Advances in Stripping Analysis of Metals

ANASTASIOS ECONOMOU AND CHRISTOS KOKKINOS

1.1 Introduction

Electrochemical stripping analysis (SA) has been established as a successful trace analysis technique for more than three decades. Stripping analysis is a general term that encompasses a wide family of electroanalytical techniques that make use of a "preconcentration" step of the analyte on the surface of the working electrode. In SA, the preconcentration step is followed by the detection step in which the accumulated analyte is detected and quantified by means of a voltammetric or chronopotentiometric scan. While in solution-phase electrochemical detection the redox signal is determined by the rate of mass transport of the electroactive analyte to the electrode surface, in SA the redox signal is determined by the amount of the accumulated analyte. Therefore, the accumulation step is responsible for the high sensitivity of SA whereas the different potential methods of accumulation of the analyte on the working electrode (discussed in Section 1.2) and the multi-parametric nature of SA provide versatility, wide applicability and enhanced selectivity. Typical metals and semi-metals that can be determined with SA are listed in Table 1.1. The theory and practice of SA are described in specialized monographs and general review articles.

A brief account of the birth and gradual evolution of SA is provided in earlier work. SA was first conceptualized and applied in 1931 by Zbinden to determine low concentrations of copper at a platinum electrode. However, not until the 1950s was SA revisited when the hanging mercury drop electrode was developed by Kemula. In the same decade, the remarkable sensitivity of SA was stressed by many workers and new types of thin mercury-film electrodes were introduced. In the 1960s, the theory for SA and pulsed potential–time waveforms was developed. The commercial availability of low cost multi-purpose apparatus and the expanding use of SA to many new analytical applications took place in the 1970s. The development of non-electrolytic preconcentration approaches and of automated flow-through manifolds, together with the introduction of microelectrodes for SA, dominated the 1980s and 1990s. Recent advances in SA are concentrated on research on new eco-friendly electrode materials, the extended use of microfabrication technologies and miniaturization and the development of integrated devices for on-site monitoring. Therefore, this chapter will not focus on specific applications but will try to highlight developments in sensor types, fabrication and materials.

1.2 The Principle of Stripping Analysis

The distinctive feature of SA in comparison to other electroanalytical techniques is the accumulation/preconcentration step. In most applications involving analysis of metals, preconcentration is carried out with electrolysis of the target metal cations to the respective metals (and some special cases to metal oxides) and deposition on the surface of the working electrode (Figure 1.1A). Following accumulation, the accumulated metals are oxidized (or the metal oxides reduced) to the respective cations and stripped back into the solution. Depending on the nature of the working electrode surface, the deposited metal can form a thin film, an amalgam or an alloy with the electrode. Reduction of metals on inert electrodes (such as carbon, platinum or boron-doped diamond) proceeds via the formation of a monolayer of the target metal (underpotential deposition, UPD) followed by the deposition of the bulk-phase metal. Although simple and potentially useful, electrolytic preconcentration on inert solid electrodes is characterized by multiple stripping peaks that are hard to interpret. This is due to the easy formation of intermetallic compounds on the electrode surface and to the different oxidation (stripping) potentials associated with the different states of the metal deposits. Amalgams are typically generated when heavy metals deposit on mercury electrodes or during the deposition of mercury on gold electrodes. Finally, alloys are formed when bismuth, tin or antimony electrodes are used for electrolytic accumulation of alloy-forming metals. Amalgamation and alloying facilitate the electrolytic deposition and stripping of metals and lead to stripping signals that are easier to interpret. Following electrolytic accumulation, the stripping step can be performed by an anodic voltammetric scan, a constant oxidation current or by a chemical oxidizing agent.

Non-electrolytic accumulation can be alternatively employed for metals that are not amenable to electrolytic deposition, are not readily oxidized during the stripping step or produce overlapping oxidation stripping peaks with other target or interfering species when accumulated by electrolysis. The adsorptive mode of accumulation involves addition of a selective complexing ligand (such as dimethylglyoxime, catechol, 8-hydroxyquinoline) with surface-active properties in the sample (Figure 1.1B-a). The metal cations form a complex with the ligand and the complex is then physisorbed on the electrode surface. The stripping step is usually based on the reduction of the metal cation in the accumulated complex but the reduction of the ligand or catalytic effects can be also exploited. A second strategy is based on the utilization of chemically-modified electrodes. In this case, the working electrode is bulk- or surface-modified with a suitable agent that can selectively bind the target metal cation. In the context of SA of metals, accumulation at chemically-modified electrodes can proceed by electrostatic interactions (with a clay or ion-exchanger modifier) or complexation (with a complexing ligand as the modifier). When the electrode is immersed in the sample, the modifier on the electrode interacts and binds with the target metal cations, which are accumulated on the electrode surface in their native oxidation state (Figure 1.1B-b). In the simplest scenario, the stripping step can be a reductive scan in which the accumulated metal cations are reduced. A more complex protocol involves potentiostatic reduction of the accumulated metal cations followed by an anodic stripping step in which the target metals are oxidized. Again, stripping can be performed by either a voltammetric scan or a (reductive or oxidizing) constant current.

The nomenclature in SA is not standardized and usually more or less established empirical abbreviations define a technique by reference to the accumulation mode, the stripping step or both. To avoid the confusion caused by this practice, the guidelines proposed earlier are adopted here. When voltammetry is used in the stripping step, the terms anodic stripping voltammetry (ASV, anodic scan) and cathodic stripping voltammetry (CSV, cathodic scan) are used. In ASV and CSV, the peak-shaped current–potential (i–E) response is recorded and the oxidation peak current is measured and related to the respective metal concentration in the sample. The voltammetric scan is normally performed in the differential-pulse (DP) or the square-wave (SW) mode, which offer better rejection of the background current and lower limits of detection. When the stripping step is performed by an oxidant in solution or a constant current (irrespective of its polarity), the term stripping chronopotentiometry (SCP) is used. In SCP, the potential of the working electrode is recorded with respect to time as the target analytes are oxidized (by the application of current or chemically) and the residence time (time during which the potential of the working electrode remains stable) is related to the respective metal concentration in the sample. In modern instruments, the response is converted into a peak-shape by plotting the differential dt/dE with respect to potential. The relative strengths of the potentiometric and voltammetric stripping modes have been reviewed. In both cases, the mode of accumulation of the target analyte(s) should be explicitly defined as electrolytic or nonelectrolytic.

Some more specialized variations of SA, particularly useful for metal speciation, include the "competitive ligand equilibration–adsorptive cathodic stripping voltammetry" and the "absence of gradients and Nernstian equilibrium stripping (ANGES)" approaches. Abrasive SA is widely used for the elemental analysis of solid samples of archeological or forensic interest.

In SA, the sensitivity critically depends on the amount of the analyte accumulated onto the surface of the electrode during the preconcentration step and, therefore, on the rate of mass transport. Efficient mass transfer of the analyte to the surface of the working electrodes during the preconcentration step is normally carried out under conditions of forced convection by mechanical stirring of the solution or rotation of the electrode. However, in addition to these conventional methods of enhancing the mass transport efficiency, some unconventional techniques to enhance mass transfer (ultrasound, microwaves or heating of the working electrode) have been developed. The principles, enhancing mechanisms, instrumentation and applications of these approaches have been reviewed. In "sonoelectrochemistry", ultrasound is applied with frequencies between 20 and 100 kHz and power densities from ca. 8 to 500 W cm-2 The high energy density transferred by the sound wave promotes turbulent solution flow and cavitational effects that lead to reduction of the diffusion layer thickness and in situ cleaning/activation of the electrode. Heating of electrodes causes heat to stream out of the heated surface and warm up the solution near the electrode. Density gradients between hot and cold regions induce laminar flow. This "micro-stirring" effect is more efficient than mechanical stirring and can be exploited during the accumulation step in SA. Electrode heating can be achieved by means of a high frequency AC current or microwave heating. Finally, application of constant or high-frequency magnetic fields have been reported for SA.

1.3 Advances in Electrodes, Sensors and Devices

1.3.1 Carbon Electrodes

Carbon still remains one of the most important and widely used electrode materials in SA. Applications of carbon-based electrodes in SA of metals have been reviewed previously. Glassy carbon, impregnated graphite, carbon paste, carbon fibres, carbon-containing composite materials, carbon nano-materials or screen-printed inks can be used; graphite ink screen-printed electrodes will be covered in Section 1.3.2.

Glassy carbon (GC) is still a widely used material in SA. Since the sensitivity of bare GC in the SA of metals is low, the GC surface is usually subject to preliminary (ex situ) surface modification and/or in situ modification to achieve the desired surface properties. Thin films of metals, conducting polymers, selective organic reagents, clay minerals and ion-exchangers have been reported for surface modification of GC. Occasionally, more than one modifying layers is used. Metal films (mercury, bismuth, gold, antimony, copper) facilitate the accumulation of metal cations and the stripping procedure of deposited metals. Conducting polymers (polyaniline, cellulose acetate, Nafion) can provide selectivity against macromolecules in complex samples and may serve as cation exchangers, thus facilitating the accumulation of metals. Organic ligands such as crown ethers and EDTA (ethylene-diaminetetraacetic acid) (sometimes immobilized in polymers) are used to fabricate chemically-modified electrodes suitable for non-electrolytic accumulation of metal cations.

Carbon paste electrodes (CPEs) are made by homogenizing graphite and a water-immiscible binding liquid (parafin, silicon oil, tricresyl phosphate). These CPEs are suitable for in situ or ex situ surface modification with metal films (mercury, bismuth, antimony) or metal oxides (which are converted into the respective metals by reduction). CPEs are also very convenient for bulk modification by incorporating organic ligands (crown compounds, calixarenes), clays (vermiculite and montmorillonite) and ion exchangers (resins) within the paste material in order to facilitate metal preconcentration.

Carbon composite electrodes are made of graphite, carbon, glassy, carbon or diamond powders mixed with polymeric binders such as epoxy resins, methacrylate, silicon, styrene–acrylonitrile and polyester. Although there are some reports of unmodified composite electrodes in SA of metals, most applications involve bulk or surface modification of composite materials by metal films or organic ligands that lead to higher sensitivity and better selectivity.

Nanoscale carbon-based materials (single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), fullerenes and graphene) exhibit many characteristics that make them ideal as electrode materials. Due to their strong adsorption ability, nanoscale carbon-based materials can be used as sorbents for metal ions. Therefore, there are many recent reports of the detection of multiple heavy metal ions by SA following a sorption/preconcentration step on nanoscale carbon-based electrodes either unmodified or modified with chelators, metal films or clays.

Boron-doped diamond (BDD) consists of a diamond lattice p-type doped with boron typically to 1020 cm-3 (which corresponds to approximately 1 boron atom per 1000 carbon atoms) ensuring good conductivity and electrochemical properties. Over the last 15 years diamond electrodes have been used for a wide range of electrochemical applications and one of the main areas on which research has focused is the use of unmodified or metal film-modified boron-doped diamond as a mercury-free alternative for analysis using SA. Due to their inertness, BDD electrodes are compatible with ultrasound-, microwave-, heating- and laser-enhanced deposition in SA.

1.3.2 Screen-Printed Electrodes

The fabrication of screen-printed electrodes (SPEs) involves squeezing the printing material (an ink paste) through a screen mesh (which defines the electrode's geometrical pattern) onto an electrically insulating substrate. The advantages of screen printing as a method for electrode fabrication are the low cost (in terms of the capital cost of the printing apparatus and the cost of consumables), the flexibility in selecting and modifying the electrode material, the potential for miniaturization and the scope for mass production of disposable sensors. SPEs have been widely used in SA of metals and their fabrication and applications have been covered in comprehensive reviews. The printing material is normally some type of graphite-containing ink but gold, platinum and silver SPEs can be produced by using ink loaded with these metals. Additionally, screen-printed microelectrodes or microelectrode arrays with various configurations can also be prepared by different technologies. Bare SPEs have been reported with some success but these are normally of limited utility due to the complicated voltammo-grams obtained. SPEs covered with mercury films and, more recently, with bismuth and antimony films are the most popular such sensors combined with electrolytic ASV and SCP. SPEs made by using gold-containing ink or coated with gold are also frequently used for the determination of mercury and arsenic while silver and platinum SPEs have been occasionally used. Numerous applications employing chemically-modified SPEs have been reported for non-electrolytic accumulation of metals. A chelating ligand can be incorporated on the surface of the SPE by drop-coating or into the bulk of the SPE by mixing in with the carbon ink prior to printing.

1.3.3 Solid Amalgam Electrodes

Recent research has indicated that solid amalgam electrodes can, in many cases, successfully substitute mercury electrodes in SA. Amalgam electrodes are essentially solid electrodes consisting of an amalgam of mercury with another metal (such as silver) or of an amalgam dispersed in a conductive polymer or pasting liquid. The classification, fabrication methods and analytical applications of amalgam electrodes in SA of metals have been extensively reviewed. Amalgams are claimed to be non-toxic since the mercury contained in the amalgam is not readily released. The main challenges in developing solid amalgam electrodes approaching the performance of their mercury counterparts are: (a) to obtain a sufficiently wide cathodic potential window (since the presence of various metals in the amalgam normally causes a reduction of the cathodic overpotential of mercury) and (b) to achieve a reproducible and stable surface. In practical terms, silver amalgam electrodes in conjunction with ASV and electrolytic deposition are the most widely used for SA of metals.