Analytica Chimica Acta (v.427, #1)

Sol–gel based thermal biosensor for glucose by Kumaran Ramanathan; Birgitta Rees Jönsson; Bengt Danielsson (1-10).
A sol–gel (SG) based glucose biosensor using thermometric measurement is reported. The enzymes (glucose oxidase, GOD and catalase, CAT) were entrapped on the surface of reticulated vitreous carbon cylinder (RVC cartridge) using SG as a binder. This ‘RVC cartridge’ was placed within the column of an enzyme thermistor (ET) device. Injection of various d-glucose concentrations resulted in changing the heat content of the circulating buffer, recorded as a thermometric peak by a sensitive thermistor. Independent calibration curves between 10 and 50 mM and between 0.2 and 1 mM d-glucose was obtained by plotting the d-glucose concentration versus the thermometric peak height. The sensitivity of the response was optimized to 1 ml min−1 flow rate of the buffer. The stability of the entrapped GOD/CAT stored at room temperature (25°C) or 4–10°C was 3 or 6 months, respectively. The effect of dissolved oxygen and other interferents such as acetaminophen, ascorbic acid, aspartic acid, glutamic acid, urea and uric acid, on the catalytic activity of the enzyme was also investigated. This system was employed to detect glucose in samples of fruit juice, coca cola and human blood serum.
Keywords: d-glucose; Thermometriy; Sol–gel; Glucose oxidase;

Criterion for Hill equation validity for description of biosensor calibration curves by B.I Kurganov; A.V Lobanov; I.A Borisov; A.N Reshetilov (11-19).
The applicability of well-known three-parameter Hill equation for description of calibration curves for potentiometric (detection of glucose, pesticides, urea) and amperometric (detection of surfactants, biphenyl, nitrite) biosensors has been analyzed. The criterion for validity of the Hill equation has been proposed. The sources of errors at the determination of concentration of the substance being analyzed have been considered.
Keywords: Biosensors; Hill equation; Calibration curve;

Novel solid-state ammonium ion potentiometric sensor based on zirconium titanium phosphate ion exchanger by Saad S.M Hassan; Sayed A Marei; Ibrahim H Badr; Hassan A Arida (21-28).
Preparation, characterization and applications of a novel long life ammonium ion membrane sensor based on zirconium titanium phosphate (ZTP) ion exchanger are described. The sensor displays a useful analytical response with excellent reproducibility, good precision, low detection limit and applicability over a wide range of ammonium ion concentration (1.2×10−5–1.0 M). It also offers the advantages of significantly high thermal stability and resistivity towards redox agents and strong mineral acids. Using a gas diffusion dialyzer/flow-injection system, under optimized conditions, the ZTP membrane based sensor provides a throughput of 30 samples per hour with excellent selectivity for ammonium ion over most common cations. The average recovery of 7–140 μg ml−1 NH3–N is 101.3±1.6% and the overall system has the advantages of simple construction, low cost, fast response time, reliability and automation feasibility.
Keywords: Ammonium; Ammonia; Solid state membrane sensor; Flow injection analysis; Gas dialysis; Potentiometric determination; Zirconium titanium phosphate; Inorganic ion exchangers;

With the title new luminescent ionophore MDC-MAC(O5) as host, the cation recognition properties with Li+, Na+, K+, Mg2+, Ba2+ and Pb2+ in acetonitrile were studied by fluorimetry method and the corresponding stable constants and the sensitive factors of the guest–host complexes were determined by the modified H-B equation. The recognition mechanism between MDC-MAC(O5) and metal ion was discussed based on the chemical construction of MDC-MAC(O5) and the fluorescence enhancement effect during they interaction each other. Because the carbonyl group of the coumarin participates in the complexation between the crown ether and the cation, and N electron of oxygen atom of CH3-group can promote the complexation by the way of the resonance, the stability of the complexes was increased obviously.
Keywords: 4-Methene-6,7-dimethoxycoumarin-monoaza-18-crown-6; Cation recognition; Stable constant; Recognition mechanism;

A method is presented that permits the microfabrication of glass chips for micro total analytical systems and micro chemical reactors in the general laboratory. The method does not require any specialist facilities, and for the first time provides a reliable and reproducible method for fabricating such devices without access to a clean room. The predictability of the process is achieved by utilising commercially available starting materials.
Keywords: Microfabrication; Photolithography; Miniaturization; μTAS;

The fabrication and application of a novel electrochemical detector (ED) with the new ion-exchange polymer-overoxidized polypyrrole (OPPy) film chemically modified electrode (CME) for liquid chromatography (LC) are described. The electrochemical behavior of dopamine (DA) and ascorbate (AA) at the OPPy CME is investigated by cyclic voltammetry (CV). It is found that the CME can permselective dopamine cations and exile the ascorbate anions, which could be used to determine the monoamine neurotransmitters and avoid the interference of ascorbate in liquid chromatography. The thickness of the OPPy film has an important effect on the sensitivity and selectivity for dopamine with respect to ascorbate. In liquid chromatography with electrochemical detection (LC-ED), DA, norephinephrine (NE), 5-hydroxytryptamine (5-HT), epinephrine (E) and 3,4-dihydroxyphenylacetic acid (DOPAC) have good and stable current responses at the CME. The linear ranges of five analytes are over three orders of magnitude and the limits of detection are 0.05 pmol for DA, 0.05 pmol for NE, 0.05 pmol E, 0.1 pmol for 5-HT and 1.0 pmol for DOPAC. The applications of this method coupled with microdialysis sampling for the in vivo determination of the monoamine neurotransmitters in the different areas of rat brain is satisfactory.
Keywords: Liquid chromatography; Microdialysis; Overoxidized polypyrrole; In vivo;

Titanium(IV) forms the complexes with methylthymol blue (MTB), xylenol orange and calcein in the weakly acidic solutions (pH 2.4–3.8). These complexes can be used for a highly selective and sensitive determination of titanium by adsorptive-catalytic stripping voltammetry (AdCtSV) with increase of the reduction peak current, thanks to the presence of chlorate ions. The detection limits (3σ of the blank value) obtained under the optimal experimental conditions are 1.10 nM for MTB, 0.41 nM for xylenol orange and 0.12 nM for calcein after 30, 30 and 60 s of the accumulation time, respectively. The peak current is proportional to the concentration of titanium(IV) in the range of 8.7–93 nM for MTB, 8.7–690 nM for xylenol orange and 1.7–160 nM for calcein. Elements such as iron, aluminium or molybdenum do not interfere with the determination of titanium. Described reagents were applied to the determination of titanium in crystalline quartz and silica glass samples with good results. Electrothermal atomic absorption spectrometry (ET-AAS) was used as a reference method to the AdCtSV measurements.
Keywords: Titanium; Methylthymol blue; Xylenol orange; Calcein; Adsorptive-catalytic stripping voltammetry;

In this paper, the diffusion of the angina therapeutic agent, nitroglycerin through a skin mimetic is described. The mimetic is a poly(ethylene) glycol loaded microfibre filter to which a transdermal drug patch is attached. The diffusion process is monitored using dynamic scanning Fourier transform infrared (FTIR)-photoacoustic spectrometry in which the IR beam illuminates the filter with the transdermal patch on the underside. Thus, the nitroglycerin diffuses up through the filter. Using different thermal diffusion lengths, as determined by the wavelength dependent modulation frequency of the FTIR spectrometer, the diffusion process is monitored. A convenient model is developed to provide a direct measurement of the diffusion coefficient of nitroglycerin through the poly(ethylene) glycol loaded filter.
Keywords: Diffusion coefficient; Nitroglycerin; Transdermal drug delivery; FTIR-photoacoustic spectrometry;

A rapid method for determining the iodine value, IV, of vegetable oils was developed. The method is based on using derivative Fourier transform infrared measurements. The infrared derivative spectrum of pure vegetable oils was measured between 4000–400 cm−1 and the heights of the derivative spectrum for functional group band maxima were determined. Pure vegetable oils as samples were used throughout this study. The method was used for determination of iodine value of 12 edible vegetable oils as well as castor and linseed oils. Oils with IV ranging from 10 to 190 were tested and found to give satisfactory values. Results were obtained with good precision and accuracy, typically exhibiting 5% relative standard deviation.
Keywords: FTIR spectroscopy; Fats and oils; Fourier transform infrared spectroscopy; Iodine value (IV); Lipid analysis; Oil analysis; Triglycerides;

A study of the behaviour of fuberidazole (FBZ) (2-(2′-furyl)benzimidazole), and thiabendazole (TBZ) (2-(thiazol-4-yl)benzimidazole) in a sequential injection system with a fluorimetric detector has been performed. Several variables such as pH, composition of buffers, volumes of sample and reagents, addition order, geometry of mixing coil and spectrofluorimetric parameters have been optimised. A pH 2 buffer solution was found to be optimal for both FBZ and TBZ. The calibration graphs are linear between 0.04 and 10 μg l−1 for FBZ and between 0.08 and 20 μg l−1 for TBZ with 3σ detection limits of 0.012 and 0.02 μg l−1, respectively. The relative standard deviation (n=10) was 0.3% for 1 μg l−1 of FBZ and 0.5% for 2 μg l−1 of TBZ. Resolution of the system was carried out by applying a multiple linear regression (MLR) calibration model. The method was applied to the determination of both fungicides added to natural waters. Recoveries between 96 and 106% were achieved.
Keywords: Sequential injection; Spectrofluorimetry; Fuberidazole; Thiabendazole; Water analysis; Variable-angle fluorescence spectrometry; Multiple linear regression analysis;

Flow-injection spectrophotometric determination of nicotinic acid in micellar medium of N-cetylpyridinium chloride by Elisa Capella-Peiró; Llorenç Monferrer-Pons; Celia Garcı́a-Alvarez-Coque; Josep Esteve-Romero (93-100).
A simple flow-injection (FI) method with spectrophotometric detection at 440 nm is proposed for the determination of nicotinic acid, which is based on the formation of a polymethine dye at room temperature. In the FI system, nicotinic acid is hydrolyzed with 0.2 M cyanogen bromide at pH 5 (acetic-acetate buffer) in a non-micellar medium. The glutaconic aldehyde formed is injected into a stream containing 0.1 M aniline at pH 7, in a 0.1 M micellar medium of the cationic surfactant N-cetylpyridinium chloride (NCPC), to produce the polymethine. The performance of NCPC is compared with that of other surfactants: cetyltrimethylammonium bromide (CTAB, cationic), sodium dodecyl sulphate (anionic), and Triton X-100 (non-ionic). In the presence of NCPC and CTAB, the coupling reaction increases its sensitivity in ×3.3 and ×2.6 factors, respectively. This effect is explained by the higher concentration of glutaconic aldehyde and aniline on the micelle surface due to electrostatic attraction and hydrophobic association to the cationic surfactant. The dispersion coefficient of the FI system was D=2.4, and the sampling rate, 95 samples per hour. The limit of detection was 1.2×10−6  M and the coefficient of variation 1.0 and 0.8% for 1×10−5 and 1×10−4  M nicotic acid, respectively. The optimized manifold was applied to the determination of commercial pharmaceuticals containing nicotinic acid, with recoveries in the 96.7–101% range.
Keywords: Nicotinic acid; Polymethine dye; N-cetylpyridinium chloride micellar medium; Flow-injection analysis; Spectrophotometry; Pharmaceuticals;

Techniques of solid and solution investigation such as nuclear microprobe using proton induced X-ray emission (PIXE), instrumental neutron activation analysis (INAA), inductively coupled plasma mass spectrometry (ICP-MS), total organic carbon (TOC) analysis and photon correlation spectroscopy (PCS) are associated to follow the repartition of metallic trace elements associated with humic acids (HA) colloids of different sizes. This paper illustrates the innovative character and the capabilities of these complementary techniques and confirms the associations between HA and numerous trace elements (both cations and anions), and evidences a strong affinity of the smallest colloids (<15 nm) for these elements.
Keywords: Humic substances; Colloids; Trace element; Nuclear microprobe; PIXE; NAA; ICP-MS; TOC analysis; PCS; Sequential filtration;

A technique is described for the measurement of total uronic acids in coastal seawater using a spectrophotometric method after separation and pre-concentration. Uronic acids in seawater are separated through cation-exchange and pre-concentrated by freeze drying. The concentrations of total uronic acids are determined based on a sulfamate/m-hydroxydiphenyl assay in concentrated sulfuric acid. This modified approach has been tested in estuarine and coastal water of Galveston Bay where the concentrations of total uronic acids ranged from 3.2 to 6.5 μM-C. Uronic acids accounted for 7.9–12.3% of the total carbohydrate content. These results suggest that uronic acids are one of the important compound classes of carbohydrates in seawater. The detection limit was 0.08 μM for uronic acids or 0.5 μM-C, and the precision was ≤10% for values close to the detection limit.

A new approach to the error sources in the spectrophotometric determination of total phenols in foods has been performed. The choice of the suitable phenolic standard and the influence of sugars and proteins as interfering compounds were carefully studied. The results obtained by the spectrophotometric method were compared with those found from the chromatographic method which was taken as reference method because it was free of interferences. The spectrophotometric method overestimates the phenolic content except in some fruit samples with a high polyphenolic content. Sugars did not show interference whereas protein showed a high influence on the total phenols at the concentration ranges found in the extracts. In green bean samples both methods gave the same total phenols when the interference was masked. This fact could constitute an useful way to find the real content of phenolics in foods.
Keywords: HPLC; Spectrophotometric; Phenolics; Foods;

Two multivariate methods, the classical partial least squares (PLS-1) and a modification of the novel hybrid linear analysis (HLA/XS method), are described for the simultaneous determination of rifampicin, pyrazinamide and isoniazid. A calibration set is constructed, using a combined central composite and factorial design (15 objects), in the range of concentrations comprised between 0 and 15 μg/ml for rifampicin and pyrazinamide, and 0–10 μg/ml for isoniazid. The proposed methods are satisfactorily applied to the determination of mixtures of these drugs. The precision expressed as R.S.D. (%) is calculated for both models. HLA/XS and PLS optimized calibrations are tested and compared in the analysis of several pharmaceutical formulations. The calculation of the net analytical signal using a modified HLA/XS method, allows us the determination of several figures of merit, as selectivity, sensitivity, analytical sensitivity and limit of detection of the proposed multivariate calibration. PLS-1 and HLA/XS calibration show similar ability to determine rifampicin and pyrazinamide but slightly worse results for isoniazid are obtained when using HLA/XS.
Keywords: Spectrophotometry; Multivariate analysis; PLS; HLA/XS; Rifampicin; Pyrazinamide; Isoniazid;

Standard methods for the determination of association constants by nuclear magnetic resonance spectroscopy are adapted for weak complexes that interact only by van der Waals forces. The association constants and the corresponding changes in Gibbs free energy are determined for the self-association of anthracene and the mixed association of anthracene with dodecane in deuterated methanol.
Keywords: Nuclear magnetic resonance spectroscopy; Association constants; van der Waals complexes; Anthracene; Dodecane;