Juan J. Pinto

A bulk liquid membrane-flow injection (BLM-FI) coupled system for the preconcentration and determination of vanadium in saline waters.

A bulk liquid membrane-flow injection (BLM-FI) system has been developed for the preconcentration and spectrophotometric determination of vanadium in saline waters. The preconcentration step was based on a bulk liquid membrane containing Aliquat 336 (acting as a carrier) dissolved in dodecane/dodecanol. Vanadium species were chemically pumped due to the pH gradient between the sample (pH 3.2) and the receiving solution (pH 9.8). Vanadium transport through the membrane was monitored by a new and sensitive spectrophotometric method based on its reaction with di-2-pyridyl ketone benzoylhydrazone (dPKBH) in an acidic medium. As a consequence of membrane transport, vanadium was recovered in an ammonium solution, where total vanadium concentration was spectrophotometrically determined at 375 nm, as the pentavalent species, by using a flow injection analysis (FIA) system. Under optimal conditions, this FIA system provided a detection limit of 4.7 μg L(-1) (3s(blank)/m) and RSD 2.72%, for vanadium determination in saline samples. Both preconcentration and determination steps were previously optimized by modified simplex methodologies. The proposed coupled method was successfully applied to the determination of vanadium in a certified reference material (TMDA-62) and in two seawater samples.

Solvent bar micro-extraction: Improving hollow fiber liquid phase micro-extraction applicability in the determination of Ni in seawater samples

During the last decade, hollow fiber liquid phase micro-extraction (HF-LPME) has become an attractive alternative in sample treatment for the analysis of trace metals in seawater. If compared with other similar methodologies, its main advantages are associated to a higher stability of the organic solution contained into the pores of the fiber, which acts as a lipophilic membrane during the extraction process. However there are some remaining problems that makes its use difficult, mostly related to the need of increasing the rate of analysis and improving portability. In this paper a novel three phase solvent bar micro-extraction (3PSBME) for the fiber device has been proposed. Its main advantage is that the 3PSBME device can be left free in the sample. This way the system is portable, and no special support is needed leading to the possibility of simultaneous extraction of several samples. In this work, multivariate central composite design of experiment has been carried out to optimize Ni pre-concentration using di-2-ethylhexyl phosphoric acid (DEHPA) as extractant and HNO3 as acceptor agent. Factors influencing extraction have been the pH in the sample and the fiber length. For seawater samples, Ni can be pre-concentrated 11 times in 140 min. The method presents RSD 9.42% and limit of detection 44 ng L(-1), using GFAAS for instrumental determination. It has been applied for determination of Ni in seawater, including a reference material CRM-403 proving its applicability.

Enhanced spectrophotometric methods for trace metal determination in waters: zinc as an example

Although spectrophotometric methods seem to be outdated as analytical tools for the analysis of trace metals in waters, there is no doubt that they still present a series of advantages over other advanced techniques (simplicity, speed, low cost and maintenance, portable instrumentation, etc.). Even more, if experimental conditions are strictly controlled, it is possible to develop highly competitive methods. In this sense, method validation turns into a key feature to improve and demonstrate its applicability. Bearing this in mind, a simple and very sensitive spectrophotometric method for the direct determination of μg L−1 levels of zinc in natural waters has been developed and in-house validated. It is based on the reaction of Zn(II) with di-2-pyridyl ketone benzoylhydrazone (dPKBH) under slightly acidic conditions (pH 6.4) and 15% (v/v) ethanol to produce a 1 : 2 (Zn : dPKBH) complex (λmax 370 nm). Beers law is obeyed in the range 6.3–3000 μg L−1Zn(II) with a detection limit of 0.7 μg L−1. Different parameters such as selectivity, recovery, linearity, limits of detection and quantification, precision, and uncertainty of the measurements were evaluated in order to validate the proposed method. Then, the new method was applied to the analysis of different water samples (e.g., tap and river water), demonstrating its applicability to the determination of Zn at environmental levels.

Solvent extraction with organophosphorus extractants in environmental samples: determination of cadmium(II) in natural water

AbstractIn this work, Cd(II) extraction in natural waters by organophosphorus extractants as organic phase, as well as its back-extraction in an acidic media, has been studied. Cadmium extraction behavior at natural waters’ pH conditions (values in the range 7–8) was studied with two different extractants and co-ions, obtaining the highest extraction efficiency when using 0.1M Cyanex 272 in kerosene as organic phase and 0.1 M NO3− as co-ion. Once they were selected, the effect on the extraction efficiency of sample pH, buffer concentration, extraction time, Cyanex 272 concentration as well as back-extractant concentration, was studied. The presence of the main inorganic and organic ligands in the sample was also studied, observing that extraction efficiency was affected most significantly when chlorides were present, with a decrease of about 14%, proving negligible for the others. Under the selected conditions, spiked real samples were successfully analyzed.

Miniaturized and direct spectrophotometric multi-sample analysis of trace metals in natural waters.

Trends in the analysis of trace metals in natural waters are mainly based on the development of sample treatment methods to isolate and pre-concentrate the metal from the matrix in a simpler extract for further instrumental analysis. However, direct analysis is often possible using more accessible techniques such as spectrophotometry. In this case a proper ligand is required to form a complex that absorbs radiation in the ultraviolet-visible (UV-Vis) spectrum. In this sense, the hydrazone derivative, di-2-pyridylketone benzoylhydrazone (dPKBH), forms complexes with copper (Cu) and vanadium (V) that absorb light at 370 and 395 nm, respectively. Although spectrophotometric methods are considered as time- and reagent-consuming, this work focused on its miniaturization by reducing the volume of sample as well as time and cost of analysis. In both methods, a micro-amount of sample is placed into a microplate reader with a capacity for 96 samples, which can be analyzed in times ranging from 5 to 10 min. The proposed methods have been optimized using a Box-Behnken design of experiments. For Cu determination, concentration of phosphate buffer solution at pH 8.33, masking agents (ammonium fluoride and sodium citrate), and dPKBH were optimized. For V analysis, sample (pH 4.5) was obtained using acetic acid/sodium acetate buffer, and masking agents were ammonium fluoride and 1,2-cyclohexanediaminetetraacetic acid. Under optimal conditions, both methods were applied to the analysis of certified reference materials TMDA-62 (lake water), LGC-6016 (estuarine water), and LGC-6019 (river water). In all cases, results proved the accuracy of the method.

Determination of silver in seawater by the direct analysis of solvent bars by high resolution continuum source solid sampling graphite furnace atomic absorption spectrometry

The direct analysis of solvent bars used for silver micro-extraction from seawater samples by high resolution continuum source solid sampling graphite furnace atomic absorption spectroscopy (HR-CS-SS-GF-AAS) has been achieved. Three successive pyrolysis steps at 290 °C, 700 °C and 1000 °C were applied to ensure the elimination of the fumes released during the calcination of the bar prior to the atomization at 1800 °C. Silver micro-extraction with solvent bars was optimised by applying a multivariate strategy. The concentrations of the extracting agent (tri-isobutylphosphine sulphide), reextracting reagent (sodium thiosulfate) and nitrate were studied by using a central composite design based on the surface response methodology. The optimum values were 0.168 mol L−1, 0.033 mol L−1 and 0.159 mol L−1, respectively. Under these conditions, a stirring rate of 1000 rpm and an extraction time of 20 minutes were optimum. Additionally, the effect of chlorides and humic acids was also studied. The method exhibited a repeatability and reproducibility of 1.54% and 2.68%, respectively, with a limit of detection of 2.52 ng L−1 and a linear range up to 334 ng L−1. For the first time, an ultra-trace element (silver at the ng L−1 level) was pre-concentrated and determined in seawater by using a direct solid analysis-based method, simplifying the sample preparation process and avoiding its manipulation. The method was successfully applied to dissolved silver determination in real seawater samples after the micro-extraction process and was validated by comparison with the reference method of liquid–liquid extraction with APDC/DDDC and ICP-MS determination.