbed layer in the same manner as DPASV. The influence of the adsorbed NOM on pseudopolarographic measurements of copper and on determination of copper complexing capacity (CuCC) was demonstrated. A shift of the peak potential and the change of the half-peak width on the accumulation potential (for pseudopolarography) and on copper concentration in solution (for CuCC) were observed. By applying a desorption step these effects vanished, yielding different final results.

Keywords: Anodic stripping voltammetry; Metal complexing capacity; Organic matter; Pseudopolarography; Speciation; Trace metals

Sensitive and stable amperometric measurements at ionic liquid–carbon paste microelectrodes by Mustafa Musameh; Joseph Wang (pp. 45-49).

Ionic liquids (ILs) were utilized in preparing carbon paste electrodes with improved sensitivity, linearity, and stability. In order to overcome the large capacitance encountered with these pastes which affects the measured signal, microelectrodes were used to minimize the background current and improve the signal to background response. A number of ILs were tested including those having the same anions, such as butyl, hexyl, and octyl hexafluorophosphate (PF₆), and those having the same cation, including hexyl-imide, trifluorophosphate (PF₃) and PF₆. Based on the structure of each ILs, different responses were obtained. Several probes were utilized for those studies including hydrogen peroxide, acetaminophen, and ascorbic acid. The butyl-methyl-imidazolium (BMI)-PF₆ displayed the highest response among other ILs with similar anions and the smallest peak separation. The optimum ILs loading in terms of electrode resistance, capacitance, and peak separation was 30–40%. Improved resistance to surface fouling was observed at the ILs–carbon paste electrode compared to that using mineral oil when tested for NADH detection. Very good linearity up to 1mM hydrazine (correlation coefficient 0.996) was obtained using the ILs–carbon paste electrode.

Keywords: Ionic liquids; Carbon paste; Mineral oil; Surface fouling

Improving prediction selectivity for on-line near-infrared monitoring of components in etchant solution by spectral range optimization by Hankyu Namkung; Youngbok Lee; Hoeil Chung (pp. 50-56).

The components (H₃PO₄, HNO₃, CH₃COOH and water) in an etchant solution have been accurately measured in an on-line manner using near-infrared (NIR) spectroscopy by directly illuminating NIR radiation through a Teflon line. In particular, the spectral features according to the change of H₃PO₄ or HNO₃ concentrations were not mainly from NIR absorption themselves, but from the perturbation (or displacement) of water bands; therefore, the resulting spectral variations were quite similar to each other. Consequently partial least squares (PLS) prediction selectivity among the components should be the most critical issue for continuous on-line compositional monitoring by NIR spectroscopy. To improve selectivity of the calibration model, we have optimized the calibration models by finding selective spectral ranges with the use of moving window PLS. Using the optimized PLS models for each component, the resulting prediction accuracies were substantially improved. Furthermore, on-line prediction selectivity was evaluated by spiking individual pure components step by step and examining the resulting prediction trends. When optimized PLS models were used, each concentration was selectively and sensitively varied at each spike; meanwhile, when whole or non-optimized ranges were used for PLS, the prediction selectivity was greatly degraded. This study verifies that the selection of an optimal spectral range for PLS is the most important factor to make Teflon-based NIR measurements successful for on-line and real-time monitoring of etching solutions.

Keywords: Near-infrared spectroscopy; Moving window Partial Least Squares; Etching solution; On-line monitoring

Vicinal diaminobenzoacridine used as the fluorescent probe for trace nitric oxide determination by flow injection spectrofluorimetry and macrophage cells imaging by Jixi Hu; Lingling Yin; Kehua Xu; Jingjing Gao; Lili Tong; Bo Tang (pp. 57-62).

Based on a photoelectron transfer (PET) mechanism, vicinal diaminobenzoacridine (VDABA), a fluorescent probe for the determination of trace amounts of nitric oxide radical in biological sample, was synthesized and characterized by elemental analysis, IR,¹H NMR and¹³C NMR spectrum. Combining a flow injection with spectrofluorimetry, a high-throughput method for detecting NO was obtained, which was successfully applied to the determination of NO in the human serum. The proposed method was simple, rapid, precise and automatic. Under optimum conditions, the linear calibration range was from 1.1×10⁻⁷ to 5.0×10⁻⁶M and the detection limit was 3.1×10⁻⁸M. Furthermore, the probe could make cell-derived NO “visible” by using confocal laser scanning microscope.

Keywords: Vicinal diaminobenzoacridine; Fluorescent probe; Nitric oxide; Flow injection spectrofluorimetry; Cell imaging

A high-precision ratiometric fluorosensor for pH: Implementing time-dependent non-linear calibration protocols for drift compensation by Aron Hakonen; Stefan Hulth (pp. 63-71).

We present a versatile time-dependent non-linear calibration protocol for optical sensors, implemented on the pH sensitive ratiometric fluorophore 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) immobilized in ethyl-cellulose. The calibration protocol individually compensated for the progressive drift of calibration parameters, whereby sensor precision and accuracy, as well as applicable lifetime were improved. A severely reduced photoacidity was observed for the immobilized fluorophore, for which excited state dynamics was characterized and benefited from during measurements. Due to the significantly reduced photoacidity of HPTS immobilized in the ethyl-cellulose sensing membrane, a dual excitation/dual emission ( F₁, ex/em: 405/440nm and F₂, ex/em: 465/510nm) ratiometric (RF1,F2= F₁/ F₂) sensing scheme could be used to amplify sensor response. The signal to noise (S/N) ratio was enhanced by ∼400% utilizing the dual excitation/dual emission ratiometric sensing scheme, rather than the more commonly used protocol of dual excitation/single emission for HPTS fluorescence. Apparent p K_a of the fluorophore ranged from 6.74 to 8.50, mainly determined by the immobilization procedure. The repeatability (IUPAC, pooled standard deviation) over three pH values (6.986, 7.702 and 7.828) was 0.0044 pH units for the optical sensor, compared to 0.0046 for the electrode used for standardization. Sensor analytical characteristics were thereby in principle limited by the performance of the standardization procedure.

Keywords: pH; Optode; 8-hydroxypyrene-1,3,6-trisulfonic acid; Photoacidity; Non-linear calibration; Signal drift; Fluorescence ratio

Particulate and soluble Eu(III)-chelates as donor labels in homogeneous fluorescence resonance energy transfer based immunoassay by Leena Kokko; Tiina Kokko; Timo Lövgren; Tero Soukka (pp. 72-79).

Many well-established homogeneous separation free immunoassays rely on particulate label technologies. Particles generally contain a high concentration of the embedded label and they have a large surface area, which enables conjugation of a large amount of protein per particle. Eu(III)-chelate dyed nanoparticles have been successfully used as labels in heterogeneous and homogeneous immunoassays. In this study, we compared the characteristics of two homogeneous competitive immunoassays using either soluble Eu(III)-chelates or polystyrene particles containing Eu(III)-chelates as donors in a fluorescence resonance energy transfer based assay. The use of the particulate label significantly increased the obtained sensitized emission, which was generated by a single binding event. This was due to the extremely high specific activity of the nanoparticle label and also in some extent the longer Förster radius between the donor and the acceptor. The amount of the binder protein used in the assay could be decreased by 10-fold without impairing the obtainable sensitized emission, which subsequently led to improved assay sensitivity. The optimized assay using particulate donor had the lowest limit of detection (calculated using 3×S.D. of the 0nM standard) 50pM of estradiol in the assay well, which was approximately 20-fold more sensitive than assays using soluble Eu(III)-chelates.

Keywords: Europium; Chelate; Nanoparticle label; Homogeneous; Time-resolved fluorescence

Enantioselective analysis of mirtazapine and its two major metabolites in human plasma by liquid chromatography–mass spectrometry after three-phase liquid-phase microextraction by Fernando José Malagueño de Santana; Pierina Sueli Bonato (pp. 80-91).

A three-phase liquid-phase microextraction (LPME) method using porous polypropylene hollow fibre membrane with a sealed end was developed for the extraction of mirtazapine (MRT) and its two major metabolites, 8-hydroxymirtazapine (8-OHM) and demethylmirtazapine (DMR), from human plasma. The analytes were extracted from 1.0mL of plasma, previously diluted and alkalinized with 3.0mL 0.5molL⁻¹ pH 8 phosphate buffer solution and supplemented with 15% sodium chloride (NaCl), using n-hexyl ether as organic solvent and 0.01moLL⁻¹ acetic acid solution as the acceptor phase. Haloperidol was used as internal standard. The chromatographic analyses were carried out on a chiral column, using acetonitrile–methanol–ethanol (98:1:1, v/v/v) plus 0.2% diethylamine as mobile phase, at a flow rate of 1.0mLmin⁻¹. Multi-reaction monitoring (MRM) detection was performed by mass spectrometry (MS–MS) using a triple-stage quadrupole and electrospray ionization interface operating in the positive ion mode. The mean recoveries were in 18.3–45.5% range with linear responses over the 1.25–125ngmL⁻¹ concentration range for all enantiomers evaluated. The quantification limit (LOQ) was 1.25ngmL⁻¹. Within-day and between-day assay precision and accuracy (2.5, 50 and 100ngmL⁻¹) showed relative standard deviation and the relative error lower than 11.9% for all enantiomers evaluated. Finally, the method was successfully used for the determination of mirtazapine and its metabolite enantiomers in plasma samples obtained after single drug administration of mirtazapine to a healthy volunteer.

Keywords: Three-phase liquid-phase microextraction; Hollow fibre sealed end; Liquid chromatography–mass spectrometry; Enantiomeric resolution; Mirtazapine; Metabolites

Molecularly imprinted polymer using β-cyclodextrin as functional monomer for the efficient recognition of bilirubin by Yu Yang; Yuanyuan Long; Qing Cao; Kean Li; Feng Liu (pp. 92-97).

Bilirubin (BR) imprinted polymer was successfully prepared using supramolecular host compound β-cyclodextrin (β-CD) as functional monomer. The adsorption equilibrium was attained in about 4h, which indicated that the adsorption kinetics was comparatively fast. The results of adsorption and selectivity experiments indicated that BR-imprinted β-CD polymer was able to bind BR specifically and reversibly. The specific recognition of BR-imprinted β-CD polymer for BR may be due to the cooperative effects of inclusion interaction and hydrogen bonding. This BR-imprinted β-CD polymer was further applied to eliminate BR in human serum sample. It was verified that the binding specificity of the BR-imprinted polymer for BR was essentially sufficient in the presence of other compounds coexisting in serum sample. Therefore, as a reusable material possessing high affinity and selectivity, BR-imprinted β-CD polymer has a potential application perspective as a clinical hemoperfusion material.

Keywords: Molecularly imprinted polymer; β-Cyclodextrin; Bilirubin; Selectivity

Multiplexed high-throughput electrokinetically-controlled immunoassay for the detection of specific bacterial antibodies in human serum by Yali Gao; Philip M. Sherman; Yu Sun; Dongqing Li (pp. 98-107).

In previous studies we have developed a simple electrokinetically-controlled lab-on-a-chip for heterogeneous immunoassay. In that method, all the sequential operations in an immunoassay, such as reagent loading and washing, were performed automatically by electrokinetically controlling the flow in an H-shaped microchannel. Here, we demonstrated further development of a high-throughput immunoassay microfluidic chip, and the application of the new immunoassay microfluidic chip in assaying human serum. The microfluidic immunoassay analyzed ten samples in parallel in 22min. Bacterial antibodies in samples were captured by antigens pre-patterned on the bottom wall of a microchannel and then bound with TRITC-labeled detection antibodies to generate fluorescent signals. With optimized surface concentration of antigen, the assay detected Escherichia coli O157:H7 antibody and Helicobacter pylori antibody from buffer solutions in concentration ranges of 0.02–10μgmL⁻¹ and 0.1–50μgmL⁻¹, respectively. Human sera that were E. coli-positive or H. pylori-positive were accurately distinguished from respective negative controls. Moreover, the two antibodies, anti- E. coli and anti- H. pylori antibodies, could be simultaneously detected from human serum. This electrokinetically-controlled immunoassay shows an excellent potential for efficiently detecting multiple pathogenic infections in clinical environments.

Keywords: Electrokinetic; Microfluidics; Immunoassay; Lab-on-chip; Escherichia coli; O157:H7; Helicobacter pylori

The design and characteristics of direct current glow discharge atomic emission source operated with plain and hollow cathodes by A. Qayyum; M.I. Mahmood (pp. 108-111).

A compact direct current glow discharge atomic emission source has been designed and constructed for analytical applications. This atomic emission source works very efficiently at a low-input electrical power. The design has some features that make it distinct from that of the conventional Grimm glow discharge source. The peculiar cathode design offered greater flexibility on size and shape of the sample. As a result the source can be easily adopted to operate in Plain or Hollow Cathode configuration. I– V and spectroscopic characteristics of the source were compared while operating it with plain and hollow copper cathodes. It was observed that with hollow cathode, the source can be operated at a less input power and generates greater Cu I and Cu II line intensities. Also, the intensity of Cu II line rise faster than Cu I line with argon pressure for both cathodes. But the influence of pressure on Cu II lines was more significant when the source is operated with hollow cathode.

Keywords: Hollow cathode discharge; Emission spectroscopy; Grimm glow discharge

Prediction of soluble solids content, firmness and pH of pear by signals of electronic nose sensors by Hongmei Zhang; Jun Wang; Sheng Ye (pp. 112-118).

The objective of this study was to investigate the predictability of an electronic nose for fruit quality indices. Responses signal of sensor array in electronic nose were employed to establish quality indices model for “xueqing” pear. The relationships were established between signal of electronic nose and the quality indices of fruit (firmness, soluble solids content (SSC) and pH) by multiple linear regressions (MLR) and artificial neural network (ANN). The prediction models for firmness and soluble solids content indicated a good prediction performance. The SSC model by ANN had a standard error of prediction (SEP) of 0.41 and correlation coefficient 0.93 between predicted and measured values, the model by ANN for the penetrating force (CF) had a 3.12 SEP and 0.94 coefficient, respectively. The results imply that it is possible to predict “xueqing” pear quality characteristics from signal of E-nose.

Keywords: Electronic nose; Pear; pH; Firmness; Soluble solids content

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Analytica Chimica Acta (v.606, #1)