Joanna L. Hardcastle

Anodic stripping voltammetry of copper at insonated glassy carbon-based electrodes: application to the determination of copper in beer

The suitability of ultrasound-assisted anodic stripping voltammetry (sono-ASV) for the detection of total copper content in beer using both mercury thin film and glassy carbon electrodes has been investigated. An immersion horn probe is introduced into a small thermostatted conventional three electrode cell (20 cm3) opposite the working electrode: an ex situ mercury plated Nafion®-coated mercury film electrode or a bare glassy carbon electrode. Minimal sample pre-treatment is required which consists of acidification of the beer with dilute nitric acid and out-gassing with argon. After the deposition of copper (as the metal or its amalgam) on the electrode in the presence of ultrasound, a square wave scan is employed to get the analytical signal. In the absence of ultrasound, electrode passivation by organic species and lower rates of mass transport prevent the observation of any measurable signals. In situ cavitational cleaning of the electrode by insonation maintains the electrode activity. Total copper content levels in the range of 100 to 300 µg Cu L–1 were determined by sono-ASV using both electrode substrates and showed excellent agreement with values provided by an independent method. This highlights the validity of the sono-ASV method as a useful electroanalytical technique in hostile media.

Sonoelectrochemical and sonochemical effects of cavitation: correlation with interfacial cavitation induced by 20 kHz ultrasound

Sonoelectrochemical measurements at macro-electrodes under extreme conditions with a very short distance between ultrasonic horn tip and electrode and different ultrasound intensity levels are shown to result in violent cavitation detected in form of current peaks superimposed on the average limiting current. Analysis of the current data obtained for the oxidation of ferrocene in dimethylformamide (0.1 M NBu4PF6) at a 4 mm diameter Pt disc electrode and for the reduction of Ru(NH3)6(3+) in aqueous 0.1 M KCl at a 6 mm diameter Pt disc electrode consistently indicate a change of the physicochemical nature of sonoelectrochemical processes under extreme conditions. The sonoelectrochemical measurement of the rate constant for the carbon bromide bond cleavage of a 3-bromobenzophenone radical anion electrogenerated at a glassy carbon electrode in dimethylformamide solution in the presence of power ultrasound is shown to yield evidence for a breakdown of the conventional mass transport model of a planar diffusion layer under extreme conditions. The change can be correlated to the number of current data points deviating more than 10% from the mean of the current due to violent cavitation processes superimposed onto the average limiting current. Further, a study of the sonochemical destruction of aqueous dilute cyanide solution (in 0.1 M NaOH) demonstrates a correlation between the electrochemically detected cavitation violence and the sonochemical activity. Factors that govern the violence of interfacial cavitation appear to be directly proportional to the factors that make cavitation in the bulk solution chemically efficient.

Biphasic Sonoelectroanalysis: Simultaneous Extraction from, and Determination of Vanillin in Food Flavoring

Biphasic sonoelectroanalysis is employed in the detection of vanillin (4-hydroxy-3-methoxybenzaldehyde). Ethyl acetoacetate is characterized as an electrochemical and sonoelectrochemical solvent and it is shown that quantitative electrochemical measurements can be made in this solvent of relatively low dielectric constant. The electrochemical oxidation of vanillin in this medium is shown to give oxidative linear sweep voltammetric signals sensitive to microadditions. In addition, the near reversible kinetics of vanillin in ethyl aceto-acetate facilitate sensitive square-wave voltammetry. Electrode passivation leads to typically diminishing peak heights for the same concentration of vanillin, but by employing ultrasound between each scan, peak heights are maintained by quantitative depassivation of the electrode surface. Microadditions of 0.05 mM vanillin to ethyl acetoacetate yield a linear calibration plot with r=0.9987 and a detection limit of 0.016 mM. The determination of vanillin in natural vanilla essence is then achieved by the exploitation of biphasic sonoelectrovoltammetry in aqueous ethanolic vanilla pod extract and ethyl acetoacetate. In contrast with silent voltammetry, ultrasound facilitates emulsification and extraction of vanillin in the extract permitting an analytical square-wave voltammetric signal to be obtained. Microadditions to two separate samples of vanilla essence facilitate quantification of vanillin in the extract. 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. Biphasic sonoelectroanalysis removes the need for sample degradation or a separation step, which would lengthen and complicate the analytical protocol. It can therefore be concluded that biphasic sonoelectroanalysis demonstrates an attractive alternative to currently accepted techniques.

The 20 kHz sonochemical degradation of trace cyanide and dye stuffs in aqueous media

Cyanide in alkaline aqueous solution is reported to be destroyed by a facile sonochemical process. In the presence of power ultrasound emitted from a horn probe with a frequency of 20 kHz and up to 60 W cm-2 intensity a decay in the concentration with first order kinetics is monitored by ion selective electrode techniques. Characteristic features of the process are (i) a rate constant inversely proportional to the solution volume, (ii) a decrease in rate upon changing the gas phase in the reactor from air to argon, and (iii) an increase in rate in the presence of higher intensity ultrasound. With these characteristic features the process is interpreted in terms of a sonochemical degradation initiated by a high energy intermediate such as OH produced in violent cavitation events. Further evidence for this mechanistic conclusion comes from complementary experiments employing low concentrations of dyes (alizarin and Procion Blue) in aqueous solutions. The sonochemical degradation of these dye solutions with 20 kHz power ultrasound is monitored by following the UV–VIS absorption. Characteristic features for the dye degradation process are in agreement with those observed for the cyanide sonolysis and therefore attributed to the same kind of sonochemical mechanism. The sonochemical degradation of cyanide proceeds with a rate an order of magnitude faster than that of the dye bleaching, possibly due in part to the faster rate of diffusion of cyanide compared to the larger organic dye molecules.

Sonoelectroanalysis: Ultrasonically Facilitated Liberation and Determination of Copper in Whole Blood

Sonoelectroanalysis is applied to the detection of copper bound within human blood protein and whole blood. A solid glassy carbon electrode in a face on horn-to-electrode arrangement was first used to obtain silent and insonated square-wave anodic stripping voltammetry (SWASV) of a 10 % solution of whole blood in 0.1 M HNO3 electrolyte and the detection of copper in whole blood was shown to be possible. Analogous results were seen using a solution (3.3 mg dL–1) of ceruloplasmin in 0.1 M HNO3;; in blood over 90 % of the copper is bound to this protein. It was shown that the enhancement of 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 sites in which it is bound prior to preconcentration. The determination of copper status using whole blood was verified by using the microaddition technique to determine the copper content of two sample of laked horse blood. The results were within one standard deviation of an independent blind analysis and it can be concluded that sono-SWASV presents an accurate and desirable alternative to conventional techniques due to its rapidity, lack of complex pretreatment and use of mercury free electrodes.

Sono‐Electroanalytical Determination of Lead in Saliva

Lead poisoning is recognized as a major environmental health risk and blood tests are used to assess occupational or domestic exposure to lead. Evidence has suggested that salivary lead concentration is closely related to recent lead exposure and may provide a useful and non-invasive indicator of lead status. Sono-electroanalysis has been shown to enhance detection of heavy metals in biological and environmental samples via the phenomena of acoustic streaming and cavitational shearing. This article therefore discusses the applicability of ultrasound enhanced square-wave anodic stripping voltammetry for the detection and determination of lead in saliva at an in situ plated Nafion coated mercury thin film glassy carbon electrode. Quantitative analysis of lead in artificial saliva from a realistic sputum volume, 220 μL, introduced to acetate buffer was shown to be possible. Employing an insonated preconcentration obviated the need for lengthy or degradative sample pretreatment by liberating the lead from the glycoproteins and other materials to which it binds in solution. Rapid experimental time was facilitated by short preconcentration periods of 120 s, due to the enhanced mass transport afforded by acoustic streaming. 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. Linear standard addition plots intersect the origin as predicted for samples uncontaminated with lead and exhibit R2 values between 0.988 and 0.994. The detection limit in the analyte is 0.25 μg L−1 which for a 220 μL sputum size corresponds to 38 μg L−1. With the onset of lead poisoning at a blood lead concentration of 100 μg L−1 this technique accesses biologically relevant lead levels.

Sono-electroanalysis of copper: enhanced detection and determination in the presence of surfactants

Surfactant adsorption has been shown to have a passivating effect on the electrode surface during anodic stripping voltammetric measurements. In the present work the feasibility of sono-anodic stripping analysis for the determination of copper in aqueous media contaminated with surfactant has been studied at an unmodified bare glassy carbon electrode. We illustrate the deleterious effect of three common surfactants, sodium dodecyl sulfate (SDS), dodecyl pyridinium chloride (DPC) and Triton-X 100 (TX-100) on conventional electroanalysis. The analogous sono-electroanalytical response was investigated for each surfactant at ultrasound intensities above and below the cavitation threshold. The enhancement in the stripping signal observed is attributed to the increased mass transport due to acoustic streaming and above the cavitation threshold the intensity of cavitational events is significantly increased leading to shearing of adsorbed surfactant molecules from the surface. As a result accurate analyses for SDS concentrations up to 100 ppm are possible, with analytical signals visible in solutions of SDS and TX-100 of 1000 ppm. Analysis is reported in high concentration of surfactant with use of sono-solvent double extraction. The power of this technique is clearly illustrated by the ability to obtain accurate measurements of copper concentration from starting solutions containing 1000 ppm SDS or TX-100. This was also exemplified by analysis of the low concentration 0.3 microM Cu(II) solution giving a percentage recovery of 94% in the presence of 1000 ppm SDS or TX-100.

The Electroanalytical Detection and Determination of Copper in Blood: Ultrasonically Enhanced Solvent Extraction Coupled with Electrochemical Detection by Sono-Square-Wave Stripping Voltammetry Analysis

Solvent extraction of copper using the N-benzoyl-N-phenyl-hydroxylamine ligand was employed as sample preparation for the sono-square-wave anodic stripping voltammetric (sono-SWASV) determination of total copper in diluted laked horse blood. The methodology involved sono-emulsification of the target aqueous phase with the ligand dissolved in ethyl acetate. Copper is then recovered into a fresh “clean” aqueous phase by emulsification with acid which “back-extracts” it from the organic phase. During emulsification ultrasound is helpful in maintaining a high surface area of organic to aqueous phase in contact enabling extraction to take place at 25 °C. Ultrasound also facilitates the liberation of copper from the active sites in which it is bound ensuring that slow release of bound copper from the protein ceruloplasmin does not compromise the results. During the deposition step of the analytical sono-SWASV ultrasound was effective in maintaining a clean electrode by cavitationally removing any residual ethyl acetate from the vicinity of the surface. Acoustic streaming and cavitation promoted the mass transport of copper to the surface of the electrode, facilitating measurements in solutions of low copper concentration, where silent measurements failed to yield an analytical signal. The methodology presented within required a sample volume of 100 μL giving a total copper determination of 1613 μg L−1 with a standard deviation of 25 μg L−1. The limit of detection for the clean test medium obtained using solvent extraction, 0.16 μg L−1, suggests that even lower sample volumes can be quantitatively interrogated with a pinprick test not unfeasible.

Sonoelectroanalytical Determination of Heavy Metals in Fish Gill Mucous

Fish gill mucous is used as a nondestructive biomarker for the detection of heavy metals by sono-square-wave anodic stripping voltammetry (sono-SWASV). The mucous sample was obtained by gently rinsing the rainbow trout (Salmo gairdneri) with a solution of borate buffer at pH 7. A glassy carbon electrode plated in situ with mercury was used in a face on arrangement with the ultrasonic horn to first demonstrate that voltammetric measurements can accurately be made in borate buffer. The optimum deposition potential and ultrasound power for copper from a dilute mucous solution was then determined and used to give a quantitative assessment of copper content using the microaddition technique. The results had a % RSD of less than 2 % and compared favorably with an independent blind analysis. The potential of the technique for detection of other heavy metals for example lead, was also demonstrated. We conclude that sono-SWASV of gill mucous may present a novel alternative to the destructive conventional techniques due to its accuracy, rapidity and facile methodology.