Showing papers in "Mikrochimica Acta in 1995"

Conducting polymer based amperometric enzyme electrodes

Abstract: The construction and the properties of conducting-polymer based amperometric enzyme electrodes are reviewed. The main aim is to focus on the properties of conducting polymer films which are important for the construction of amperometric enzyme electrodes. Additionally, the review is focused on electron-transfer pathways between conducting-polymer integrated immobilized enzyme molecules and the modified electrode using free-diffusing redox mediators as well as direct electron transfer via the conducting-polymer wires. Possible future applications using microstructured conducting-polymer films will be discussed.

Electrochemical biosensors based on enzymes immobilized in electropolymerized films

Abstract: A review based on 135 references concerns the design and properties of electrochemical biosensors for 13 different substrates of enzymatic reactions. In the sensors discussed the enzymes are immobilized within or on the top of electropolymerized films, mostly of conducting polymers. Amperometric detection is most often used for internal electrochemical sensing.

Thermal characterization of biomass smoke particles

Abstract: In this paper we present results on characterization of filter-collected redwood (Sequoia sempevirens)-needle and eucalyptus smoke particles by thermal, optical, and solvent extraction methods. Our results show that black carbon and a significant fraction of organic carbon in biomass smoke particles have similar volatility and combustion temperatures. Combustion temperatures of both black carbon and this organic fraction critically depend on the concentrations of common constituents of biomass smoke such as Na and K. These species are also known to catalyze the combustion and therefore lower the combustion temperatures by more than 100 °C. Consequently, thermal methods that rely on a specific temperature to separate organic from black carbon may either underestimate or overestimate the black and organic carbon concentrations, depending on the amounts of Na and K and on the composition and concentration of organic material present in a sample. These results suggest that more than one method must be used to accurately determine the organic and black carbon concentrations in biomass smoke particles.

Spectral and non-spectral interferences in inductively coupled plasma mass-spectrometry

Abstract: Inductively coupled plasma mass spectrometry of environmental and biological samples is often hampered by spectral and non-spectral interferences Spectral interferences, caused by the limited resolution of the quadrupole mass spectrometer, can be eliminated in a variety of ways For their identification inspection of a signal versus carrier gas flowrate is useful Anion exchange allows the removal of most S and Cl containing compounds, which are at the origin of the majority of spectral interferences Matrix modification, for example the addition of ethanol and subsequent optimization of the gas flow rates in a number of cases enables the reduction of the interferences to insignificant values Often a mathematical correction based on isotopic signal ratios can be applied Non-spectral interferences can be divided in reversible, that is occurring while the sample is being measured, and irreversible matrix effects, that is clogging of the nebulizer and sampling orifices or deposition on the torch or in the ion lens stack The errors associated with non-spectral interferences can be eliminated by appropriate calibration procedures, adapted sample preparation or limitation of the amount of sample delivered to nebulizer, plasma and sampling devices, for example by the application of flow injection Applications of all the elimination procedures are described for the analysis of sea-water, estuarine water, soil and sewage extracts, percolate water, urine, serum and wine

On-line glucose monitoring by using microdialysis sampling and amperometric detection based on ‘wired’ glucose oxidase in carbon paste

Abstract: In-vitro on-line glucose monitoring is described, based on microdialysis sampling and amperometric detection operated in a flow-injection system. Samples were injected into a two-electrode microcell containing an Ag/AgCl quasi-reference electrode and a glucose enzyme electrode as the working electrode, operated at + 0.15 Vvs. Ag/AgCl. The enzyme electrode is constructed by mixing the ‘wired’ glucose oxidase into carbon paste. {Poly[1-vinylimidazole osmium(4,4′-dimethylbipyridine)2Cl)]}+/2+ was used to ‘wire’ the enzyme. The non-coated electrodes, cross-linked with poly(ethylene glycol) diglycidyl ether, responded linearly to glucose concentrations up to 60 mM, and were characterized by a sensitivity of 0.23 μA mM−1 cm−2, when operated in flow injection mode and of 5.4 μAmM –1 cm–2 in steady-state conditions. This sensitivity of the resulting enzyme electrode was 50% lower than that of similarly prepared but non-cross-linked electrodes. However, the cross-linked electrodes showed superior operational and storage stabilities, which were further improved by coating the electrodes with a negatively charged Eastman AQ film. An in-house designed microdialysis probe, equipped with a polysulphone cylindrical dialysis membrane, yielded a relative recovery of 50–60% at a perfusion rate of 2.5 μl/min–1 in a well stirred glucose solution. The on-line set up effectively rejected common interferences such as ascorbic acid and 4-acetaminophen when present at their physiological concentrations.

Plastic colorimetric film sensors for gaseous ammonia

Abstract: The preparation and characterization of three different plastic thin-film colorimetric sensors for gaseous ammonia is described. In the film sensors, the neutral form of a pH-sensitive dye (Bromophenol Blue, Bromocresol Green or Chlorophenol Red) was encapsulated in a plastic medium, either poly(vinyl butyral) or ethylcellulose plasticized with tributyl phosphate. Each of these film optodes gave a reproducible and reversible response towards gaseous ammonia. The sensitivity of the film sensors towards ammonia was found to be strongly dependent upon the pK a of the encapsulated dye. Thus, the film with Chlorophenol Red (pK a = 6.25), proved to be very insensitive (operating range: 0.29% < %NH3 < 100%), whereas the film with Bromophenol Blue (pK a = 4.1), was much more sensitive (operating range: 0.0003% < %NH3 < 0.11%). The sensitivity of a plastic film sensor decreased markedly with increasing operating temperature and the 90% response (15–38 s) and recovery (820-127 s) times were slow and activation-controlled.

The oxidation behaviour of niobium containing γ-TiAl based intermetallics in air and argon/oxygen

Abstract: The oxidation behaviour of γ-TiAl based alloys with different Nb contents (2–10 At.%) was investigated in air and in argon-20% oxygen at 900 °C using thermogravimetric analysis. The oxide scales were characterized by a combination of optical microscopy, SEM/EDX and X-ray diffraction analyses. Although in all studied cases the presence of niobium improves the oxidation resistance of γ-TiAl, the oxidation kinetics, scale morphology and composition in air differed strongly from that in argon-oxygen. In air the oxidation resistance increases with increasing niobium-content. In Ar/O2 the niobium dependence is far more complex because internal oxidation occurs which is favoured by the presence of niobium. SNMS analysis revealed that the differences in behaviour in the two atmospheres are related to the formation of Ti-rich nitride at the scale/alloy interface during air oxidation. The positive effect of niobium on the oxidation resistance of γ-TiAl is mainly caused by a decrease of the transport processes in the heterogeneous TiO2/Al2O3-surface scale. Nitride formation and/or niobium enrichment at the scale/alloy interface also affect the oxidation behaviour, however these factors are believed to be the result of the decreased transport processes rather than the main reason for the niobium effect.

Method comparison study on soot-selective techniques

Abstract: In a long-term field study at two locations with different air pollution levels several soot-selective measurement techniques were compared with a thermochemical method which measures non-extractable carbon (NEC) detecting the evolved CO2 by means of coulometric titration. The attenuation measurement technique (aethalometer) and the aerosol photoemission method showed good correlations to NEC for concentrations ranging from 1.6μg/m3 to 40.8μg/m3. The specific mass absorption coefficient of black carbon with respect to NEC varied between 8.4m2/g and 13.7m2/g with respect to the measurement sites, but the σ value was found to remain constant at each site independent of seasonal or meteorological variations. The ratio of photoelectric signal to NEC varied between 16 fA·(μg/m3)−1 and 33 fA·(μg/m3)−1 depending on the age of the aerosol. The diurnal variations of that ratio showed strong similarities to the traffic patterns. Additionally a slight temperature dependence of this ratio was found for the aged aerosol with a proportionality factor of − 0.35 fA·(K·μg/m3)−1. With the reflectance measurement technique (smoke shade method) reliable NEC determination was not possible for NEC concentrations ⩽ 5 μg/m3.

Supramolecular detection of solvent vapours with calixarenes: Mass-sensitive sensors, molecular mechanics and BET studies

Abstract: Chemical sensors, based on highly mass sensitive QMB or SAW devices, coated with thin layers of calixarenes, enable the detection of organic solvent vapours, especially halogenated or aromatic hydrocarbons, down to a few ppm. Force field calculations allow the tailoring of these sensor materials seeing that the predicted interaction energies between the host molecules and a large variety of analytes are linearly correlated to the measured sensor effects. These correlations and also BET adsorption analysis prove the analyte recognition properties of these calixarene coatings to be mainly based on host/guest inclusion principles.

Redox enzyme linked electrochemical sensors : theory meets practice

Abstract: Information from theoretical models of redox enzyme linked biosensors highlights the importance of membrane thickness and enzyme loading on signal response in regard to both sensitivity and reproducibility. The conclusions are substantiated by examination of practical examples in the literature and, with a view to the importance of the character of the enzyme layer and overlayers, immobilization techniques are assessed which are in current use.

Immobilization of Proteins on Gold Coated Porous Membranes Via an Activated Self-Assembled Monolayer of Thioctic Acid

Abstract: A new methodology for efficient protein (e.g., antibodies, enzymes, etc.) immobilization on microporous nylon membranes for use in a variety of bioanalytical systems is introduced. The method utilizes an activated self-assembled monolayer (SAM) of thioctic acid on gold coated forms of the membranes. Via a carbodiimide mediated reaction, the protein is anchored to the gold surface through an amide bond with the terminal carboxyl group of the adsorbed thioctic acid. The immobilization efficiency is high (∼95% for a monoclonal immunoglobulin G(IgG) and the surface bound protein appears to be stable enough to resist any displacement by other proteins in a matrix as complex as serum. Immunological activity of immobilized antibody is retained as demonstrated via use of such membrances in colorimetric ELISA for human chorionic gonadatropin (hCG). The high protein immobilization efficiency, the high tensile strength of microporous nylon membranes, and the excellent electrochemical characteristics of gold make this approach very attractive for preparing biomembranes that should be useful in affinity chromatography, electrochemical immunosensing systems, flow-through enzyme reactors, etc.

New method for determining the concentration of ionic impurities in solvent polymeric membranes

Abstract: A new method is proposed for ascertaining the amount of ionic impurities in solvent polymeric membranes of ion-selective electrodes. The method is based on determining the selectivity coefficient for ions of different valences as a function of the concentration of a lipophilized tetraphenylborate salt added to the membrane phase. Thus, the concentration of anionic impurities in commercially available poly(vinyl chloride) and of cationic ones in Tecoflex® (a polyurethane) was obtained as 0.063 ± 0.016 and 0.044 ± 0.006 mmol/kg, respectively.

Ruthenized screen-printed choline oxidase-based biosensors for measurement of anticholinesterase activity

Abstract: Plastic disposable choline biosensors based on ruthenized-carbon screen-printed electrodes were prepared and their use for monitoring organophosphorus pesticides and carbamates is described. The presence of 0.5% ruthenium on activated carbon mixed to form a simple graphite-based ink for the working electrode surface increased the sensitivity towards hydrogen peroxide. The choline biosensor is based on such an electrode coupled with choline oxidase immobilized by adsorption and was used to detect the inhibition effect of carbamates and organophosphorus pesticides on acetylcholinesterase. With the optimized procedure described (pH, buffer composition, incubation time, substrate concentration), concentrations of pesticides (Carbofuran) as low as 1 nM could be detected.

The micro water/1,2-dichloroethane interface as a transducer for creatinine assay

Abstract: Facilitated ammonium ion transfer reactions, using the crown ether dibenzo-18-crown-6, at a single microhole supported water/1,2-dichloroethane interface were used as the transducer to detect the biomolecule creatinine. The ammonium ion was produced during the enzymatic action of creatinine deiminase on creatinine. A prototype sensing device incorporating a Teflon gas permeable membrane, to exclude possible interferents, yielded a linear amperometric response to added creatinine from 20 uM to 1 mM. This range is sufficient for that required for the determination of creatinine in serum.

Sample digestion procedures for trace element determination

Abstract: A survey is given of both dry and wet procedures for matrix dissolution for the purpose of trace element determination in all kinds of materials. Various methods are compared and evaluated. With wet digestion methods special attention is paid to mixtures of strong acids and oxidants. Emphasis is laid on the completeness of these digestions, for which the use of HF is a prerequisite.

Atomic absorption spectrometric determination of mercury in soil standard reference material following microwave sample pretreatment

Abstract: Several decomposition procedures and their influence on the determination of mercury by electrothermal (ET) and cold vapour (CV) atomic absorption spectrometry (AAS) have been studied. Soil samples were decomposed by microwave digestion in closed and open vessels as well as by digestion under reflux according to German standard. The use of different acids (HNO3, HCl or aqua regia) was evaluated and compared in respect to their influence on the determination of mercury by ET AAS and CV AAS. The digestion solutions were analyzed by ET AAS with a palladium modifier and by CV AAS using SnCl2 or NaBH4, as reducing agents. The detection limits obtained with different procedures were also evaluated. For the soil containing 6.25 μg/g of Hg the ET AAS measurements were possible. In the case of lower concentration of mercury the CV AAS determination following the microwave digestion procedure with HCl or aqua regia is recommended. The accuracy of the proposed procedure was confirmed by the determination of total mercury in SRM 2711 Montana Soil.

Determination of P/Al ratio in phosphorus-doped aluminium oxide thin films by XRF, RBS and FTIR

Abstract: Phosphorus-doped aluminium oxide thin films were deposited in a flow-type ALE reactor from AlCl3, H2O and from either P2O5 or trimethyl-phosphate. Structural information of the films was obtained from Fourier transform infrared (FTIR) spectra. Rutherford backscattering spectroscopy (RBS) was used to quantitatively determine the composition of the films. The P/Al intensity ratios calculated from X-ray fluorescence (XRF) results were in a linear relation with the P/Al concentration ratios calculated from RBS results. For comparison, the intensity ratios of the phosphorus peak (P=O) at about 1250 cm−1 and the aluminium peak (Al-O) at about 950 cm−1 were determined from the IR absorption spectra. The calibration of FTIR peak intensities was done by plotting the intensity ratios of phosphorus and aluminium peaks against the P/Al concentration ratios measured by RBS. FTIR gave also a linear calibration curve with RBS but the method is less suitable for routine analysis of P/Al ratio than XRF.

Study of some palladium-containing chemical modifiers in graphite furnace atomic absorption spectrometry

Abstract: Several chemical modifiers based on palladium have been evaluated: the individual Pd(II) and the mixed modifiers Pd + Zr, Pd + W, Pd + Zr + citric acid, Pd + W + citric acid. The mechanisms by which these chemical modifiers stabilize analytes and control atomization have been suggested. Factors that might have an influence on the characteristic mass and non-spectral interferences are discussed. The advantages and limitations of the palladium-tungsten modifiers are shown.

Determination of mercury(II), methylmercury and ethylmercury in the ng/ml range with an electrochemical enzyme glucose probe

Abstract: Hg(II), methylmercury and ethylmercury have been determined with an electrochemical glucose probe. Mercury and its compounds inhibit the enzyme invertase which, in presence of its substrate, sucrose, produces glucose. When invertase is in presence of mercury its activity decreases; this causes a decrease of glucose production, which is monitored by the glucose sensor and correlated to the concentration of mercury in solution. Parameters such as pH, enzyme concentration, substrate concentration, and reaction and incubation time were optimized. Results showed that mercury, methylmercury and ethylmercury can be detected directly in aqueous solution in the range 2–10 ng/ml.

Biocatalysis and biorecognition in nonaqueous media. Some perspectives in analytical biochemistry

Abstract: Biocatalysis and, to a lesser extent, biorecognition in non-aqueous media (including organic solvents as well as supercritical fluids and gases) constitute at present an exciting research area which has already demonstrated its biotechnological potential in numerous, varied applications. Less attention, however, has been paid to its analytical possibilities, even though many advantages have been postulated and a wide range of poorly water-soluble analytes are present in samples (or waste materials) from food and drink, petrochemical, pharmaceutical, military and other industries. The main approaches, developed in recent years to exploit the use of enzymes, antibodies or antibody mimics in water-restricted environments for analytical purposes, as well as possible future directions are briefly discussed.

Determination of chelating agents and metal chelates by capillary zone electrophoresis

Abstract: Aminopolycarboxylic acids such as diethylenetriaminepentaacetic acid (DTPA) are commonly used as chelating agents in many pulp and paper industries, particularly as scavengers of metal ions which catalyze the decomposition of hydrogen peroxide used as a bleaching agent. Concern for the effect of waste DTPA in the aquatic environment has led to a need for the development of methods to determine its levels in waste water. This paper describes the determination of free DTPA and several metal-DTPA complexes in water and waste water by capillary zone electrophoresis. The optimization of separation conditions included the selection of an appropriate carrier electrolyte composition (pH, organic solvents, ion-pairing reagents) and the systematic investigations of selective complexation of free DTPA as well as metal exchange reactions for metal-DTPA complexes in order to achieve selective and sensitive direct UV detection. The determination of DTPA in waste water from a paper mill was possible in the low ppm range.

A weak acid extraction method as a tool for the metal pollution assessment in surface sediments

Abstract: A simple technique has been evaluated for the initial assessment of heavy metal pollution in coastal sediments, in order to overcome many difficulties involved in routine monitoring of such analytes. The leaching of sediment samples with a cold dilute HC1, which affects only the non-residual part of the metals, gives the “anthropogenic fingerprints” on the bottom deposits and provides more data on the extent of heavy metal pollution relatively to the total sediment analysis. The proposed technique has been applied in most Hellenic coastal regions, both polluted and unpolluted. Comparisons between the various regions with the use of an “enrichment factor” have been made, which establish the gross degree to which a sediment population has been subjected to heavy metal pollution from the overlying waters.

Evaluation of a purge-and-trap injection system for capillary gas chromatography-microwave induced plasma-atomic emission spectrometry for the determination of volatile selenium compounds in water

Abstract: A method is presented for the selective determination of the volatile selenium species dimethylselenide and dimethyldiselenide, using a commercially available purge-and-trap injection system coupled to capillary gas chromatography-microwave induced plasma-atomic emission spectrometry. The efficiency of the purging step was evaluated and the parameters affecting the purge and trap processes were optimized. The method was applied to the determination of volatile selenium compounds in lake water. Relative detection limits of 2ng/l for dimethylselenide and dimethyldiselenide, corresponding to an absolute detection limit of 10 pg, were achieved.

Oxygen-sensitive layers for optical fibre devices

Abstract: The spectroscopic features and photoreactivity with oxygen of a novel, highly lipophilic, luminescent metal complex Ru(5-odap) 3 2+ (where 5-odap stands for 5-octadecanamide-1,10-phenanthroline), are reported and compared to those of the well-known tris(1,10-phenanthroline) analogue. An array of silicone membranes, containing Ru(5-odap) 3 2+ or Ru(φ2p) 3 2+ (where φ2p stands for 4,7-diphenyl-1,10-phenanthroline) as solid-supported luminescent indicators, have been fabricated. Their (non-linear) response to oxygen was evaluated with fibre-optics as a function of their adsorbent nature (silica gel, glass, reverse-phase silica, styrene/divinyl benzene copolymer, or none), surface coverage by the deposited metal complex, and loading of dyed support into the sensitive layer. The largest oxygen sensitivity (I 0/I ca. 6 for a partial pressure of 130 Torr) is achieved with the highest amount of silica gel- or glass-supported Ru(φ2p) 3 2+ and the highest membrane loading. Ru(5-odap) 3 2+ dissolved directly into silicone yields a linear dose/response plot, but with a small slope (I 0 /I ca. 1.4 for a partial pressure of 720 Torr). These results are discussed in terms of dynamic surface quenching between the co-adsorbed indicator and oxygen molecules.

Potentiometric study using chemically modified silica gel with pyridinium as membrane for ClO4− ions

Abstract: A potentiometric sensor for the perchlorate anion was developed by mixing chemically modified silicagel with pyridinium perchlorate, with an epoxy polymer and graphite. The electrode showed Nernstian response between 1.0 × 10−2 and 1.0 × 10−3M perchlorate concentrations. The electrode showed high selectivity to this ion at solutions pH between 5.5 and 8.0. The presence of IO 4 − , NO 3 − ,Br−, IO 3 − , Cl− and SO 4 2− ions in the solutions, had only small interference in the electrode response in the range mentioned.

The use of inductively coupled plasma mass spectrometry in the production of environmental certified reference materials

Abstract: The use of inductively coupled plasma mass spectrometry (ICP-MS) in the production of environmental certified reference materials by the National Research Council of Canada is reviewed. Particular emphasis is placed on the use of isotope dilution ICP-MS. Results for fresh and saline natural waters, fish tissues and sediments are presented to illustrate the impressive capabilities of this technique.

HPLC-AAS hybrid technique for studying the speciation of trace metals (Al, Fe, Si, Hg) in biological fluids: A review of applications, recent experiences and perspectives

Abstract: A method developed using the combination of HPLC and AAS enabling both qualitative and quantitative study of the protein binding and speciation of trace metals in biological fluids at clinical relevant levels is reviewed. The whole system was made metal-free by using polymer-based columns, column holders and tubing and by the insertion of a silica-based scavenger column placed immediately before the injection valve to selectively retain any trace of aluminum and iron originating from buffer solutions and recipients. ETAAS instrumental conditions were carefully selected to eliminate interferences secondary to the salt gradient elution. Particular attention was paid to the choice of HPLC columns. Protein recoveries varied between 95 and 105% and trace metal recoveries were close to 100%. Intra-assay and inter-assay CVs of the HPLC/AAS hybrid technique were below 10%. Because of its high sensitivity, the method can be used at clinically relevant concentrations and was applied successfully to study (i) the interaction between iron and aluminum for binding to transferrin, (ii) the influence of citrate on the transferrin binding of aluminum, (iii) the speciation of silicon in the serum of dialysis patients and (iv) the toxicity of mercury compounds in cell cultures.

Catalase biosensor for the determination of hydrogen peroxide, fluoride and cyanide

Abstract: The enzyme catalase, which catalyses the decomposition of hydrogen peroxide to oxygen and water, was immobilized in a membrane by entrapping it in polyacryl amide and contacted to a Clark-type oxygen electrode. With the resulting catalase biosensor it was possible to detect the substrate hydrogen peroxide and the inhibitors fluoride and cyanide in phosphate buffer.

Evaluation of flow-injection in lead hydride generation-atomic absorption spectrometry

Abstract: The present study explores the attractiveness of combining flow-injection (FI) with lead hydride generation atomic absorption spectrometry (AAS) to improve the selectivity and sensitivity of analysis. Lead hydride was generated in three acid-oxidant media: HNO3-(NH4)2S2O8, lactic acid-K2Cr2O7 and HNO3-H2O2. The effect of chemical parameters (acid-oxidant concentration and NaBH4 concentration) was investigated and the performance of each generation medium in terms of interferences, sensitivity and detection limits was compared with that obtained in batch mode. In all cases improved sensitivity (HNO3-H2O2, 0.8 ng Pb; lactic acid-K2Cr2O7, 0.2 ng Pb; (NH4)2S2O8-HNO3, 4ng Pb) was obtained, most notably in HNO3-H2O2, which provided 12 times higher sensitivity than in batch mode and sharper absorption peaks. Furthermore, interference by Cu and Ni was lower in the proposed FI-HG system. Compared with the batch mode, about 10 to 100 times higher concentrations of interferent are tolerated in the sample. The use of FI also allows work at a lower NaBH4 concentration. The method was applied to the determination of lead in water samples with a sampling frequency of 180 samples per hour. In terms of both sensitivity and freedom from interferences, lactic acid-K2Cr2O7 was the best of the generation media tested.

Biochemical sensors: The state of the art

Abstract: The basic components of a (bio)chemical sensor and the main concepts involved in the (bio)chemical sensor methodology are considered in order to depict the state of the art of the development of research in this field, paying special attention to the evolution of the published scientific literature in analytical chemistry.