High flow-rate cells for continuous monitoring of low concentrations of electroactive species by polarography and stripping voltammetry at the static mercury drop electrode

A high-capacity flow-through cell which can be used at a maximum flow rate of 300 ml min^-1 has been developed for continuous monitoring of electroactive substances. The cell is compatible with the recently developed static mercury drop electrode. Comparative studies with a cell employing a conventional dropping mercury electrode are described. A wide range of polarographic techniques is applied, and it is demonstrated that the static mercury drop electrode improves the limits of detection, that laminar flow conditions are essential for low noise levels of operation, and that solution flow through a sulphite bed is a more effective method of oxygen removal than nitrogen bubbling. The combination of a microprocessor-controlled polarographic system, static mercury drop electrode and high-volume flow cell is very versatile for the determination of trace levels of electroactive species in flow streams. Preliminary results on anodic stripping voltammetry in flow streams are reported.     

An electrochemical detector with a dropping mercury electrode for high-performance liquid chromatography

The electrochemical flow-through cell described has an active volume of less than 1 μl, and incorporates a dropping mercury electrode with drop times of about 0.05 s. Its performance as a detector for high-performance liquid chromatography is assessed for p-nitrophenol and nitrobenzene. The detection limits are 4–5 ng; the relative standard deviation of the peak height is better than 5% for the range 5–150 ng. The dependence of the response on flow rate, mercury pressure and drop time is described.     

Automated square-wave anodic-stripping voltammetry with a flow-through cell and matrix exchange

An extremely sensitive and versatile instrument is described that performs square-wave anodic-stripping voltammetry. The instrument incorporates a flow-through cell and is capable of changing the solution matrix between the deposition and stripping steps. The working electrode is a static mercury drop electrode constructed in the authors' laboratories. The entire system is controlled by a microcomputer that allows the usual variation of the square-wave parameters, as well as setting of the initial and final scan potentials, the deposition time, the scan-rate, the instrument sensitivity and the drop size. To show the performance of the instrument, calibration graphs for Cd in the ranges 0.2-40 and 0.1-1 ng/ml are described and the reproducibility of the drop is discussed. Analysis of a NaCl sample for Cd is given as an example of application of the method.     

Automated polarographic analysis : Part I. Design of flow-through cells and deaeration unit

The design of polarographic flow-through cells for automated analysis of discrete samples is discussed. A simple way of fitting the DME capillary into the cell is described, and the problems associated with the use of a mercury pool counter electrode are discussed. A platinum wire or gauze counter electrode downstream from the DME is more satisfactory. The reference electrode may be placed outside the flow cell, e.g. in the mercury waste bottle. A new method for the deaeration of sample solutions is also described; argon is bubbled through the solutions in the sample cups just before aspiration. The effect of various experimental parameters on the deaeration is described. Pump stoppage during sample shifts is advantageous. The adaptation of the deaeration method to commercial samplers is explained. The application of the proposed automated system for the determination of chlordiazepoxide is discussed.     

Flow-through voltammetry (FTV) with the hanging mercury drop electrode (HMDE)

A flow-through cell with integrated hanging mercury drop electrode for voltammetric, inverse voltammetric, adsorptive voltammetric, and chronoamperometric methods is described. HMDE droplets are automatically formed and renewed. By optimizing the flow-channel the sensitivity could be increased approximately five times in comparison with batch methods using the same deposition conditions.Dedicated to Professor Dr. Rolf Neeb on the occasion of his 65th birthday     

The determination of copper,lead and cadmium in sea water by differential pulse anodic stripping voltammetry

The effect of various instrumental parameters is investigated and optimized conditions established. The results are in accordance with the theory of differential pulse anodic stripping voltammetry. Both a hanging mercury drop electrode, and a rotating glassy carbon electrode mercury plated in situ were used. The best detection limit is obtained with the mercury film electrode, but the hanging mercury drop electrode is more reproducible. The differential pulse stripping technique is compared to linear sweep stripping, and increased sensitivity and better peak separation is demonstrated for the former technique, particularly when a hanging mercury drop electrode is used. However, the differential pulse technique will also improve the detection limit for a mercury film electrode, if the electrode has a non-ideal response with a corresponding high background current.     

In situ voltammetric measurement of trace elements in lakes and oceans

A submersible probe with a flow-through cell allowing in situ voltammetric measurements by means of either a mercury film or a mercury drop electrode has been developed. The various possibilities of voltammetric measurements in the presence of dissolved oxygen have been reviewed and tested. Optimum conditions for the determination of trace metals in oxygenated waters have been found. In situ determinations of Mn(II) in anoxic lake water and of trace metals (Cu²⁺, Pb²⁺, Cd²⁺, Zn²⁺) in oxygen-saturated sea water are reported.     

Thallium Determination at the Single Picomole per Liter Level by Flow‐Injection Differential‐Pulse Anodic Stripping Voltammetry

The flow‐injection differential‐pulse anodic stripping voltammetric (FI‐DP‐ASV) procedure has been developed for the determination of thallium concentration of the order of the single pM. Due to the analyte preconcentration at the electrode, DPASV is among the leaders in competition for achieving a detection limit as low as possible. The enrichment factor can be easily regulated by the duration of the preconcentration time. A DPASV measurement in a flow‐injection system enables the circulation of an analyzed sample and a medium exchange when preconcentration is completed. This approach ensures significant improvement of a background current. The measurements were performed in a cell of the wall‐jet type containing the mercury film electrode. One hour preconcentration was used for the determination of the lowest thallium concentrations. The developed method provides the opportunity to determine thallium in real environmental samples at the single pM level with the detection limit equal to 0.25 pM and RSD equal to 8.2%. This is by more than one order of magnitude better than the lowest detection limit reported in the literature (5 pM). Additionally, the medium exchange resulted in the improvement of the measuring accuracy, which was evidenced by the application of a certified reference material.     

The flat surfaced membrane coated mercury electrode as analytical tool in the continuous voltammetric analysis

Specific analytical problems often arise which can be easily solved if a mercury electrode of constant surface area is used as voltammetric sensor. In this work a membrane covered mercury electrode was prepared and used in solving various analytical tasks. One of them was the determination of water hardness. A sample injection method was developed applying EDTA reagent and a flow-through analytical system. Two different voltammetric enzyme electrodes (measuring glucose and L-amino acid) were prepared by using the membrane covered mercury electrode as base sensor. The application of the enzyme electrode in flow-through circumstances was proved to be very advantageous in determining selectively L-DOPA in the presence of the D-form.     

2.1 Anorganische industrielle Produkte

Summary Digestion-free determination of trace-metals (Zn, Cd, Pb, Cu) in beverages by inverse voltammetry in a flow-through cell equipped with a mercury film-electrode is described. Optimal enrichment potentials are obtained by inspection of the pseudopolarograms of the elements in the original diluted sample solution. After the deposition step in the untreated sample solution stripping is effected in a proper supporting electrolyte after medium exchange. Values obtained by this techniques with various samples compare well with those obtained by usual procedures including wet digestion of the samples.     

Digestion-free determination of heavy metals (Pb, Cd, Cu) in honey using anodic stripping differential pulse voltammetry and potentiometric stripping analysis

Experiments have been carried out to assess the potential of differential pulse voltammetry and potential stripping analysis for determining Pb, Cu and Cd directly in dissolved honey samples using a flow-through cell. With the hanging mercury drop electrode Pb alone can be determined only if the electrode is first modified in-situ with Triton X 100 to increase the separation between the Pb peak and a broad, interfering adsorption peak which overlaps the Cu peak. If the (more sensitive) thin film mercury electrode is used the interference encountered is less so also Cu and Cd can be determined. With potentiometric stripping analysis Cu and Pb can be determined using normal procedures. The determination of Cd, however, can only be carried out if the concentration of the oxidizing agent [Hg(II)] in solution is decreased. A good agreement has been obtained between the values found and those obtained after high pressure digestion of the samples.     

Determination of cadmium and lead in the low ng/l range by stripping potentiometry employing medium exchange in batch mode and multiple stripping in a hanging stripping medium drop

A novel three-in-one electrode assembly, permitting medium exchange under a controlled potential in batch mode by exploiting electrolytic conductance through a drop hanging under the electrode, has been used for the simultaneous determination of cadmium(II) and lead(II) at trace levels. Potentiostatic reduction and amalgamation of these ions are carried out in the sample and the subsequent stripping, in the stripping potentiometry mode, in a drop of a suitable medium. Several such media have been investigated, a mixture of 2.5M acetic acid and 7.5M ammonium acetate having been found to be the most suitable with respect to sensitivity, potential resolution and trace metal purity. The quiescent conditions in the drop of medium facilitated multiple stripping and thus increased sensitivity. Detection limits for cadmium(II) and lead(II) were found to be around 0.5 ng/l. (5 and 2.5pM) for an electrolysis time of 10 min. The relative precision at the concentration level 20 ng/l. was 6.4% for cadmium and 5.4% for lead. The procedure has been used for the determination of cadmium(II) and lead(II) in reference seawater samples.     

Combination of flow injection analysis and voltammetry

The flow injection analyzer for students, teaching and research (FIAstar) is used in conjunction with a new micropool mercury flow-through electrode, to explore the possibilities offered by using d.c. polarography, rapid scan and amperometric titrations on a dispersed sample zone in motion. The work outlines the principles of the use of a scanning detector for investigation of the concentration gradients and chemical reactions taking place during the dispersion of sample solution within a carrier stream. The data, which are collected in real time, are displayed in three-dimensional diagrams.     

Comparison of different flow-through adsorptive stripping voltammetric methods for the determination of molybdenum(VI)

Four different complexing reagents namely chloranilic acid, oxine, tropolone, and cupferron were applied for AdSV determinations of molybdenum. The parameters for the determination using a flow-through cell with a hanging mercury drop (HMDE) as working electrode were examined systematically for all four systems and evaluated. Cyclic voltammograms were recorded to examine the electrode reaction, alternating current (AC) voltammetry was used to determine adsorption processes. The comparison includes sensitivity, detection limit, linear concentration range, the susceptibility to interference by organic compounds or foreign ions, and the applicability to sea and tap water samples. The interpretation of the electrode reaction mechanism for the reduction of the Mo cupferron complex published from Jiao et al. [10] was improved. Received: 12 February 1999 / Revised: 10 May 1999 / Accepted: 14 May 1999     

Comparison of different flow-through adsorptive stripping voltammetric methods for the determination of molybdenum(VI)

Four different complexing reagents namely chloranilic acid, oxine, tropolone, and cupferron were applied for AdSV determinations of molybdenum. The parameters for the determination using a flow-through cell with a hanging mercury drop (HMDE) as working electrode were examined systematically for all four systems and evaluated. Cyclic voltammograms were recorded to examine the electrode reaction, alternating current (AC) voltammetry was used to determine adsorption processes. The comparison includes sensitivity, detection limit, linear concentration range, the susceptibility to interference by organic compounds or foreign ions, and the applicability to sea and tap water samples. The interpretation of the electrode reaction mechanism for the reduction of the Mo cupferron complex published from Jiao et al. [10] was improved. Received: 12 February 1999 / Revised: 10 May 1999 / Accepted: 14 May 1999     

Determination of Co(II) by chemiluminescence after in situ electrochemical pre-separation on a flow-through mercury film electrode

A novel method of electrochemical pre-separation of Co(II) before detection by chemiluminescence is reported together with the associated instrumentation. The Co(II) ions were selectively pre-separated on a mercury film electrode (MFE) by on-line reduction, then the accumulated metal was oxidised and selectively stripped back into the flowing solution as Co(II). These secondary ions were quantified as a result of their catalytic activity on the chemiluminescent reaction between luminol and hydrogen peroxide that was also induced on-line. The whole sequence was carried out in an automated flow-through system, in which the electrochemical pre-separation of metals was performed in either continuous flow or flow injection analysis (FIA) regimes. The scope of the method, both in terms of selectivity and sensitivity, has been demonstrated and the quantitative determination of Co(II) by the proposed method has been investigated. For a period of continuous flow pre-separation of 4 min, the calibration curve for Co(II) was linear up to a concentration of 100 micrograms l-1, the relative standard deviation was 4% at the 20 micrograms l-1 level and the limit of detection was 0.5 microgram l-1 (at the 3 sigma level). The method was applied to the determination of the cobalt content in a high purity iron sample.     

Differential pulse polarography and voltammetry with a microprocessor-controlled polarograph and a pressurized mercury electrode

The performance of a microprocessor-controlled polarograph with a pressurized mercury electrode system has been evaluated. For the technique of differential pulse polarography, the theory applying to the pressurized mercury electrode in the dropping mercury format is shown to be the same as for a conventional gravity-controlled mercury electrode system. At the short drop times used (0.2–0.4 s), faradaic distortion terms are shown to influence the shape of the observed differential pulse polarograms. A substantial decrease in sensitivity is also incurred in using these short drop times, compared with the longer ones generally employed in differential pulse polarography. Results for differential pulse anodic stripping conform to the usual expectations.     

Construction and evaluation of a flow-through cell adapted to a commercial static mercury drop electrode (SMDE) to study the adsorption of Cd(II) and Pb(II) on vermiculite

This paper describes the construction and application of a robust flow-through cell for use with the capillary of a commercial static mercury drop electrode. Linearity of peak current was observed up to 0.50 mumol l(-1) for Cd(II) or Pb(II) in anodic stripping voltammetry experiments performed under continuous flow during the deposition step, using 120 s of deposition time and flow rate of 4.0 ml min(-1). Under these conditions the limits of detection for Cd(II) and Pb(II) were 13 and 17 nmol l(-1), respectively. An analytical throughput of 20 analyses per h was possible using 10 s for cleaning the cell between two samples and including the time needed for the potential scan, which was performed with the flow stopped, using the differential pulse mode for current sampling. The linear dynamic range can be extended up to 5 mumol l(-1) for both cations if the deposition time is decreased to 30 s, a condition in which the sampling throughput is 35 analyses per h. The proposed manifold was used to study the adsorption rates of Cd(II) and Pb(II) onto vermiculite at different pHs, allowing one to perform high sensitivity measurements at high sampling frequency, using low cost instrumentation.     

Anodic stripping voltammetry in a flow-through cell with a fixed mercury film glassy carbon disc electrode. Part I

Anodic stripping voltammetry at a glassy carbon disc electrode covered by a thin mercury film was adapted for use in a flow-through cell. The resulting system is characterized by extreme simplicity of set-up and operation, high sensitivity and excellent precision and stability. Its performance was tested via the determination of hydrated or labile complex ions of heavy metal ions in sea water, using short (2–10 min) deposition periods. The dependence of the stripping peak charge on metal ion concentration, length of deposition period, solution flow rate and other variables was examined and the reliability of the results obtained were evaluated under conditions resembling continuous monitoring.     

Voltammetric analysis of ceftazidime after preconcentration at various mercury and carbon electrodes: application to sub-ppb level determination in urine samples

The electrochemical behavior of ceftazidime (CFZ) at four different kinds of electrodes viz. static mercury drop electrode (SMDE), controlled growth mercury electrode (CGME), glassy carbon electrode (GCE) and carbon paste electrode (CPE) has been presented. Optimal operational parameters have been selected for the drug preconcentration and determination in aqueous medium. Down to 2x10(-10) M CFZ is achieved as detection limit at the CGME. Modification of the CPE with polyvinyl alcohol (PVA) enhances both the sensitivity and selectivity for the drug accumulation and, therefore, its determination at very low levels. Application of the proposed method for CFZ analysis in spiked urine samples or those taken after metabolism has been easily assessed. Down to 1x10(-9) M CFZ (0.695 ng ml(-1)) could be easily achieved in such samples.