Db Hibbert

Electrodeposited polytyramine as an immobilisation matrix for enzyme biosensors

The application of an electrodeposited polytyramine film as an immobilisation matrix for the construction of enzyme biosensors is described. Glucose oxidase (used as a model enzyme) is covalently attached to free amine groups on the polytyramine film using the coupling reagents 1-ethyl-3(3-dimethylaminopropyl) carbodiimide hydrochloride and N-hydroxysuccinimide. The resultant recognition interface consisted of multilayers of GOx immobilised onto the polymer surface. This method of constructing enzyme biosensors is shown to produce a highly reproducible and stable device. The biosensor showed no loss in electrode response after four months of dry storage and exhibited only minor loss in response after 20 days of repeated use. The resultant biosensor had a linear range of 0.1-28 mM glucose and a detection limit of 0.01 mM.

Linearity and the limitations of least squares calibration

The magnitude of errors that can arise in practice from the limitations of the least squares method of calibration is estimated. Data generated from y = xn (0.7 < or = n < or = 1.3 and 1 < or = x < or = 30, or < or = 60) was analysed by least squares regression. Each y-value was then presented to the linear model and an x-value predicted. The relative errors on small x-values reached 70% of the concentration value even when r2 exceeded 0.999. Estimates of the errors on each predicted x-value, determined from the standard errors of the slope and intercept failed to reveal large errors at small x-values. Reducing the range over which linear regression is performed improved the errors. Other data sets with a heteroscedastic error distribution show that linear regression by least squares can also lead to the rejection of methods that performed sufficiently well for their application. Heteroscedastic data may be treated by repeated measurements at the lower end of the range. Data from a validation of an HPLC method for isoflavones in legumes is used to show the errors in recovery when a check sample is presented to the instrument using a calibration which satisfies the linearity tests. It is recommended that both y- and relative x-residuals are inspected. It is proposed that over-reliance on linear calibration supported by r2 may make a major contribution to large, hitherto unexplained, inter-laboratory errors.

The electrogenerated chemiluminescent behavior of hemin and its catalytic activity for the electrogenerated chemiluminescence of lucigenin

The electrogenerated chemiluminescent (ECL) behavior of hemin at a platinum electrode in the alkaline solution has been investigated in detail. Under the optimum conditions the linear response range of hemin is 1.0 x 10(-5)-1.0 x 10(-8) g ml(-1), the detection limit was 1.0 x 10(-8) g ml(-1), and the relative standard derivation for 1 x 10(-7) g ml(-1) hemin was 2.8%. It has been also found that hemin would catalyze the ECL of lucigenin at a platinum electrode in a neutral solution in the presence of hydrogen peroxide, the catalytic ECL intensity was linear with the concentration of hemin in the range of 1.0 x 10(-14)-1.0 x 10(-10) g ml(-1). IgG labeled with hemin was used to examine the ECL catalytic activity of hemin after conjugating to protein, and the results showed that hemin retained ECL catalytic activity when conjugated to protein.

Flow injection potentiometry for enzymatic assay of cholesterol with a tungsten electrode sensor

Flow injection potentiometry (FIP) for the enzymatic determination of cholesterol is reported. The assay utilises a combination of three enzymes: cholesterol esterase (CE), cholesterol oxidase (COD) and peroxidase (POD). The method is developed by the use of a tungsten wire electrode as a sensor vs. Ag/AgCl in conjunction with a redox mediator ferrocyanide. CE converts esterified cholesterol to free cholesterol, which is then oxidised by COD with hydrogen peroxide as product. Ferrocyanide is converted to ferricyanide by hydrogen peroxide, catalysed by POD, and the tungsten electrode responds to the ratio of ferricyanide to ferrocyanide. Flow injection potentiometry gave well-defined peaks for cholesterol samples with a fast response (30 s). Linear calibration was obtained from 0.05 to 3.0 mM cholesterol, with a slope of 60.2 mV/decade change in cholesterol concentration, and detection limit 0.01 mM cholesterol (S/N=3). Repeatability was 3% (CV). Interferences from commonly found species were shown to be negligible. The sensor cell is simple to construct, and it was free from surface contamination problems over long periods of use. The application of the sensor for the determination of serum cholesterol was demonstrated.

Structures and properties of La1?x , Sr x CoO3?y , prepared by freeze drying

Several samples of La1−xSrxCoO3−y (x = 1, 0.8, 0.5, 0.2) were prepared by calcining freezedried, stoichiometric mixtures of nitrates at different temperatures and for different lengths of time. Selected samples were characterized by powder X-ray diffraction, conductivity measurements, Brunauer-Emmett-Teller surface area measurement, and by electrochemical cyclic voltammetry in alkaline solutions and alkaline solutions containing methanol. The structures of each composition were complex and varied with composition and preparation conditions. There is evidence for non-stoichiometry caused by oxygen anion vacancies. For La0.5Sr0.5CoO3−y the electrochemical activity towards the oxidation of methanol is a maximum at the fully formed cubic perovskite. The oxidation currents at La0.8Sr0.2CoO3−y and La0.2Sr0.8CoO3−y go through maxima at more distorted structures.

Stripping Voltammetry of Pb(II), Cu(II), and Hg(II) at a Nafion-Coated Glassy Carbon Electrode Modified by Neutral Ionophores

The analysis of Pb2+, Cu2+ and Hg2+ by stripping voltammetry using a Nafion-coated glassy carbon electrode modified with diethyldithiocarbamic acid (DDTC) or 1,4,7,10,13,16-hexaoxacyclooctadecane(18-crown-6) is described. Metal ions were accumulated at the electrode by complexing with the ionophore, reduction and detected by differential pulse anodic stripping voltammetry. A five-fold increase in the voltammetric response was achieved at the modified electrode compared to a bare glassy electrode. Parameters and conditions such as the modifier composition, the pH of supporting electrolyte, deposition potential, deposition time, and stripping mode were optimized. For five minutes preconcentration at DDTC in 100 mM KNO3, pH 2.5, linear calibration plots in the range 0.5 μM to 50 μM were obtained with a detection limit of 10 nM. For 18-crown-6, after accumulation at open circuit for 1 minute, a linear plot between 2 and 40 μM was achieved. The lifetimes of each electrode were more than three weeks.

Continuous flow analysis of lead (II) and mercury (II) with substituted diazacrown ionophore membrane electrodes.

A novel flow cell for use with ion-selective membrane electrodes is reported in which the carrier stream is drawn through a tube that suppresses the pump noise. PVC membrane electrodes based on 7,16-dithenoyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTODC), and 7,16-di-(2-thiopheneacetyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTAODC) for lead (II), and 7,16-dithenyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTDC) and 7,16-di-(2-methylquinolyl)-1,4,10,13-tertraoxa-7,16-diazacyclootadecane (DQDC), for mercury (II) were prepared and evaluated. The linear ranges were pPb: 5.5-3.0 (DTODC) and 6.0-2.0 (DTAODC); pHg: 5.5-3.0 (DTDC) and 4.5-2.5 (DQDC). With flow rate of 3 ml min(-1) the repeatability of measurements was less than 5% RSD (n = 3). The system was applied to the determination of lead (II) and mercury (II) in spiked natural water samples.

A COMPARISON OF CLASSIFICATION IN ARTIFICIAL INTELLIGENCE, INDUCTION VERSUS A SELF-ORGANISING NEURAL NETWORKS

Abstract Three methods of classification (machine learning) were used to produce a program to choose a detector for ion chromatography (IC). The selected classification systems were: C4.5, an induction method based on an information theory algorithm; INDUCT, which is based on a probability algorithm and a self-organising neural network developed specifically for this application. They differ both in the learning strategy employed to structure the knowledge, and the representation of knowledge acquired by the system, i.e., rules, decision trees and a neural network. A database of almost 4000 cases, that covered most IC experiments reported in the chemical literature in the period 1979 to 1989, comprised the basis for the development of the system. Generally, all three algorithms performed very well for this application. They managed to induce rules, or produce a network that had about a 70% success rate for the prediction of detectors reported in the publication and over 90% success for choosing a detector that could be used for the described method. This was considered acceptable due to the nature of the problem domain and that of the training set. Each method effectively handled the very high noise levels in the training set and was able to select the relevant attributes.

A critical examination of a cryochemical method for the preparation of high surface area semiconducting powders

In the preparation of high surface area powders by the method of freeze-drying, a solution of the dissolved substance is rapidly frozen in liquid nitrogen, followed by vacuum sublimation of the ice. A computer analysis of the vacuum sublimation step showed that the heat supply to the surface ice was less than that for maximum sublimation. An infra-red lamp was used to supply extra heat and, in agreement with the computer model, the time required for sublimation of 100 g ice fell from 42 h to 3.8 h. The ice did not melt. Conductivity measurements of samples of freeze dried 10 at % Li doped NiSO4 prepared with and without the extra heat supply showed that in both cases, a very uniform distribution of Li+ in the NiSO4 lattice had been achieved. The surface areas of the freeze-dried samples were 3 times that of a sample of 10 at % Li doped NiSO4 which had been prepared by boiling to dryness a solution of LiSO4 and NiSO4.

Model forensic science

This article provides an explanation of the duties and responsibilities owed by forensic practitioners (and other expert witnesses) when preparing for and presenting evidence in criminal proceedings. It is written in the shadow of reports by the National Academy of Sciences (US), the National Institute of Standards and Technology (US), the Scottish Fingerprint Inquiry and a recent publication entitled ‘How to cross-examine forensic scientists: A guide for Lawyers’. The article examines potential responses to questions focused on the need for scientific research, validation, uncertainties, limitations and error, contextual bias and the way expert opinions are expressed in reports and oral testimony. Responses and the discussion is developed around thematics such as disclosure, transparency, epistemic modesty and impartiality derived from modern admissibility and procedure rules, codes of conduct, ethical and professional responsibilities and employment contracts. The article explains why forensic practitioners must respond to the rules and expectations of adversarial legal institutions. Simultaneously, in line with accusatorial principles, it suggests that forensic practitioners employed by the state ought to conduct themselves as model forensic scientists.