M.D. Gelado-Caballero

Rapid determination of copper, lead and cadmium in unpurged seawater by adsorptive stripping voltammetry

Abstract A rapid method to determine copper, cadmium and lead directly in seawater is presented. The procedure uses adsorptive cathodic stripping voltammetry and rapid (> 1 V s−1) potential scans. The ligand concentration, pH, deposition potentials and electrochemical variables are optimised for these conditions. The method is especially insensitive to the turbulence of the solution during the scan. No perturbations are produced by stirring during the scan. Oxygen interference is also drastically decreased since adsorbed metal peaks are favoured at high scan speeds. This interference together with capacitative components are eliminated by blank subtraction. The blank is measured under the same conditions, but without collection. A procedure to avoid the overlapping, especially between copper and lead peaks, is also worked out. This uses subtraction of repetitive scans in which some of the metal peaks are removed. The replicate scans are made in the same drop after the first scan, using a combination of times and reoxidation potentials. This procedure successfully results in resolving overlapping peaks without introducing delays or losses of sensitivity. The reproducibility and accuracy of the method were calculated by reference to open oceanic waters. Applications of the method to the determination of copper, lead and cadmium in an oceanic profile (Central Atlantic Ocean) is given.

Direct determination of cobalt in unpurged oceanic seawater by high speed adsorptive cathodic stripping voltammetry

Abstract A fast, sensitive method to determine Co (II) in unpurged ocean seawater is presented. The method combines high scan speed staircase voltammetry (HSACSV) and nitrite catalytic effect to enhance Co(II)-dimethylglyoxime reduction currents. The catalytic effect was magnified using high speed adsorptive cathodic stripping voltammetry and peak currents of ca. 0.1 μA were obtained for pM cobalt samples using 30-s deposition times. The effect of buffer, pH, nitrite concentration and scan speed on cobalt sensitivity was studied. Capacitance currents developed at high scan speeds were filtered by background subtraction. The same procedure was successfully applied to remove oxygen interference in unpurged solutions. Blank voltammograms were obtained in different ways yielding similar results without variation in peak reproducibility or accuracy. Nickel and zinc interferences were removed using a cleaning potential before the scan at −1.03 V. Ratios over 10 000 fold were eliminated using more negative cleaning potential steps. Nickel can also be measured by this method. Total analysis time of unpurged seawater (pM levels of cobalt) was decreased to reagents addition time, 30-s adsorption period and 10-s quiescence time. A detection limit of 5.9 pM Co was achieved in seawater. Short analysis time, sensitivity and portable hardware recommend this method for aboard determinations of cobalt. An oceanic profile of cobalt was measured at the Central East Atlantic Ocean.

Aluminium distributions in Central East Atlantic waters (Canary Islands)

Abstract DASA-reactive aluminium and nutrients have been determined on a grid located in Central East Atlantic waters (Canary Islands) during early spring. The sample area shows major features of biogeochemical interest, such as elevated aeolian (dust) inputs from the Sahara desert, proximity to upwelling areas (150–200 km) and mesoscale features induced by the effect of the islands on the course of the Canary Current. The aluminium distributions show a marked latitudinal E-W gradient. The low values towards the east are explained by the intrusion of cold waters originated in the upwelling region off northwest Africa, where aluminium is scavenged by organic matter. High values towards the west suggest aeolian inputs that may stretch to below the seasonal thermocline. Nutrients are shown to be inefficient to trace these features, as they are quickly used by biological uptake. The mesoscale circulation south of the islands produces cyclonic and anticyclonic eddies that, respectively, rise and sink the thermocline over 60 m. Aluminium appears to be a useful tracer for this mixing process as well as for the intrusion of cold waters coming from the eastern boundary.

Rapid adsorptive cathodic stripping voltammetry of zinc complexes in sea water

Abstract The effects of the potential scan speed on the adsorptive cathodic stripping voltammetric determination of zinc-tetramethylenedithiocarbamate complexes in sea water were investigated. It was observed that an increase in scan speed increases the peak height and peak width, shifts the peak potential towards negative values and increases the capacitance background current. These effects were explained by taking into account the ohmic drop at high scan speeds. Several advantages of using a high scan speed in the determination of zinc were found. The faradaic current and sensitivity of the determination are noticeably increased as the whole electroactive substance is reduced in a shorter time. The stirring or turbulent motion of the solution during the scan does not perturb the zinc peak as the complete scan is done in a few milliseconds. Oxygen interference is less severe as the metal and oxygen reduction currents are now in the same current range. The entire analysis time can be markedly reduced as the purging, collection and quiescence time can be decreased. The method is proposed for use on-board oceanographic vessels where its advantages are especially valuable. All results were obtained using a laboratory-designed polarograph able to operate at very high scan speeds.