Pablo Richter

Screening and determination of pesticides in soil using continuous subcritical water extraction and gas chromatography-mass spectrometry.

In the present work the efficiency of water under subcritical conditions for the extraction of pesticides having a broad spectrum of polarities from soils was evaluated. The pesticides under study were carbofuran, hexachlorobenzene, dimethoate, simazine, atrazine, lindane, diazinon, methylparathion, alachlor, aldrin-R, metholachlor, chlorpyrifos, heptachlor epoxide, dieldrin, endrin, 4,4-DDT and metoxichlor. Optimization studies were carried out using a blank soil (Non-Polluted Soil 1, CLN-1, RTC) and a real soil which were previously spiked with the pesticide mixture and aged for 60 days. A laboratory-made aluminum oven with controlled temperature was used to carry out the leaching process with subcritical water, where it is placed a pre-heater and the extraction cell. The following variables were studied, keeping the pressure controlled about 1200 p.s.i.: the extraction temperature, the time of static and dynamic extraction and the flow-rate of water (1 p.s.i. = 6894.76 Pa). The extraction efficiency of the pesticides increases with the temperature trending to the quantitative extraction at temperatures near to 300 degrees C. After the extraction process, the analytes were transferred quantitatively to 5 ml dichloromethane, before the determination by GC-MS. The results indicate that under the optimized conditions mostly of the analytes are extracted quantitatively in 90 min with recoveries quite similar to those obtained by the standard Soxhlet extraction procedure. Alternatively, by using an extraction time of 25 min, the method can be used as screening for all the pesticides, with recoveries depending on their polarity.

Rotating-disk sorptive extraction of nonylphenol from water samples

In this study the sorption of nonylphenol was implemented on a rotating Teflon disk coated with a PDMS film on one of its surfaces. In this way, the disk, which has a high surface area, contacts only the liquid sample, which can be stirred at higher velocity than with the stir bar used in stir-bar sorptive extraction (SBSE), without damaging the phase while at the same time facilitating analyte mass transfer to the PDMS surface. We refer to the procedure as rotating-disk sorptive extraction (RDSE). Extraction variables such as disk rotational velocity, extraction time, and surface area of PDMS film were studied to establish the best conditions for extraction. With increasing rotational velocity, the amount of extracted analyte significantly increases because the stagnant layer concomitantly decreases. On the other hand, the extracted amount concomitantly increases with extraction time, reaching equilibrium at approximately 20 min, which can be reduced to 10 min when the surface area of PDMS increases from 1.74 to 6.97 cm(2). Precision of the method was determined by using the same disk (n=6) and different disks (n=3), showing relative standard deviations for the analyte of 3.7% and 10%, respectively. The detection limit of the method was 0.09 microg/L NP, defined at a signal to noise ratio of 3. The method was applied to a real sample, achieving quantitative recovery. The PDMS phase on the disk could be used for at least 50 experiments. In any case, replacement of the PDMS film on the disk is very easy and inexpensive, as compared to the commercial alternative SBSE.

Polarographic behaviour and determination of ranitidine in pharmaceutical formulations and urine

A polarographic method for the determination of ranitidine is described, based on the reduction of the nitro group at a dropping-mercury electrode. The proposed method permits the drug to be determined, without any prior separation or extraction, in pharmaceutical formulations and urine at levels at which the unchanged drug is excreted. The current is diffusion controlled and proportional to concentration from 3.58 x 10(-3) to 1.50 mmol l-1. The detection limit is 1.07 x 10(-3) mmol l-1. The reduction process was studied at different pH values and a reduction mechanism is proposed in which the importance of the homogeneous chemical reactions associated with the electron-transfer steps are indicated.

Determination of pesticides in river water using rotating disk sorptive extraction and gas chromatography-mass spectrometry.

The rotating disk sorptive extraction (RDSE) technique was applied to the determination of pesticides in aqueous samples. Pesticides of different polarities were considered in this study: chlorpyrifos, diazinon, fenvalarate, cyhalothrin, cypermethrin, lindane and malathion. The sorptive/desorptive behavior of the pesticides was studied using a rotating disk containing a polydimethylsiloxane (PDMS) phase on one of its surfaces. The analyte polarity was a significant factor in the extraction time; shorter extraction times were required for the more apolar pesticides. The optimum variables for the extraction of all analytes were: extraction time of 3h, sample volume of 25 mL, rotational velocity of the disk 1250 rpm, desorption time of 30 min using methanol. For pesticides with values of Log K(ow)>4, the extraction time can be reduced to 30 min for a quantitative extraction. Under these conditions, recoveries between 76% and 101% were obtained for the target pesticides, and the repeatability of the methodology, expressed as relative standard deviation, was determined to be between 10% and 20%. Additionally, the limits of detection of the analytes were lower than 3.1 μg L(-1). The extraction method developed using the RDSE was compared to a stir bar sorptive extraction (SBSE) under the same conditions. It can be observed that the extraction using the rotating disk offers higher recoveries because of its higher PDMS volume and its higher surface area to volume ratio that allows for improved mass transfer.

Simultaneous determination of amiloride and furosemide in pharmaceutical formulations by first digital derivative spectrophotometry

This work presents a simple and fast method for the simultaneous determination of amiloride and furosemide by digital derivative spectrophotometry. HCl 1 x 10(-2) mol/l dissolved in ethanol was used as solvent and to extract drugs from formulations. Subsequently the samples were evaluated directly by first digital derivative spectrophotometry, using a smoothing factor of 8 and scale factor of 1 x 10(-4). The simultaneous determination of furosemide and amiloride can be carried out at 241.4 and 343.6 nm, respectively. In both cases, the zero crossing approach was used. When both compounds are present together in a sample, it is possible to quantify one in the presence of the other, without mutual interference. The determination range was found to be of 6.9 x 10(-8) to 16 x 10(-5) and 6.8 x 10(-8) to 8 x 10(-5) mol/l, for amiloride and furosemide, respectively. A good level of repeatability (RSD) of 0.9 and 0.6% was observed for amiloride and furosemide, respectively. The ingredients commonly found in commercial pharmaceutical formulations do not interfere. The proposed method was applied to the determination of these drugs in pharmaceutical formulations.

Influence of redox potential (Eh) on the availability of arsenic species in soils and soils amended with biosolid.

A study was done on the influence of redox potential on the mobility and availability of the various arsenic chemical forms in a Mollisol soil from central Chile amended with biosolid. Arsenic availability was strongly dependent on the applied redox potential. As expected, under reducing conditions (-200 mV vs Hg/Hg(2)Cl(2)) arsenic availability increased significantly, and arsenic was found mainly as arsenite. On the contrary under oxidizing conditions (200 mV vs Hg/Hg(2)Cl(2)) arsenic solubility decreased markedly and was governed by the presence of arsenate. The greatest concentration of organic arsenic species was found under reducing conditions, which would indicate that methylated species may participate in the transformation of arsenate to arsenite. In biosolid-amended soils the concentrations of methylated species increased as a function of time under reducing conditions, which can be attributed to the greater microbial activity resulting from the organic matter supply from the biosolid to soil. In all the systems, a high concentration of As(V) was found under reducing conditions, indicating that the chemical kinetics for the conversion of arsenate to arsenite is slow. Along time, the content of As(V) increased in the control soils, which may be attributed to the possible dissolution of iron oxides and hydroxides under reducing conditions.

Extractability of arsenic, copper, and lead in soils of a mining and agricultural zone in central Chile

Abstract Copper (Cu) mining activities are very important in central Chile. The present study consists in determining the distribution and availability of copper, arsenic (As), and lead (Pb) in soils near a smelting facility, in the VI Administrative Region of Chile, which are subjected to risk of contamination by emissions and by irrigation with contaminated water. In order to determine the distribution of these elements in their various chemical forms in soils, two sequential extraction methods (SEM) were applied. A five stage SEM was used for Cu and Pb, and a four stage one for As. Single extraction methods were also carried out using 1 M HCl, 0.005 M DTPA and 0.005 M EDTA for As, Cu, and Pb, respectively. Furthermore, a 0.1 M malic-citric acid solution (MC) was used as simple extractant for the three elements to simulate the conditions of root environment of the plant. The soils showed the following order of total metal concentration: Cu > As > Pb. Copper was extracted in higher amount than the other metals, both with the sequential and with the simple extraction method. Single correlation analysis was applied between the amount of metal in the single extraction method and the different fraction obtained with the sequential technique. Significant positive correlations were obtained, especially for copper and lead.

Voltammetric behaviour of nitrofurazone, furazolidone and other nitro derivatives of biological importance

In pyridine-formic acid buffer and tetramethylammonium chloride solution of pH 4.5 at a dropping mercury or a glassy carbon electrode, nitrofurazone, furazolidone and nitrofurantion are reduced in a single six-electron wave, while chloramphenicol and other structurally related nitro derivatives are reduced in only one four-electron wave, the nitro group being reduced to the amine or to the hydroxylamine, respectively. The electrochemical behaviour of these compounds depends mainly on the nature and position of the substituents. Reduction to the primary amine occurs when the substituents possess available π electrons to conjugate with the nitro group of the aromatic ring, which determines the transformation of the hydroxylamine into the amine via formation of a highly reducible intermediate imine or a quinonoid structure. In contrast, if the formation of the intermediate imine is made impossible by an adverse effect of the substituent, the hydroxylamine does not undergo further reduction.Cyclic voltammograms were recorded at different pH values and at different scan rates in order to identify certain relatively unstable species. The effect of pH on the diffusion-limited current and on the E½ values of the polarographic waves was also studied and the results obtained were compared with those obtained by cyclic voltammetry.On this basis, and according to the polarographic and cyclic voltammetric data, a reduction mechanism for the nitrofuran derivatives is suggested, in which the importance of the homogeneous chemical reactions associated with the electron-transfer steps is examined.

Flow Injection Photometric Determination of Zinc and Copper With Zincon Based on the Variation of the Stability of the Complexes With pH

A flow injection photometric method for the sequential determination of zinc and copper in mixtures was developed based on the variation of the stability of the chromogenic complexes between the analytes and the reagent zincon with pH. At pH 5.0 only the Cu-zincon complex exists, whereas at pH 9.0 the copper and zinc chelates co-exist. A three-channel manifold was implemented containing two alternating buffer streams (pH 5 and 9) which permit the colored reaction products to be formed sequentially at both pH values, and consequently the mixtures can be resolved. A continuous preconcentration unit (Chelex-100) was used in order to increase the sensitivity of the method, thus allowing the analysis of water samples in which the analytes are present at the ng ml-1 level. On the other hand, preconcentration was not required when the analytes were determined in brass. Under the optimum conditions and using a preconcentration time of 2 min, the detection limits (3 sigma) were found to be 0.35 and 0.80 ng ml-1 for zinc and copper, respectively. The repeatability of the method, expressed as the RSD, was in all instances less than 3.1%. Considering the sequential determination of both species, a sampling rate of 70 h-1 was obtained if preconcentration of the samples was not required.

Rotating disk sorbent extraction for pre-concentration of chromogenic organic compounds and direct determination by solid phase spectrophotometry

A novel and very simple microextraction approach for pre-concentration and direct solid phase spectrophotometric measurement has been developed for the determination of chromogenic analytes. The model analyte to assess this approach was the chromophore malachite green (MG). The analyte was extracted from water samples onto a small rotating disk made of Teflon containing a sorbent phase of polydimethylsiloxane (PDMS) on one of its surfaces. We refer to the extraction procedure as rotating disk sorptive extraction (RDSE). After extraction, the sorbent phase with the concentrated analyte was separated from the Teflon disk and used directly for MG determination by solid phase spectrophotometry at 624 nm, without the necessity of a desorption step. Chemical and extraction variables such as concentration of sodium sulfate, pH, disk rotational velocity, extraction time, and temperature were studied in order to establish the best conditions for extraction. Under optimum conditions, the extraction of MG was carried out in 18 min and 90 min, for sample volumes of 100mL or 1000 mL, respectively. The detection limit, based on three times the standard deviation of the blank phase (3σ(b)), was 1.4 μg L⁻¹ and the repeatability, expressed as relative standard deviation (RSD), for 20 μg L⁻¹ MG was 8.1%. This study also applied the method to real samples, obtaining quantitative recovery (mean recovery of 99.3%). The PDMS phases could be reused after desorbing the MG into methanol for 3h. Replacement of the PDMS film onto the disk is very easy and low cost.