Detection of metals using bi-modified glassy carbon electrodes : an insight into their performance

Abstract

Heavy metals are well known as a source of several contamination problems for humans, marine life, and animals. Their detection and quantification are extremely important to suppress negative effects such as diseases caused by them. Conventionally, spectrometric techniques such as AAS and ICPs are the most recommended due to their sensitivity and accuracy. However, the main drawbacks of those techniques are the cost and the specialized training required for its operator. Electroanalytical techniques have been an important substitute for spectrometric methods due to their cost-effectiveness and simplicity of their operation. Bi-modified electrodes introduced recently produced results that can be comparable to those obtained by spectrometric techniques. Glassy carbon has been widely used as the substrate on which Bi is electrodeposited, and in this work glassy carbon Bimodified electrodes have been explored. The roughness factor of the bare electrode has been estimated using two different methods, however, only the recalibration technique provided values in the expected range which is from 1.5 to 2.0. In this research, two heavy metals are targeted namely Zn and Pb. Trials with several reference electrodes have been done such as mercury-mercurous sulfate electrode MSE (K2SO4 saturated) and MSE (1 M H2SO4) and finally calomel (1M KCl). the latter showed the capacity for the detection of both Zn and Pb. Acetate buffer, pH 4.5, was employed since it is the most used and recommended. The study of the influence of the addition of another electrolyte to increase the ionic strength and consequently the improvement of the sensitivity has been realized in this work. NaClO4 and Na2SO4 chosen to undergo this study showed the peak shift in the case of Zn. Higher Zn peaks are mainly obtained under stirring conditions, particularly in the case of NaClO4. Among other factors, the plating solution preparation also influences the performance of the stripping analysis, Zn stripping peak heights obtained from Bi prepared in acetate buffer are lower than those from Bi prepared in HNO3. Although the irreproducibility problem was consistently observed throughout this research, the calibration curve obtained for the 1-6 µM range for both Pb and Zn requires a series of treatments until it reaches the one that fulfills the requirements, for Zn two calibration curves were produced, a drop in sensitivity is recorded on the second, therefore the first and an average curve are maintained for sample calculations showing r= 0.979 and r= 0.943 respectively. The detection of Pb has been extremely difficult due to the issue of multiple peaks which was addressed by applying an extensive electrochemical cleaning. The calibration curve generated presented a correlation factor of r = 0.969. Real samples were used to fulfil the purpose of this work, natural and tap water from different sources are used. Samples analysed have not shown any Pb peak, only Zn peaks could be detected. Zn stripping peaks have suffered some shifts compared to the position of the peaks from the standard solution. The concentration of Zn(II) was estimated based on the first and average calibration curves respectively, for tap water: Stanton Street presented 13.5±6.8 and 14.3±7.2 µg/L, and Morpeth Street 24.6±8.3 and 25.7±8.7 µg/L, for natural water: Blaydon Bridge 5.9±0.5 and 6.3±0.5 µg/L and Blaydon Burn 14.5±3.5 to 15.4±3.7 µg/L The results obtained from the electroanalytical technique through calibration curve calculation technique are comparable to the ones obtained in standard addition, ICP-OES and ICP-MS. The matrix effect is observed as the differences in charges registered during calibration curves experiments are extremely lower than in standard addition. Additionally, the LOD and LOQ were estimated for Pb as 1.17 µg/L and 3.56 µg/L, and for Zn as 0.252 µg/L and 0.764 µg/L respectively, using the regression parameters and the conditions of the sample applied in this research work.