Análisis de plaguicidas en hortalizas de hojas verdes mediante espectroscopía de plasmas producidos por láser
Authors: Martino Lucila 
Published: June 2021

La presencia de plaguicidas en los alimentos representa una amenaza importante para los consumidores, sin embargo, la detección rápida directamente en las muestras implica una tarea compleja. En este trabajo de investigación se evaluó la aplicación potencial de la técnica Laser Induced Breakdown Spectroscopy (LIBS) para la medición rápida de determinados plaguicidas en hortalizas de hojas verdes, identificándolos a partir del registro de las líneas de emisión que componen los agentes activos. Los resultados obtenidos fueron exitosos, pudiendo encontrar condiciones experimentales óptimas, posibilitando la identificación de los elementos en las muestras pertinentes. A su vez, mediante la aplicación de análisis quimiométricos al conjunto de datos LIBS, se pudo determinar que las muestras tienden a agruparse de acuerdo al plaguicida que las compone, lo que posibilitó su clasificación. Por otro lado, fue posible cuantificar residuos de plaguicidas mediante la medición de Cl, P y S. Se estableció una clara evidencia de la aplicabilidad de esta técnica como una herramienta potencial para el estudio de plaguicidas en matrices alimentarias. Esto se logró a partir de una preparación mínima de muestras, garantizando la rapidez del análisis, bajos costos de aplicación, y sin producción de desechos tóxicos, evidenciando su uso como técnica preliminar de evaluación o en forma complementaria a los métodos analíticos tradicionalmente ya usados.
The use of pesticides in production systems is carried out in order to control pests and weeds. However, due to use and abuse in their application, they cause adverse effects on white and nonwhite organisms, reaching water bodies and air due to their drift. In turn, the presence of pesticide residues in food represents an important threat to consumers, mainly due to the toxicity that they can present in humans, finding evidence that determines the relationship of many diseases and even deaths in the population by ingestion, contact, or mishandling of them. From of the increasing knowledge about the potential dangers associated with the use of pesticides, and the availability of data evidencing their presence in the environment, the need to introduce controls on pesticide residues has become a matter of central concern for society. Against this background, global environmental legislation fixes Maximum Residue Levels (MRLs), which are increasingly stringent regulations for pesticide residues in water and food. These limits are consistently exceeded as a result of the unreasonable application of pesticides, though. Noncompliance with MRLs results in the presence of excessive amounts of pesticide residues in crop products, leading to the accumulation of pesticide traces in the environment, the resurgence and outbreak of resistant pests, and adverse health outcomes due to contaminated food consumption. However, the techniques used for the detection and quantification of these residues require a long procedure, use highly hazardous reagents, and offer no possibility of re-measuring the same sample. Knowing the effects produced by the use of pesticides and with the premise that the maximum permissible limits are exceeded in several cases, it is necessary to have rapid detection and quantification methods that allow the analysis and evaluation of their presence, being the first step to determine if the maximum allowed limits are exceeded with the consequent identification of risks and decision making. The Laser-Induced Breakdown Spectroscopy (LIBS) technique has made its way as a simple diagnostic technique that allows rapid detection of interest elements in samples, regardless of their state of aggregation. It is an analytical technique that relies on the formation and study of a plasma induced by a focused high-power laser pulse on the surface of a sample to generate a plasma of high temperature and electronic density around 10x1017 cm-3 , hence providing information about the elements that it is composed of at the atomic level. The plasma emission produced, due to electronic recombination, generates a spectrum. That spectrum is collected and its spectroscopic analysis allows carrying out qualitative and/or quantitative analysis. LIBS has become established as an important spectroscopic technique with enormous analytical potential due to its particular characteristics and advantages over other techniques, such as no or minimal sample preparation, iv the capability of multielement analysis, no waste production, rapidity, affordable costs, and remote sensing capability. At present, the LIBS technique coupled with multivariate chemometric methods and data analysis is being used as a new tool in the classification of samples. Chemometric techniques such as principal component analysis (PCA) and linear discriminant analysis (LDA) are multivariate statistical methods applied to: reduction of dimensional problems (number of variables), avoiding the loss of essential information; pattern recognition; and characterization of two or more sample groups. At the same time, the creation of calibration protocols using relatively simple experimental setups allows finding detection and quantification limits with high precision. With this, the objective of the present study was to evaluate and analyze the feasibility of using the LIBS technique as a potential tool for the detection and quantification of some pesticides, including herbicides and fungicides, in chard samples. The objectives proposed in this work were: i) to identify the pesticides in contaminated chard samples based on the detection of the elements that make up the active agents, ii) to characterize and classify the pesticides by chemometric methods, and iii) establish routines for quantitative analysis. With the main purpose of contributing a new and/or complementary analytical technique to the identification of pesticides in food or other matrices. The research that has been carried out included systematic studies of laser-induced plasmas in order to obtain advantageous information. An exhaustive study of the elements of difficult determination that make up the active agents of pesticides was carried out, at atmospheric pressure and low pressure with different experimental schemes, to obtain an optimal database that allows the identification and characterization of pesticides starting from the application of chemometric analysis. In turn, calibration protocols were carried out to study the sensitivity and precision of the analytical technique, establishing the optimal plasma parameters. Finally, the plasmas produced were characterized to evaluate their homogeneity and repeatability. The results obtained from the LIBS signal records were successful, being able to find optimal experimental conditions and allowing the identification of P, S, C, and Cl in all standard samples and in the sample used for control purposes. A new signal registration acquisition method was also developed, this guarantees the obtaining of several signal registrations with minimal sample destruction and in a fast and versatile way. By applying multivariate analysis, it was observed that the samples tend to group according to the pesticide that composes them and differ from the control sample, in turn, differences in the classification of groups could be observed with a confidence level of 95 %. From the application of the LIBS technique, it was possible to quantify pesticide residues by measuring Cl, P, and S, from a minimum preparation of the sample, guaranteeing the speed of the analysis, low application costs, and no production of toxic waste. The proposed calibration models were validated and it is concluded that they can be used in a complementary way to the analytical methods traditionally used in the detection of pesticides. Through this work, it has been shown that the LIBS technique can be used as a complementary tool in agricultural applications and can in turn be applied to other matrices in addition to the one presented in this work. Starting from new simple experimental configurations, with short execution times, without the production of toxic waste and low application costs.