J. R. Castillo

In situ preparation of overoxidized PPy/oPPD bilayer biosensors for the determination of glucose and cholesterol in serum

Overoxidized polypyrrole (PPy)/poly(o-phenylenediamine) (oPPD) bilayer biosensors have been developed with the enzymes glucose oxidase or cholesterol oxidase entrapped within an inner PPy layer onto which a non-conducting oPPD film was then electrodeposited. Electrosynthesis of the polymer layers was carried out in situ in a flow injection system with continuous circulation of a solution containing the monomer (plus the enzyme in the case of pyrrole). The analytical performance of this type of biosensor in the amperometric determination of glucose and cholesterol was compared with that of a PPy single layer sensor and their selectivity against electroactive substances frequently present in biological media was assessed by using them to determine both substrates in reference artificial serum.

An optical glucose biosensor based on derived glucose oxidase immobilised onto a sol–gel matrix

Abstract Glucose oxidase, firstly covalently bonded to a fluorescein derivative, is immobilised onto a sol–gel matrix. During the enzymatic reaction, glucose reacts with the labelled enzyme and when the oxygen dissolved in the solution is consumed, an increase in the fluorescence intensity of the labelled enzyme is observed. The time at which the variation in the fluorescence intensity is observed is directly related to the glucose concentration, and it was therefore used as the analytical parameter for glucose quantification. The protocol of sol–gel formation is also optimised. The sol–gel presents a dynamic response range from 100 to 1000 mg/l with a lifetime of 15 days.

Metal associations to microparticles, nanocolloids and macromolecules in compost leachates: Size characterization by asymmetrical flow field-flow fractionation coupled to ICP-MS

A methodological approach based on the size characterization of environmental microparticles (size larger than 1 microm), nanocolloids (1 microm to 15 nm) and macromolecules (lower than 1000 kDa) by asymmetrical flow field-flow fractionation (AsFlFFF), taking advantage of both normal and steric elution modes, is presented. The procedure was optimized to minimize the potential alteration of the size distribution and metal associations of the species characterized. Prior to separation by AsFlFFF, samples are subjected to gravitational settling of the solid suspension, followed by a centrifugation of the settled sample. The comparison between the fractograms of the settled and the centrifuged samples allows the characterization of the microparticles, which are eluted in steric mode in the AsFlFFF system. The characterization of nanocolloids and macromolecules is carried out on the centrifuged sample by applying different operational conditions under normal mode in the AsFlFFF system. A comparison with the conventional frontal filtration through 0.45 microm pore size membranes have shown that filtration removes particles below their nominal pore size, modifying the size distribution of the samples respect to the centrifugation. The methodology proposed has been applied to the size characterization of compost leachates. The contribution of these three differentiated fractions to the mobilization of metals has been determined by coupling the AsFlFFF system to an inductively coupled plasma mass spectrometer (ICP-MS).

Silicon determination by inductively coupled plasma atomic emission spectrometry after generation of volatile silicon tetrafluoride.

A method for the determination of silicon by inductively coupled plasma atomic emission spectrometry (ICP-AES) is described. The procedure is based on a discontinuous generation of volatile silicon tetrafluoride in concentrated sulphuric acid medium after injecting 125 mul of 0.1%, w/v sodium fluoride solution into 100 mul of the sample. The gaseous silicon tetrafluoride is fed directly into the ICP torch by a flow of 250 ml min(-1) Ar carrier gas. The calibration curve was linear up to at least 100 mug ml(-1) of Si(IV) and the absolute detection limit was 9.8 ng working with a solution volume of 100 mul. The relative standard deviation for six measurements of 10 mug ml(-1) of Si(IV) was 2.32%. The method was applied to the determination of silicon in water and iron ores.

The use of chemical modifiers in the determination of cadmium in sewage sludge and tin in PVC by solid sampling-graphite furnace atomic absorption spectrometry

Abstract The effects of NH 4 H 2 PO 4 and Pd as chemical modifiers in SS-GFAAS are studied in the determination of Cd in sewage sludge and of Sn in PVC samples. Whereas for Cd both modifiers act equally well whether Cd is in solution or in the solid sample, the effect of Pd on tin is different. To make the Pd modifier as effective on Sn in PVC as on Sn in solution a two-step pyrolysis must be introduced into the temperature programme. It is also shown that the contribution of the application of chemical modifiers to the final imprecision of the determination is negligible compared to that introduced by the sample inhomogeneity.

Three approaches to the development of selective bilayer amperometric biosensors for glucose by in situ electropolymerization

Three different glucose oxidase biosensors for the enzymatic determination of glucose, based on bilayer polymer coatings consisting of polypyrrole (PPy) and poly(o-phenylenediamine), were developed. The electrode substrates are Pt metal, carbon paste and an organic conducting salt (tetrathiafulvalene-tetracyanoquinodimethane), which introduces appreciable variability in the electrode construction process. The three sensors were compared with one another and with bare and PPy monolayer-coated sensors in terms of performance in the determination of glucose in a synthetic serum sample. These configurations provide improved selectivity against the interferences of electroactive species such as ascorbic acid and uric acid, frequently present in biological samples, and the differences between them can be taken advantage of in application to different kinds of samples.

Interferences in electrochemical hydride generation of hydrogen selenide

Abstract Interferences from Cu(II), Zn(II), Pt(IV), As(III) and nitrate on electrochemical hydride generation of hydrogen selenide were studied using a tubular flow-through generator, flow injection sample introduction and quartz tube atomic absorption spectrometry. Comparison with conventional chemical generation using tetrahydroborate was also performed. Lead and reticulated vitreous carbon (RVC), both in particulate form, were used as cathode materials. Signal supressions up to 60–75%, depending on the cathode material, were obtained in the presence of up to 200 mg l−1 of nitrate due to the competitive reduction of the anion. Interference from As(III) was similar in electrochemical and chemical generation, being related to the quartz tube atomization process. Zinc did not interfere up to Se/Zn ratios 1:100, whereas copper and platinum showed suppression levels up to 50% for Se/interferent ratios 1:100. Total signal suppression was observed in presence of Se/Cu ratios 1:100 when RVC cathodes were used. No memory effects were observed in any case. Scanning electron microscopy and squared wave voltametry studies supported the interference mechanism based on the decomposition of the hydride on the dispersed particles of the reduced metal.

Influence of Oxidising Agents in Lead Determination by Hydride Generation Direct Flame Atomic Absorption Spectroscopy

SummaryThe influence of Cr2O72−, MnO4−, Ce4+, H2O2 and S2O82− as oxidising agents in lead determination as volatile covalent hydride using NaBH4 is reported in this paper.The reaction conditions for every oxidising agents (pH, quantity and concentration of the oxidising agent, quantity and concentration of the NaBH4 and reaction time) are optimized, determining sensitivity and detection limit by measuring the peak height.The recovery of lead from solution has been measured by flameless atomic absorption spectroscopy.A linear relationship the logarithm of absorbance values and the redox potential of the system is obtained with a correlation coefficient of 0.999. The results and their interpretation are given in this paper.ZusammenfassungEs wird über den Einfluß der Oxidantien Cr2O7, MnO4−, Ce4+, H2O2 und S2O82− auf die Bleibestimmung als kovalentes Hydrid berichtet. Für jede oxidierende Substanz werden die Reaktionsbedingungen optimiert (pH, Menge und Konzentration des Oxidans, Menge und Konzentration von NaBH4 und Reaktionszeit), um Empfindlichkeit und Nachweisgrenze durch Peakhöhenauswertung zu bestimmen.Die Wiederfindungsrate wurde durch elektrothermale Atomabsorptionsspektrometrie von Blei in der Lösung bestimmt.Der Zusammenhang zwischen dem Logarithmus der Extinktion und dem Redoxpotential des Oxidans ergibt eine lineare Beziehung mit einem Korrelationskoeffizienten von 0.999. Die Ergebnisse und deren Interpretation werden in dieser Arbeit beschrieben.

Direct determination of copper and lead in sewage sludge by solid sampling-graphite furnace atomic absorption spectrometry — study of the interference reduction in the gaseous phase working in non-stop flow conditions

Abstract Graphite furnace atomic absorption spectrometry is most useful for the determination of metals in solid samples when samples with a slow or complicated dissolution process are used. Its application to inorganic matrixes (especially geological material), therefore, appears to be a field of great interest. However, most of the papers published to date deal with organic samples. In the present paper two different strategies for the direct determination of metals in inorganic samples are proposed depending on the relationship between the analyte atomization temperature and the vaporization temperature of the matrix. For the more complicated case where both temperatures are similar, working in non-stop flow conditions is proposed because it permits a considerable reduction of interferences in the gas phase. This way the determination of lead and copper in a sewage sludge sample was carried out. The results can be considered as satisfactory with regard to both accuracy and precision (R.S.D. of 7.8% and 5.7%, for Pb and Cu, respectively), as they are similar to previous ones obtained for mainly organic less problematic samples.

Identification of iron(III) oxides and hydroxy-oxides by voltammetry of immobilised microparticles

In this paper, we propose the use of an electroanalytical technique based on the direct oxidation or reduction of the electroactive components of the sample on the surface of the working electrode, called voltammetry of immobilised microparticles (VMPs). The sample is easily deposited on the electrode by abrasion and then the electrode is transferred to the electrochemical cell where the square wave potential scan is performed. Electroactive species showed peaks whose peak potential is related to the standard formal potentials. We applied this technique to the identification of iron oxides and hydroxy-oxides in cosmetics. To characterise and identify the iron(III) oxides and hydroxy-oxides VMP was performed in two different media: oxalic acid and hydrochloric acid, that is, a complexing and a slightly complexing media. Two electrode processes were observed. They were influenced by the media and the synthesis procedure of the oxides. The reduction peak at negative potentials ( −0.50 V in hydrochloric acid and −0.60 V in oxalic acid) is related to the direct reduction of the iron(III) oxide and it does not appear in the case of the more reactive phases (hydroxy-oxides). The peak at positive potentials (0.90 V in hydrochloric acid and 0.60 V in oxalic acid) involves the reduction of iron(III) in solution. The same electrode process were observed for binary mixtures but the peak potentials are shifted from the pure components peak potentials. This allowed us to distinguish between their mixtures. Finally, VMP was used to characterise iron oxides in cosmetic powders.