David C. Stone

Characterization of Polymer Films of Pyrrole Derivatives for Chemical Sensing by Cyclic Voltammetry, X-ray Photoelectron Spectroscopy and Vapour Sorption Studies

Eight different conducting polymer films formed from pyrrole and N-substituted pyrrole derivatives were characterized by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy. In particular, the XPS of poly[N-butylpyrrole], poly[N-(2-carboxyethyl)pyrrole], poly[N-(6-hydroxyhexyl)pyrrole] and poly[N-(6-tetrahydropyranylhexyl)pyrrole] is reported for the first time. The vapour sorption properties of these films were also examined by forming the films onto the electrodes of thickness-shear mode acoustic wave sensors. The influence of the pendant side chain is apparent in both the electrochemical behaviour, composition, doping level, morphology and the nature and extent of polymer-vapour interactions. The latter can be rationalized by consideration of vapour physical properties and solvatochromic parameters.

Selective detection of aroma components by acoustic wave sensors coated with conducting polymer films

Eight conducting polymer films of polypyrrole and its derivatives were used as sensitive and selective coatings for thickness-shear mode (TSM) acoustic wave sensors. They were applied to the detection of volatile alcohols and carbonyl compounds, which are important fish freshness determinants. The conducting polymers were synthesized and coated on TSM devices by electrochemical oxidation. The exposure of the coated TSM sensors to four compounds (pent-1-en-3-ol, oct-1-en-3-ol, nona-3,6-dien-1-ol and nona-2,6-dienal) was investigated, and a linear response with concentration found for the different aroma components. The sensor response was also found to be proportional to the film thickness. The response patterns obtained by grouping the data from the individual sensors are characteristic for each aroma molecule, and were investigated using both cluster and principal components analysis. The results demonstrate the feasibility of fish freshness determination through the use of a TSM sensor array combined with pattern recognition techniques.

Models for dispersion in flow injection analysis. Part 1. Basic requirements and study of factors affecting dispersion

The various approaches adopted for accounting for dispersion behaviour in flow injection analysis are examined and the advantages of a modelling approach are discussed. The variation of the dispersion coefficient as a function of (a) flow-rate, (b) tube length, (c) tube inner diameter and (d) method of injection obtained under typical flow injection conditions are studied and discussed. Explanations for some of the effects observed are presented in terms of molecular diffusion and convective flow patterns. The advantages of “time” injection over the more usual “slug” injection are clearly demonstrated and the use of the single well stirred tank model to describe the relationship between volume injected and dispersion coefficient under conditions of time injection are examined. The applications of this model to describing the entire curve shape for slug injection and of the potentially more versatile two-tank model are briefly introduced.

Flow cell and diffusion coefficient effects in flow injection analysis

Abstract The effects of flow cell geometry and nature of the solute on peak shape and dispersion/flow rate relationship are described. Flow cells produce significant effects caused by the finite volume samples by the light beam and the disruption of laminar flow conditions. At low flow rates, the larger the molecules, the greater the dispersion.

Models for dispersion in flow injection. Part 2. Two tanks in parallel model

The application of two models based on the stirred mixing tanks, the well stirred tank and the two tanks in parallel models, is discussed, and results are presented for flow manifolds with both flame atomic absorption spectrometric and solution spectrophotometric detectors. Both models can be applied to the description of the dispersion generated by the nebuliser/spray chamber of a flame atomic absorption spectrometer. They can also be applied to flow injection manifolds under conditions giving rise to peaks having exponential-like rise and fall curves.

Blood platelet adhesion to protein studied by on-line acoustic wave sensor.

The attachment of blood platelets to the surface of bare and protein-coated thickness-shear mode acoustic wave devices operating in a flow-through configuration has been studied. Platelets in washed from bind to the gold electrodes of such sensors, but the resulting frequency shifts are far less than predicted by the conventional mass-based model of device operation. Adherence to albumin and various types of collagen can be produced by on-line introduction of protein or by a pre-coating strategy. Differences in attachment of platelets to collagen types I and IV and the Horm variety can be detected. Platelets attached to collagen yield an interesting delayed, but reversible signal on exposure to a flowing medium of low pH. Scanning electron microscopy of sensor surfaces at various time points in this experiment reveals that originally intact platelets are eventually destroyed by the high acidity of the medium. The reversible frequency is attributed to the presence of removable platelet granular components at the sensor-liquid interface.

Effect of redox state on the response of poly-N-(2-cyanoethyl)pyrrole coated thickness-shear mode acoustic wave sensors to organic vapours

Different redox states of poly-N-(2-cyanoethyl)pyrrole (PCPY) films were prepared by electropolymerization and subsequent oxidation or reduction on the surface of thickness-shear mode (TSM) acoustic wave sensors. The resulting films were characterized by cyclic voltammetry, impedance spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. The response of the coated sensors to organic vapours consisting of 9-carbon chains with different functionalities was also studied. The observed responses were dependent on both film redox state and organic vapour properties. All films showed high sensitivity to nona-3,6-dien-1-ol, an aroma component characteristic of early degradation of fresh fish. Principal components analysis allowed all five analytes to be distinguished in the discriminant plane using only different redox state PCPY films.

Response selectivity of etched surface acoustic wave sensors

Abstract The etching of a quartz surface acoustic wave crystal is discussed in terms of its effects as a chemical sensor for gas-phase organic vapours. Experimental results are presented for a bare quartz surface and one having a covalently bound film of 3-aminopropyltriethoxysilane (APTES). The sensitivity of the prepared devices was examined using steady-state and transient adsorption measurements, and found to be enhanced when the surface was first subjected to mild etching. This has been attributed to an increase in the density of silanol groups on the surface, providing an increased number of sites for both hydrogen bonding and covalent attachment. Improved selectivity was observed for nitrobenzene over other organic vapours between etched and unetched APTES-coated devices.

Should students be graded on accuracy and precision? Assessment practices in analytical chemical education

The genesis of this article was a comment from a colleague seeking suggestions for a new experiment, who concluded, BAnd of course we’ll grade on accuracy and precision!^ My immediate reaction was, BWhy ‘of course’?^ I will be completely honest and admit that I too grade my students— at least in part—on accuracy and precision. Indeed, most analytical instructors do the same, to various extents; an informal show of hands at a recent conference revealed that only a small minority did not grade students on accuracy and precision at all. Further, most of us were also graded on accuracy and precision as students. It might therefore seem odd to question this practice, which amounts to a Brite of passage^ for most introductory analytical courses. Yet we subject analytical methods to considerable scrutiny before accepting them as standard practice; should we not do the same for our methods of assessing student performance? Accuracy (closeness of results to the true value) and precision (closeness of results to one another) are arguably the most fundamental concepts and essential skills in chemical analysis: without precision in a set of measurements, it is very hard to be confident about their accuracy; similarly, precise measurements that lack accuracy have limited utility. Grading students on their accuracy and precision, however, is not something we should do simply out of tradition. As physical scientists, we owe it both to our students and to ourselves to have good reasons for how, when, and to what extent we assign such grades should we decide to do so. This review will provide a framework formaking such decisions, drawing on the literature in the field of educational assessment. In its doing so, we will see strong parallels with the principles and practices used in quality assurance, method validation, and proficiency testing.

Sensor Response and Computational Molecular Modelling

This paper concerns the application to the sensor field of the various modes of computational exploration, representation and prediction of the nature of intermolecular interactions that lie at the heart of the response selectivity of chemical and biosensors to analyte species. Such a strategy is expected to be helpful in the understanding of response associated with existing sensor-receptor combinations, and in the possible design of new surfaces for the detection of environmental molecules. A key issue in this area is the role of shape complementarity between the receptor site and the analyte molecule and its connection to the interaction energies of instigated hydrogen bonds. We have used three distinct methods for the study of such molecular interactive systems; these are molecular mechanics and semi-empirical and ab initio quantum mechanical models. These methods applied to the problem of intermolecular interactions must accurately reproduce the molecular geometry of the separate receptor and analyte entities, and the hydrogen-bonded complex. In addition, reasonable values for the relative energies of these moieties must be available.