Lars Renman

Flow potentiometric and constant-current stripping analysis for arsenic(V) without prior chemical reduction to arsenic(III): Application to the Determination of Total Arsenic in Seawater and Urine

Abstract Arsenic(V) is reduced to elemental arsenic on a gold-coated platinum-fibre electrode at electrolysis potentials below −1.60 V vs. Ag/AgCl and subsequently re-oxidized, either by means of a constant current, or chemically, with gold(III) as oxidant. Total arsenic in acidified seawater can be determined by means of electrolysis for 60 s at −1.80 V vs. Ag/AgCl and subsequent stripping in 4 M hydrochloric acid containing 2.5 M calcium chloride. The detection limit obtained after 60 s of electrolysis (ca. 0.1 μg1−1) is about ten times lower than that obtained by the electrochemical stripping methods for arsenic(III) reported hitherto. Total arsenic in urine is determined after digestion with nitric acid and hydrogen peroxide.

Carbon fibre electrodes in flow potentiometric stripping analysis

Abstract The construction of carbon fibre flow electrodes suitable for use in connection with potentiometric and constant-current stripping is described, and the fibre electrodes are compared with a glassy carbon disc thin layer cell. The signal-to-background ratio is approximately 1.6 times higher for an 8–10 μm carbon fibre compared to the glassy carbon disc electrode. If an Ag/AgCl tube is used as both counter and reference electrode, the signal-to-noise ratio of the fibre electrode is approximately five times better than for a glassy carbon disc electrode with a calomel reference; the latter electrode design, however, gives slightly better precision. The dead volume and internal potential drop of the fibre electrodes are more than one order of magnitude smaller than for the glassy carbon disc electrode. Because of the simplicity of the manufacturing process and low material cost, the fibre cells can be used as disposable electrodes and the polishing process necessary in connection with glassy carbon disc electrodes can be omitted.

Computer system and flow cell for automated potentiometric and constant-current stripping analysis

Abstract A flow cell and a personal computer-based system for potentiometric and constant-current stripping analysis are described. The thin-layer cell is designed for teflon-embedded glassy carbon electrodes in a spring-regulated holder, allowing simple instalment and leak-free behaviour. The personal computer is interfaced to hardware components such as potentiostat, amplifier, peristaltic pump, six inlet valves for different solutions, autosampler and strip-chart recorder. User-accessible commands allow electrolysis, stripping current, pump flow rates, valves, stripping transient recording, derivation, filtration and integration to be controlled, and through a number of user-defined programs, to be performed in unattended operation. This is described by an example. Through a software interface to a high-level language (BASIC), results from any number of analytical runs can be further evaluated by standard addition, or from calibration curves, or by statistical methods.

Automated determination of total arsenic in sea water by flow constant-current stripping analysis with gold fibre electrodes

Abstract Total arsenic in sea water is determined in a fully automated flow system, by means of potentiostatic deposition for 4 min at a 25-μm gold fibre electrode and subsequent constant-current stripping in 5 M hydrochloric acid. Previously the sample is acidified with hydrochloric and arsenic(V) is reduced to arsenic(III) with iodide. During stripping, the potential vs. time transient is recorded with a real-time measurement rate of 26.5 kHz and a potential resolution of 1 mV. Cleaning and regeneration of the gold electrode are fully automated. The total arsenic concentrations in two reference sea waters (NASS-1 and CASS-1) were evaluated by single-point standard addition and found to be 1.58 and 1.14 μg l −1 with standard deviations of 0.39 and 0.28 μg l −1 , respectively; certified values are 1.65 ± 0.19 and 1.04 ± 0.07 μg l −1 . The arsenic(III) content in these samples was below the detection limit (0.15 μg l −1 ).

Asymmetric distribution of results in calibration curve and standard addition evaluations

Abstract The inherent asymmetry in the application of linear regression analysis to the determination of sample analyte concentrations using calibration curve or standard addition evaluations gives rise to systematic errors. On the average, this always results in an overestimation of the true analyte concentrations in standard addition evaluations, while in calibration curve evaluations, the mean relative error depends on the value of sample concentration in relation to the analyte concentrations used for the calibration curve. In both evaluation techniques, the distribution of the results will deviate from a Gaussian distribution even if all analytical signals are normally distributed. It is shown that for the standard addition technique, and for samples with low analyte concentrations evaluated by the calibration curve technique, optimum precision and accuracy is obtained by using a minimum number of calibration points or standard additions, and performing multiple measurements on these. It is also shown that the linear regression correlation coefficient is a very poor indicator of the accuracy and precision in multiple-point standard addition evaluations. Weighted linear regression can be used to decrease the magnitudes of the systematic errors, but due to the inherent asymmetry, the distribution of the results will nevertheless be non-Gaussian. A publically available, in-house constructed Windows 3.1/Windows 95 program, capable of simulating all kinds of calibration curve and standard addition evaluations, was used for all calculations.

Determination of selenium by means of computerized flow constant-current stripping at carbon fibre electrodes. : Application to human whole blood and milk powder.

Abstract The main features of the flow constant-current stripping analysis for selenium(IV) are formation of a mercury film on a carbon fibre sensor in a chloride medium containing mercury(II), electrolysis in the sample at −0.20 V vs. SCE for 15–60 s, and subsequent stripping (reduction) of the mercury(II) selenide formed on the electrode surface, by means of a constant current of 0.40 μA in an acidic magnesium chloride solution containing Triton X-100. During stripping, the potential vs. time gradient is monitored at a real-time measuring rate of 25.6 kHz. All experimental parameters are under computer control. A standard addition method is used and the results are calculated and reported, both digitally and graphically. Equations relating the magnitude of the constant current to the concentration of reducible species, and, in particular, of dissolved dioxygen, are derived. Milk powder and whole blood reference samples were analyzed by high-pressure digestion in nitric acid and dilution with hydrochloric acid, in order to reduce selenium(VI) to selenium(IV), and then constant-current stripping. The results obtained by this method were lower than those obtained by the reference technique, but the values agreed within one standard deviation of the two techniques. Ions, such as iron(III) and lead(II), known to interfere with electrochemical stripping for selenium(IV) in batch analysis did not interfere in the flow approach.

Automated determination of cadmium and lead in whole blood by computerized flow potentiometric stripping with carbon fibre electrodes

Abstract A sample pretreatment involving only the dilution (1 + 19) of two 0.2–0.4 ml sample aliquots with 0.5 M hydrochloric acid, with a standard addition to one of the aliquots, precedes the injection of each sample. The computer-controlled flow system used automatically executes a pre-programmed number of cycles on each sample pair before presenting the final result. Each cycle, which has a duration of 80 or 135 s for lead(II) and cadmium(II), respectively, involves the display of the derivative stripping signal on a printer/plotter and integration of the stripping peaks generated. For lead(II), striping is done in 0.5 M hydrochloric acid, which eliminates interferences from copper, though at the cost of a relatively high stripping rate, compared to the 5 M calcium chloride used for cadmium(II). The flow cell consists of a silver chloride-lined silver tube which acts as both reference and counter electrode, and a disposable carbon-fibre working electrode mounted in a PVC tube, which normally will operate for 50–200 cycles. The method was verified for whole blood reference standards and by comparison with results obtained by atomic absorption spectrometry.

Flow constant-current stripping analysis for antimony(III) and antimony(V) with gold fibre working electrodes : Application to Natural Waters

Abstract Antimony(III) is determined by means of electrolysis at −0.40 V vs. Ag/AgCl on a gold-coated gold fibre electrode for 0.5–10 min in a redox buffer containing 0.01 M iron(II) in 0.10 M hydrochloric acid, and subsequent stripping with a constant current of 0.50μA either in 2 M hydrochloric acid or in 4 M hydrochloric acid/4 M calcium chloride. Antimony(V) is determined by the same procedure in 4 M hydrochloric acid medium. Bismuth(III) is masked by the addition of iodide to the sample prior to electrolysis. Antimony(III) and antimony(V) are determined by standard addition methods; the whole procedure including digital and graphical evaluation of the results is fully automated. The antimony(V) concentrations in the river water reference sample SLRS-1 and the seawater reference sample NASS-1 were found to be 0.63 and 0.31 μg l−1 with standard deviations of 0.046 and 0.051 μg l−1, respectively (n=15). The certified value for SLRS- 1 is 0.63±0.05 μg l−1; no certified value is available for NASS-1.

Computerized flow potentiometric stripping analysis with nafion-modified carbon fibre electrodes

Abstract Five to ten carbon fibres (10-μm diameter) are mounted perpendicularly to the flow direction in polyethylene tubes (0.8 mm inner diameter) and modified simultaneously by sucking five 10-μl portions of Nation dissolved in ethanol through the flow electrodes; after each portion, air is sucked through the tubes for some minutes. The modified electrodes can be used for several hundred electrolysis/stripping cycles in solutions containing albumin or Triton X-100. Analysis of a urine reference sample shows that zinc(II), cadmium(II) and lead(II) can be determined after acidification and electrolysis for 5, 35 and 100 s, respectively. For the determination of lead(II), 2 M hydrochloric acid is used as stripping medium but for the two other elements the sample itself is used. The electrochemically available fraction of the trace elements can be assessed by analyzing unacidified samples.

Simultaneous determination of mercury(II), copper(II) and bismuth(III) in urine by flow constant-current stripping analysis with a gold fibre electrode

Abstract Urine samples are treated with concentrated nitric acid and potassium permanganate ar 70°C for 10 min prior to injection. The flow electrode system consists of a 10-μm diameter gold fibre working electrode, a glassy carbon reference electrode and a platinum counter electrode. In the fully automated constant-current stripping procedure, the gold fibre is first covered with a fresh gold film after which the sample is electrolyzed for 1 min prior to stripping in 0.1 M hydrochloric acid with a current of 0.1μA. The procedure is repeated on a spiked sample after which the sample analyte concentrations are evaluated and presented digitally and graphically on a printer/plotter. The results obtained for bismuth, copper and mercury in a urine reference sample were 36.9, 39.7 and 47.7 μg l −1 with standard deviations ( n =10) of 3.2, 4.2 and 2.1, respectively. The certified values for copper and mercury were 45 and 51 μg l −1 ; no certified value was available for bismuth.