No abstract

The response mechanism of the conducting polymer poly(pyrro1e) to a selection of gases and vapours was investigated using two techniques: measurement of resistance change and mass changes using a piezoelectric quartz crystal microbalance with the objective of characterizing responses for incorporation in sensor arrays. Bromide-doped films were exposed to methanol, hexane, 2-2-dimethylbutane, ammonia and hydrogen sulfide. Polymers of different thicknesses were also exposed to methanol vapour and the response profiles were studied. The responses were all of a Fickian type except the piezoelectric signal, which exhibited an anomalous non-Fickian response to methanol. This suggests that the poly(pyrro1e) resistance changes frequently observed are partly due to one stage in the two-stage sorption perhaps involving the swelling of the polymer. It was concluded that the response mechanism of poly(pyrro1e) sensing of different gases and vapours is due to a mixed response involving electronic effects and physical effects.

The conducting polymer poly(pyrrole), electrochemically prepared and doped with anions, has been found to be a responsive coating for a piezoelectric gas detection system. Polymers doped with bromide, nitrate and sulphate ions were tested. It was found that samples of ammonia gas cause a measurable frequency decrease, interpreted as adsorption by the polymer coating of the quartz crystal; the linear range was 0.051% for mixtures of the gas in nitrogen. These signals were found to correspond to simultaneous conductivity changes of a similarly prepared poly(pyrro1e) sample, showing analogies in the two sensing mechanisms. The duality of the poly(pyrro1e) response increases the possibilities of using it as a gas sensor.

A new electrochemical detector, designed for use with conventional capillary electrophoresis systems, is described. The sensor incorporates a platinum microband array electrode and three larger electrodes on a silicon chip. The fluid flow is facilitated over the electrodes by an etched FI glass cap which allows simple connection to separation capillaries. The device was evaluated using a series of catecholamines with the lowest detectable amount of hydroquinone being 1.4 fmol (based on three times the standard deviation (30) of the background noise). Comparison with ultraviolet (UV) detection, using the same separation system, indicated that electrochemical detection was more sensitive for this range of analytes. The UV detection limit of hydroquinone was 58 fmol. Despite this increased sensitivity, the separation efficiency of the electrochemical method was reduced, as expected for the off-column detection mode.

The technique of rapid scanning amperometric detection is described. This method combines an ability to perform rapid potential scanning of the analyte, which facilitates the construction of a three-dimensional current trace at different potentials versus time, with the technique of pulsed amperometric detection, which allows the application of cleaning/activation pulses to the detection electrode. Unlike pulsed voltammetry with scanning voltammetric detection, which is described by other workers, this method employs a true microelectrode as the detector (an array of eight 5 pm band electrodes). Rapid scanning and pulsing is possible without interference from non-faradaic charging currents that may mask the faradaic signals. Catecholamine separation by high-performance liquid chromatography was selected as a model system to demonstrate the detection method in flowing streams. Measurements were made using a series of different pulsing rates; no distortion of the voltammetric signal was observed up to a pulsing rate of 20 ms, 50 Hz (effective scan rate, 4.4 V s-I). Comparison with a series of d.c. amperometric measurements at a set of different potentials demonstrates the scope of analyte information that this technique makes available. Analyte data can be obtained in both electrochemical and chromatographic domains.