Ma Esther Fernández Laespada

Detection of adulterants in olive oil by headspace-mass spectrometry.

In the present work, we propose the use of direct coupling of a headspace sampler to a mass spectrometer for the detection of adulterants in olive oil. Samples of olive oils were mixed with different proportions of sunflower oil and olive-pomace oil, respectively, and patterns of the volatile compounds in the original and mixed samples were generated. Application of the linear discriminant analysis technique to the data from the signals was sufficient to differentiate the adulterated from the non-adulterated oils and to discriminate the type of adulteration. The results obtained revealed 100% success in classification and close to 100% in prediction. The main advantages of the proposed methodology are the speed of analysis (since no prior sample preparation steps are required), low cost, and the simplicity of the measuring process.

Electronic nose based on metal oxide semiconductor sensors as a fast alternative for the detection of adulteration of virgin olive oils

Abstract An “electronic nose” has been used for the detection of adulterations of virgin olive oil. The system, comprising 12 metal oxide semiconductor sensors, was used to generate a pattern of the volatile compounds present in the samples. Prior to different supervised pattern recognition treatments, feature selection techniques were employed to choose a set of optimally discriminant variables. Linear discriminant analysis (LDA), quadratic discriminant analysis (QDA) and artificial neural networks (ANN) were applied. Excellent results were obtained in the differentiation of adulterated and non-adulterated olive oils and it was even possible to identify the type of oil used in the adulteration. Promising results were also obtained as regards quantification of the percentages of adulteration.

Micelle-mediated methodology for the preconcentration of uranium prior to its determination by flow injection

Cloud point extraction has been used for the preconcentration of uranium, prior to its determination by flow injection. The non-ionic surfactant employed was Triton X-114 and the reagent chosen to form a hydrophobic chelate of uranium was 1-(2-pyridylazo)-2-naphthol. The optimum conditions for the preconcentration and determination of uranium have been studied. This methodology has been applied to the determination of trace amounts of uranium in tap and river waters from Salamanca.

Cloud Point Methodology : A New Approach for Preconcentration and Separation in Hydrodynamic Systems of Analysis

The analytical potential shown by the cloud point phenomenon for the separation and preconcentration of different analytes as an alternative method to other separation techniques is studied. We offer and discuss several examples that can be applied in flow injection analysis and high performance liquid chromatography with both optical (UV and fluorescence) and electrochemical detection.

Evaluation of surface- and ground-water pollution due to herbicides in agricultural areas of Zamora and Salamanca (Spain).

The pollution of agricultural land due to herbicides was assessed in the Guarena and Almar river basins, situated in the provinces of Zamora and Salamanca (Spain). A set of fifteen herbicides, including triazines, ureas, amides and others, was selected owing to their frequency of use, the amounts used, their toxicity and their persistence in the environment. Solid-phase extraction with polymeric cartridges, followed by HPLC with diode-array detection, were used to monitor the herbicides. This technique was chosen owing to the wide range of functionality and polarity of the analytes under study. The detection limits obtained were in the 0.004–0.025 μg/l range (λ=220 nm). Surface and ground waters, taken from different locations in the basins, were analyzed over a 6-month period. The presence of six out of the fifteen herbicides monitored — chlortoluron, atrazine, terbutryn, alachlor, diflufenican and fluazifop-butyl — was detected in several samples at levels ranging from the detection limit to 1.2 μg/l. The relationship of these herbicides to the agricultural activities of the zone is discussed.

Analytical applications of membrane extraction in chromatography and electrophoresis.

An overview of the analytical applications of membrane-based systems for sample enrichment in chromatography and capillary electrophoresis is presented. A brief introduction to the different types of membranes and the main forces related to the transport through them is also given.

Cloud point preconcentration of rather polar compounds: application to the high-performance liquid chromatographic determination of priority pollutant chlorophenols

The potential of the cloud point methodology for the preconcentration of relatively polar compounds was studied using the non-ionic surfactant Triton X-114 and five EPA chlorophenols as test analytes. Analyte determination was performed using reversed-phase gradient LC with electrochemical and spectrophotometric detection. The amount of surfactant used is a critical variable in the preconcentration factor because it determines the extraction yield and the volume of surfactant-rich phase obtained. These values were determined as a function of the Triton X-114 concentration, together with the phase ratio, which allows prediction of the maximum preconcentration factor under given conditions.

Continuous membrane extraction coupled with chromatographic analysis for the determination of phenols in fuels

On-line coupling of devices for nonchromatographic separation to chromatographic analysis systems affords a substantial improvement in sample processing and facilitates the automation of the procedure, considerably decreasing the error of the analytical methods used. In this work, a silicone membrane device has been coupled to a high-performance liquid chromatograph with electrochemical and ultraviolet detection and the system has been used to determine phenols in complex organic matrices (gasolines and kerosene) with minimum sample handling. A microcomputer controls the set of operations required in the overall automatic process. Several quantification methods, internal standard, calibration in phenol-free matrices and standard addition, have been used, all of them providing similar results.

Automated on-line membrane extraction liquid chromatographic determination of phenols in crude oils, gasolines and diesel fuels.

Determination of phenols in crude oils and derived fuels requires a sample pretreatment step, usually performed by liquid-liquid extraction or preparative chromatography. In this work, sample preparation is accomplished using a silicone membrane separation unit coupled on-line to a high performance liquid chromatograph with amperometric and ultraviolet detection. The contents of phenol, cresols and dimethylphenols were determined in thirty three samples including three crude oils, twenty gasolines and ten diesel fuels. The whole set-up is fully automated through a feed-back system that allows the microcomputer controlling the process to examine the signals in real time and to make decisions while the experiment is running.

Determination of naphthalene and total methylnaphthalenes in gasoline using direct injection-mass spectrometry.

A high-speed determination of naphthalene and total methylnaphthalenes using a non-separative method based on direct injection into the mass spectrometer was performed. The results obtained for total methylnaphthalenes were very similar to those provided with fast gas chromatography-mass spectrometry (GC-MS). However, the non-separative method afforded higher concentrations in the determination of naphthalene than those found when fast GC-MS was used. We propose a correction that removes this error very satisfactorily and allows the same results to be obtained with both methodologies. The non-separative method is rapid, simple and - in view of the results - highly suitable for the determination of naphthalene and total methylnaphthalenes in gasoline samples.