Analytica Chimica Acta (v.417, #2)

Estimation of sensitivity for refractive index and immunoreaction in a surface plasmon resonance sensor probe by Takuo Akimoto; Satoshi Sasaki; Kazunori Ikebukuro; Isao Karube (125-131).
A surface plasmon resonance (SPR) sensor probe was proposed, and its sensitivities were examined. The sensitivity for a refractive index was experimentally decided using an SPR sensor probe. The obtained sensitivity of 4.0×103  nm per refractive index was approximately the same as that obtained by theoretical calculation. The sensitivity for immunoreaction was experimentally estimated using an anti-bovine serum albumin antibody as a model analyte. The sensitivity defined in the antibody concentration’s region of 6–60 μg ml−1 was 0.13 nm ml μg−1. The sensitivity for the antibody concentration was found to be reasonable compared to that obtained by a planar SPR instrument.
Keywords: Surface plasmon resonance; Probe; Refractive index; Immunoreaction;

A novel 4-aminobenzoic acid (4-ABA) monolayer film is formed on glassy carbon electrode (GCE) by amino cation radical method. Silicotungstic heteropolyanion (SiW12O40 4−, denoted as SiW12)-containing multilayer films have been fabricated on the 4-ABA modified GCE surface by alternate deposition with a quaternized poly(4-vinylpyridine) partially complexed with [Os(bpy)2Cl]2+/+ (denoted as QPVP-Os). Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and X-ray reflectivity (XR) have been used to characterise the as-prepared multilayer films. It is proved that the multilayer films are uniform and stable. The average thickness for a bilayer of QPVP-Os/SiW12 in the multilayer film is 30.2 Å. The electrocatalytic activities of the multilayer films have been investigated on the reduction of three substrates of important analytical interests, HNO2, BrO3 and H2O2. Especially, the influence of layer number of the multilayer films on the electrocatalytic reduction of HNO2 has been investigated in detail.
Keywords: Layer-by-layer assembly; 4-Aminobenzoic acid; Silicotungstate; Electrocatalytical reduction;

The behavior of sildenafil citrate (SC) was studied by square wave technique, leading to two methods for its determination in aqueous samples (pH 2.0) and pharmaceutical formulations. The application of the square wave (SW) without the adsorptive accumulation shows the maximum response at –1.03 V. Besides, SC gave two adsorptive stripping voltammetric (AdSV) peaks at –1.03 and −1.15 V using an accumulation potential of −0.80 V. The effect of experimental parameters that affect this determination are discussed. For the stripping technique, SC proved to be more sensitive, yielding signals three or four times larger than those obtained by applying a square wave scan without the previous accumulation. The calibration graph to determine SC was linear in the range 5×10−9 to 9×10−7  M by stripping mode with a time of accumulation t acc of 10 s. The relative standard deviations obtained for concentration levels of SC as low as 1.0×10−7  M with square wave was 2.5% (n=10) and for 1.0×10−7  M with stripping square wave was 3.1% (n=10) in the same day. The two proposed methods (SW and SWAdSV) were applied to the determination of SC in three pharmaceutical products (Viagra 25, Viagra 50 and Viagra 100) with very good recoveries with respect to the labelled values.
Keywords: Voltammetry; Stripping voltammetry; Square wave; Sildenafil citrate; Viagra;

Determination of the chemical speciation of trace metals in aqueous systems by the Wageningen Donnan Membrane Technique by Erwin J.M. Temminghoff; Alexandra C.C. Plette; Rein Van Eck; Willem H. Van Riemsdijk (149-157).
In order to determine the ‘free’ metal ion concentration in aqueous solutions the so-called Wageningen Donnan Membrane Technique (WDMT) has been developed. This involves a continuous flow system in which the donor side and the acceptor side of the WDMT cell are continuously flushed with solution across the membrane. The new cell design allows pseudo equilibrium to be reached for the free metal ions via a Donnan equilibrium across a negatively charged ion-exchange membrane within a reasonable time span. The donor solution contains both ‘free’ and complexed metal ions. The concentration of the cations in the acceptor solution is either equal to the concentration of the ‘free’ cation concentration in the donor solution, or it can be calculated using simple correction factors. The optimization experiments have been performed with cadmium and copper, in the presence of various (in)organic complexing agents, at various pH values, and different salt concentrations. In multi-component systems, like the systems used for the experiments with calcium, cadmium, copper, protons and EDTA, the results are in good agreement with the speciation calculations. The transport of the different cations and anions across the membrane can be very well explained by the simplified theory presented. Effects of difference in salt concentration on metal concentrations can be corrected. For the more complicated systems with natural dissolved organic matter (e.g. humic acid) the concentrations measured in the acceptor solution are also in good agreement with the speciation calculations performed for the donor solution.
Keywords: Speciation; Donnan equilibrium; Membrane; Humic acid;

Metal affinity liquid membrane by Juan Antonio Calzado; Cristina Palet; Jan Åke Jonsson; Manuel Valiente (159-167).
A new supported liquid membrane system based on a palladium carrier (trans-benzylchloride bis-triphenylfosfine palladium(II) (BPP-Pd (II))) for the transport and pre-concentration of amino acids has been characterised. The amino acid tryptophan (Trp) has been used as a target for the membrane transport. The affinity of the anionic form of Trp for the metallic centre (Pd(II)) is the power that maintains the extraction of the amino acids. The lower pH in the receiving phase changes the speciation of the amino acid leading to its release in the receiving phase. Various parameters that characterise the Trp transport by this liquid membrane have been studied, i.e. carrier concentration, stripping agent, concentration of the stripping agent, pH in the donor phase and pre-concentration time. A valuable pre-concentration factor of 45 was achieved with an initial Trp concentration of 0.1 mM at pH 9 of the donor solution and after 40 h of running the transport experiment.
Keywords: Amino acid; Trp; Pd(II); Trans-benzylchloride bis-triphenylfosfine palladium(II); Liquid membrane;

A capillary zone electrophoresis (CZE) method for determining fluoxetine and fluvoxamine is proposed. Optimal conditions for the quantitative separation were investigated. A background electrolyte solution consisting of 40 mM borate buffer adjusted to pH 9.3, hydrodynamic injection and 8 kV of separation voltage were used, obtaining in these conditions analysis times lower than 2.5 min. Main aspects of the validation method are examined and discussed. Detection limits of 1.0 mg/l for fluoxetine and fluvoxamine were obtained. The developed method is rapid and sensitive and this is the first report on the determination of fluoxetine and fluvoxamine concentrations in pharmaceutical preparations by CZE. Furthermore, the proposed method was applied to nine pharmaceutical preparations with recoveries between 97.75 and 102.95 over their nominal contents.
Keywords: Capillary zone electrophoresis; Fluoxetine; Fluvoxamine; Selective serotonin reuptake inhibitors;

Extraction, solubility and stability of metal complexes using stainless steel supercritical fluid extraction system by Mustafa Z Özel; Keith D Bartle; Anthony A Clifford; Mark D Burford (177-184).
Conventional supercritical fluid extraction (SFE) system are constructed using stainless steel due to its relative inertness, ready availability, tensile strength and low cost. Recently, metal complex solubilities and metal extraction using chelating agents have been investigated using a stainless steel SFE system. Metal complexes are usually soluble in supercritical carbon dioxide (SC-CO2) and so this was the fluid of choice for the experiments. During the chelation step, temperature and pressure have great effect on the extraction efficiency. Using a metal salt as a sample, and hexafluoroacetylacetone (HFAcAc) as a ligand, the most efficient extraction conditions were found to be 60°C and 400 atm. However, when contaminated soil was used as a sample, extraction efficiency fell. This was thought to be related to the corrosion of the stainless steel SFE system by the ligand HFAcAc. During the 30 min extraction period, 1678 μg of iron was obtained. The corrosion of the SFE system was much greater during the soil extraction than during both the metal salt extraction and metal complex solubility experiments.
Keywords: Extraction; Solubility; Stability; Corrosion; Metal complex;

Performance enhancement in flow reversal flow injection using on-capillary detection by James Edwards; Amish Patel; Dana M Spence (185-190).
On-capillary absorbance detection is combined with flow reversal flow injection for automated determinations of glucose. The flow injection system used fused silica capillary tubing of 75 μm i.d. as the reactor tubing and a six-port injection valve to enable the sample zones to make multiple passes through a UV–Vis absorbance detector. Phenol red was used as the sample to characterize the amount of zone broadening due to the flow reversals. Results from a single zone making five passes by the detector showed a decrease in peak absorbance of only 6.1%, while the peak width (measured at half-height) increased by only 5.6%. This system was also applied to an enzyme-catalyzed kinetic determination of glucose where a calibration curve and a sample determination was performed in less than 6 min while the combined consumption of standard and sample was 300 nl.
Keywords: Flow injection; Flow reversal; Fused silica capillary; Microscale; Glucose;

This paper presents a spectrophotometric sequential injection (SI) determination of phosphate and silicate in environmental samples and cell cultivation medium using the molybdenum blue reaction. The interference of silicate in the determination of phosphate was eliminated by using a reagent composed of 5 mmol l−1 ammonium molybdate in 0.2 mol l−1 nitric acid, containing 0.25% (w/v) oxalic acid to avoid the formation of molybdosilicic acid. The interference of phosphate in the determination of silicate was avoided adding a 10% (w/v) oxalic acid solution to the reaction zone where the molybdophosphoric and molybdosilicic acids were previously formed, in order to destroy the molybdophosphoric acid. To perform this task in the single line SI system, obtaining a total sample and reagent zones penetration, it was used as a combination of sandwiching the sample zone between reagent zones and flow reversal through an auxiliary reaction coil. The method has a phosphate sampling frequency of 75 h−1, with a linear dynamic range between 0.2 and 7 mg l−1 and a detection limit of 0.1 mg l−1 PO4 3=P. For silicate, the sampling frequency is 40 h−1 with a linear dynamic range between 5 and 50 mg l−1 and a limit of detection of 1 mg l−1 SiO3 2=Si.
Keywords: Sequential injection; Phosphate; Silicate; Waters; Sediments; Cell cultivation medium;

A new method for the determination of species of antimony(III) [Sb(III)], antimony(V) [Sb(V)] and trimethylstiboxide [TMeSbO] with fluoride as modifier is introduced. A flow injection (FI) system for hydride generation was used in combination with an inductively-coupled plasma atomic emission spectrometer as detector. The pre-reduction was accomplished with potassium iodide dissolved in hydrochloric acid and the reduction with sodium borohydride. The influence of fluoride on the reduction and pre-reduction step was investigated by adding different amounts of sodium fluoride to the solvent stream. At a concentration of 100 mg/l fluoride and 1.2% potassium iodide, the hydride formation of Sb(V) and Sb(III) was suppressed below the detection limit, while TMeSbO showed no signal depression. The use of 100 mg/l fluoride without potassium iodide led to complete signal suppression for Sb(V) with apparently no influence on the signal intensity of Sb(III) and TMeSbO. The concentration of TMeSbO was measured directly, the concentrations of Sb(III) and Sb(V) were calculated on the basis of the three analyzing steps, giving detection limits and relative standard deviations of 1.1 (2.6%), 1.2 (5.3%) and 1.4 μg/l (8.1%), respectively. The method was applied to orange juice samples.
Keywords: Antimony-A; Antimony(III); Antimony(V); Trimethylstiboxide; Speciation; ICP-AES;

Characterisation of Galician (NW Spain) Ribeira Sacra wines using pattern recognition analysis by S Rebolo; R.M Peña; M.J Latorre; S Garcı́a; A.M Botana; C Herrero (211-220).
The objective of this work was to develop a classification system that would confirm the authenticity of Galician certified brand of origin (CBO) wines from the Ribeira Sacra area. Thirty-four chemical variables involving metals, volatile and phenolic compounds were determined in 39 red wines from Galicia divided into two categories: Ribeira Sacra CBO and non-Ribeira Sacra CBO. Feature selection techniques were employed to choose a set of optimally discriminating variables. Multivariate chemometric classification procedures were applied to modelling classes on the basis of the selected chemical data. The obtained results indicated good performance in terms of classification and prediction for CBO Ribeira Sacra wines in order to differentiate them from other similar wines produced in nearby geographical areas. Five key features (Fe, Li, Rb, delphinidin and epicatechin) provided sufficient information to enable classification rules to be developed for identifying wines according to their origin.
Keywords: Wine; Metals; Volatile and phenolic compounds; Pattern recognition analysis;

The application of the second most popular artificial neural networks (ANN), namely, the radial basis function (RBF) networks, have been developed for obtaining sufficient quantitative structure-formation relationships (QSFR) with improved accuracy. To this end, a data set of 17 barbiturates as guests complexing to α- and β-cyclodextrins (CDs) was examined using RBF and generalized regression neural networks (GRNN) as function approximation systems. The proposed methods led to substantial gain in both the prediction ability and the computation speed of the resulting models compared to regression models. For the development and evaluation of the ANN systems, the same (four) descriptors used by Lopata in a former study [A. Lopata, J. Pharm. Sci. 25 (1985) 777–784] were used also in the present study. Some of the proposed models diminished substantially the number of outliers, during their implementation to unseen (new) barbiturates.
Keywords: Cyclodextrin; Radial basis function networks; Multivariate analysis; Artificial neural networks; Binding constant;

Isolation and identification of phenolic compounds in Citrus sinensis by Prasan Swatsitang; Greg Tucker; Kevin Robards; Daniel Jardine (231-240).
Various methods were compared for the recovery of phenolic compounds from sweet orange. The need for careful consideration of sample preparation was demonstrated. In the case of cinnamic acids, hydroxide-treatment was essential to liberate the bound acids. Ferulic acid was the predominant phenol in alkali-treated extracts. Positive and negative ion electrospray ionisation mass spectra were systematically investigated for a range of phenols. Detection limits were significantly lower in negative ion than in positive ion mode. However, positive ion mode provided additional structural information for many of the compounds.
Keywords: Phenols; Citrus; HPLC; Mass spectrometry; Sample preparation;

Index (243-244).