Natalia Navas

Determination of bisphenol A (BPA) in water by gas chromatography-mass spectrometry

Abstract A simple method for determination of bisphenol A in waters was developed using Gas Chromatography-Mass Spectrometry with a Selected Ion Monitoring (GC/MS-SIM). A 1000 ml water sample was extracted with dichloromethane in acid medium. No clean-up was necessary. Anthracene-d10 was used as an internal standard. The applicable concentration range was 2.5 to 10 ng ml−1 in water samples. The detection limit was 0.6 ng ml−1. The method was applied satisfactorily to the determination of oestrogenic environmental pollutants in sea water from Malaga (Spain) and spring water from the fertile plain of Granada (Spain). The method was validated using the standard addition methodology. The accuracy of the proposed method was verified by applying a Student t test. The relative standard deviations established for the linear concentration range was between 8.6 and 1.5%.

Kohonen artificial neural networks as a tool for wavelength selection in multicomponent spectrofluorimetric PLS modelling: application to phenol, o-cresol, m-cresol and p-cresol mixtures

Abstract Different strategies for wavelength selection for partial least squared (PLS) calibration models have been proposed. In this article, Kohonen artificial neural networks (K-ANN) are used to select optimal sets of wavelengths for PLS calibration of mixtures with stray overlapping. This kind of variable selection appears simple and very effective due to the well known high correlation of spectroscopic data; a measure of the multivariate correlation of the different wavelength subsets is also given. This strategy has been applied to the resolution of mixtures of phenol, o -cresol, m -cresol and p -cresol by spectrofluorimetry. The number of samples to obtain the calibration matrix is also reduced with respect to the number necessary when the full spectrum is used, and the predictive ability of the PLS method is improved.

Classification of organic solvents and modelling of their physico-chemical properties by chemometric methods using different sets of molecular descriptors

Abstract Different sets of molecular descriptors using the k -nearest neighbor classification method were used to make a general classification of 152 organic solvents, the classification being further improved by performing the counter-propagation artificial neural network. An extensive investigation was made of the physico-chemical properties of 152 solvents in a search for quantitative structure–property relationships (QSPR). Wide sets of molecular descriptors were tested and regression models were obtained by selecting the best descriptor subset by genetic algorithm in order to optimize their prediction power.

Determination of five nitroimidazoles in water by liquid chromatography–mass spectrometry

A rapid liquid chromatography-mass spectrometric method for the determination of five nitroimidazoles in water from different sources is described. The extraction procedure was based on HLB solid-phase extraction with acetonitrile followed by an evaporation step. Ternidazol, another nitroimidazole, was used as the internal standard. The liquid chromatographic separation was made on a C18 bonded silica column applying a gradient with an ammonium acetate buffer solution and acetonitrile. Following electrospray ionisation, the protonated molecular ions [M+H]+ were obtained. Quantification for each nitroimidazole was carried out by monitoring its molecular ion. Calibration functions, quantification and detection limits, intra- and inter-day reproducibility and accuracy were estimated. For the confirmatory assay, several fragment ions from each nitroimidazole were obtained and monitored. The method was applied successfully to determine and identify nitroimidazoles in water from different sources at a level of 0.2 microg l(-1).

Preliminary study of UV ageing process of proteinaceous paint binder by FT-IR and principal component analysis

This work presents a preliminary study on the ageing process of proteinaceous binder materials used in painting under UV light. With this aim, two sets of model samples were prepared: samples prepared using a single protein material and complex samples prepared in a similar way to the sequence of layers in a real painting from lowest to highest complexity (protein, drying oils, pigment and varnish). The study focuses on acquiring information about the possible degradation process of proteinaceous binders due to ageing and how this process be affected by the presence of characteristic non-proteinaceous painting materials, such as lipids from linseed oil, terpenic compounds from varnish and inorganic pigments. Samples simulated the accelerated ageing process, as did the UV light exposition. The FT-IR spectra were recorded after 100, 500, 1000 and 1500 h of exposition. The study of the accelerated ageing process was performed by means of principal component analysis (PCA) using the FT-IR spectra obtained. Loadings from the significant principal components were analysed to find the FT-IR frequency (cm(-1)) involved in the degradation process. The study showed the lack of any relevant modification on the proteins in the single model samples. On the contrary, the complex model samples showed the ageing process. The accelerated ageing process can be explained by a principal component from PCA. The most affected IR region was 2900-3600 cm(-1), where the amide band was included.

Resolution of mixtures of three nonsteroidal anti-inflammatory drugs by fluorescence using partial least squares multivariate calibration with previous wavelength selection by Kohonen artificial neural networks.

A spectrofluorometric method for the quantitative determination of flufenamic, mefenamic and meclofenamic acids in mixtures has been developed by recording emission fluorescence spectra between 370 and 550 nm with an excitation wavelength of 352 nm. The excitation-emission spectra of these compounds are deeply overlapped which does not allow their direct determination without previous separation. The proposed method applies partial least squares (PLS) multivariate calibration to the resolution of this mixture using a set of wavelengths previously selected by Kohonen artificial neural networks (K-ANN). The linear calibration graphs used to construct the calibration matrix were selected in the ranges from 0.25 to 1.00 mug ml(-1) for flufenamic and meclofenamic acids, and from 1.00 to 4.00 mug ml(-1) for mefenamic acid. A cross-validation procedure was used to select the number of factors. The selected calibration model has been applied to the determination of these compounds in synthetic mixtures and pharmaceutical formulations.

High-performance liquid chromatography determination of Zn-bacitracin in animal feed by post-column derivatization and fluorescence detection.

A sensitive and selective method is presented for the determination of Zn-Bacitracin in adulterated animal feed by reversed-phase ion-pair high-performance liquid chromatography and post-column derivatization with o-phthalaldehyde prior to fluorescence detection. The calibration function was estimated to be between 8.0 and 65.0 mg l(-1) of Zn-BC. The detection and quantification limits of the chromatographic method were 2.5 and 7.5 mg 1(-1), respectively. Using the extraction procedure of Zn-Bacitracin from the feedstuff that we recently proposed and applying this new chromatographic method, it was possible to detect this antibiotic at levels below 5 mg kg(-1) in different kinds of feedstuffs with a standard deviation less than 6.0%.

Collagen-based proteinaceous binder-pigment interaction study under UV ageing conditions by MALDI-TOF-MS and principal component analysis

This study focuses on acquiring information on the degradation process of proteinaceous binders due to ultra violet (UV) radiation and possible interactions owing to the presence of historical mineral pigments. With this aim, three different paint model samples were prepared according to medieval recipes, using rabbit glue as proteinaceus binders. One of these model samples contained only the binder, and the other two were prepared by mixing each of the pigments (cinnabar or azurite) with the binder (glue tempera model samples). The model samples were studied by applying Principal Component Analysis (PCA) to their mass spectra obtained with Matrix-Assisted Laser Desorption/Ionization-Time of Flight Mass Spectrometry (MALDI-TOF-MS). The complementary use of Fourier Transform Infrared Spectroscopy to study conformational changes of secondary structure of the proteinaceous binder is also proposed. Ageing effects on the model samples after up to 3000 h of UV irradiation were periodically analyzed by the proposed approach. PCA on MS data proved capable of identifying significant changes in the model samples, and the results suggested different aging behavior based on the pigment present. This research represents the first attempt to use this approach (PCA on MALDI-TOF-MS data) in the field of Cultural Heritage and demonstrates the potential benefits in the study of proteinaceous artistic materials for purposes of conservation and restoration.

Study of the GC–MS determination of the palmitic–stearic acid ratio for the characterisation of drying oil in painting: La Encarnación by Alonso Cano as a case study

The correct identification of drying oils plays an essential role in providing an understanding of the conservation and deterioration of artistic materials in works of art. To this end, this work proposes the use of peak area ratios from fatty acids after ensuring that the linear responses of the detector are tested. A GC-MS method, previously reported in the literature, was revisited to its developed and validated in order to identify and quantify of eight fatty acids that are widely used as markers for drying oils in paintings, namely myristic acid (C(14:0)), palmitic acid (C(16:0)), stearic acid (C(18:0)), oleic acid (C(18:1)), linoleic acid (C(18:2)), suberic acid (2C(8)), azelaic acid, (2C(9)) and sebacic acid (2C(10)). The quaternary ammonium reagent m-(trifluoromethyl)phenyltrimethylammonium hydroxide (TMTFAH) was used for derivatization prior to GC-MS analysis of the oils. MS spectra were obtained for each methyl ester derivative of the fatty acids and the characteristic fragments were identified. The method was validated in terms of calibration functions, detection and quantification limits and reproducibility using the signal recorded in SIR mode, since two of the methyl derivatives were not totally separated in the chromatographic run. The proposed method was successfully applied to identify and characterise the most widely used drying oils (linseed oil, poppy seed oil and walnut oil) in the painting La Encarnación. This 17th century easel painting is located in the main chapel of the cathedral in Granada (Spain) and was painted by the well-known artist of the Spanish Golden Age, Alonso Cano (1601-1667).

Trace determination of phenol, bisphenol A and bisphenol A diglycidyl ether in mixtures by excitation fluorescence following micro liquid–liquid extraction using partial least squares regression

The determination of phenol, bisphenol A and bisphenol A diglycidyl ether in mixtures was carried out by recording excitation fluorescence spectra between 245 and 290 nm with an emission wavelength of 306 nm. The proposed method involves a micro liquid–liquid extraction of sodium chloride saturated water samples with diethyl ether followed by direct fluorimetric analysis of the extracts in a 10 mm quartz cell with a 2 mm inside width and a volume of 400 µl. The excitation–emission spectra of these compounds overlap considerably, which does not permit their direct determination without previous separation by conventional methodologies. A method is proposed for the determination of these compounds by the use of a full-spectrum multivariate calibration method, partial least squares (PLS). A comparison between two PLS algorithms with different pre-processing methodologies was made. The experimental calibration matrix was designed with 19 samples. The concentrations were varied between 3.0 and 18.0 µg l–1 for phenol, 3.0 and 13.0 µg l–1 for bisphenol A and 5.0 and 30.0 µg l–1 for bisphenol A diglycidyl ether. The cross-validation method was used to select the number of factors. The proposed method, which applied the PLS-1 model with mean centred data, was used for the determination of these compounds in previously spiked water. The predicted concentrations were satisfactory in all cases.