Novel voltammetric methods in heavily passivating media

The work contained in this thesis demonstrates that the introduction of power ultrasound into electrochemical systems can broaden the scope of techniques such as anodic stripping voltammetry. Analysis is facilitated by the well-documented phenomena associated with ultrasound, cavitation and acoustic streaming. Cavitation is caused by the collapse of voids created by ultrasonic compression and rarefaction of the solution and results in both localised high temperatures and pressures in the bulk solution and microjetting on the surface. Precise mechanistic details are still open to debate but there is evidence to suggest that shear forces at the interface when the electrode is flushed are responsible for removing organic material (particularly large species such as proteins) facilitating continuous cleaning and activation of the surface. Acoustic streaming has been shown to give diffusion layer thicknesses of the order of 1-10 μm depending on solvent and ultrasound power. The increased mass transport at such low diffusion layer thicknesses results in shorter preconcentration periods in electrochemical stripping voltammetry, greater sensitivity and a correspondingly lowered detection limit. Often ultrasound has been applied to highly passivating systems where conventional silent electroanalysis fails with the effect of transforming invisible or tiny voltammetric signals into large and quantitative responses suitable for sensitive and accurate analytical purposes.
In Section IIthe relative contributions of acoustic streaming and cavitationally induced microjetting to the sonovoltammetric response is assessed. Chronoamperometry at insonated electrodes of both micro and macro dimensions, and differential pulse voltammetry (DPV) were used to explore the frequency and violence of cavitational events and the nature of the diffusion layer prevailing under steady-state electrolysis. The results lead to a physical model of an insonated electrode which may be described as a steady diffusion layer a few microns thick brought about by acoustic streaming which is occasionally and randomly punctuated by a cavitational event. The frequency and violence of the event is dependent on the solvent and ultrasound power, except at very short electrode-to-horn separation where the cavitational contribution becomes substantial.
Section II concentrates on the implementation of the technique with applications to the detection of heavy metals in biofluids. In Chapter 4 sonoelectroanalysis is applied to the detection of copper bound within human blood protein and whole blood. It is shown that the enhancement of square wave anodic stripping peaks observed in ceruloplasmin and whole blood is not simply due to mass transport enhancement and cavitational cleaning effects alone but also the liberation of copper from the active sites in which it is bound prior to preconcentration. The results of a quantitative determination of total copper in two samples of whole blood were 1300 μg L-1 and 620 μg L-1, verified by independent blind analysis using atomic absorption spectroscopy (AAS).
The need for rapid analysis of environmental samples can also be fulfilled using sonoelectroanalysis. In Chapter 5 fish gill mucus is used as a non-destructive biomarker for the detection of heavy metals by sono-square wave anodic stripping voltammetry (sono-SWASV). A quantitative assessment of copper content yielded values of 16 μgL-1 and 21 μgL-1 which compared favourably with independent blind analysis by AAS. The potential of the technique for detection of other heavy metals for example lead, was also demonstrated.
Lead poisoning is recognized as a major environmental health risk and in Chapter 6 quantitative analysis of lead in artificial saliva from a realistic sputum volume, 220 μL, introduced to acetate buffer is investigated. An insonated preconcentration obviates the need for lengthy or degradative sample pretreatment by liberating the lead from the glycoproteins and other materials to which it binds in solution. Quantitative depassivation of the electrode surface by cavitational shearing maintained the analytical signal throughout the experiment where under silent conditions the signal diminished to zero with time. The detection limit in the analyte is 0.25 μg L-1. Following this proof of concept, Chapter 7 goes on to apply the technique to the quantitative determination of lead and cadmium in real human saliva. Close agreement between lead concentration determined by sono-SWASV and independent and blind ICP-MS is reported for human saliva samples with detection limits of 0.5 μg L-1 lead and 1 μg L-1 cadmium in saliva.
Section IV harnesses the benefits of acoustic emulsification. Power ultrasound is capable of forming droplets of micron dimensions with lower energy consumption than conventional emulsifiers. Chapter 8 acoustic emulsification is employed in the detection of vanillin (4-hydroxy-3-methoxybenzaldehyde) in vanilla essence using ethyl acetoacetate as a novel electrochemical and sonoelectrochemical solvent. Contrasting with silent voltammetry, ultrasound facilitates emulsification and extraction of vanillin in the extract permitting an analytical square wave voltammetric signal to be obtained. Close agreement with a blind analysis of the samples using HPLC-UV is observed with a limit of detection in the biphasic medium of 0.020 mM.
In Chapter 9 acoustic emulsification is utilized in the solvent extraction of copper. Ultrasonic emulsification of an aqueous phase containing copper ions with the N-benzoyl-N-phenyl-hydroxylamine ligand in an organic ethyl acetate phase was shown to facilitate the extraction of copper into the organic phase at 25°C. Subsequent emulsification with 1 M acid "back-extracts" or "re-strips" the copper into the aqueous phase prior to analysis via sono-SWASV. The technique necessarily removes contaminants present in the test solution since these will prefer to remain in the initial aqueous phase, or will transfer to the organic phase but are unlikely to be doubly transferred into the "clean" final aqueous phase. In Chapter 10 the technique is applied to the analysis of copper in the soft drink "Ribena® Light" with a detection limit of 2 μg L-1 and copper in blood with a detection limit of 0.16 μg L-1. In the latter case the analysis required a sample volume of 100 μL suggesting that a pinprick test may be feasible.
Finally in Section V, the analysis of copper in the presence of surfactants is investigated. Surfactant adsorption has been shown to have a passivating effect on the electrode surface during anodic stripping voltammetric measurements. Effluent, both industrial and domestic, is commonly contaminated with surfactant and electroanalysis of heavy metals is frequently precluded. It is therefore desirable to formulate a new analytical strategy in waste water. This is facilitated by controlled experiments with known amounts of anionic, cationic or neutral surfactant which also provides an ideal model system for comparing the techniques introduced in earlier Chapters. "Direct" sonoelectroanalysis in the medium of interest is compared with biphasic sono-solvent double extraction and both are appraised in the light of novel sonotrode technology.
This thesis concludes that sonoelectroanalysis is a powerful and versatile analytical tool that can be employed in an expanding range of applications. The examples given herein are pertinent to the major fields of medical diagnosis and the assessment of environmental pollution. They demonstrate that sonoelectroanalysis is a viable alternative to conventional analytical techniques which have the disadvantages of more lengthy sample pretreatment and cost. Sonoelectroanalysis has the potential to become a portable technique especially in the light of the encouraging results presented using the single probe sonotrode, an instrument which is now commercially available in collaboration with Windsor Scientific Ltd.

Advisor: Compton, R