Avaliação de estratégias para melhoria da sensibilidade em análises por espectrometria de emissão óptica com plasma induzido por laser
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Data
2017-08-09
Autores
Lázaro, Maisa Cristina [UNESP]
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Editor
Universidade Estadual Paulista (Unesp)
Resumo
A espectrometria de emissão óptica em plasma induzido por laser (LIBS) é uma técnica que vem se desenvolvendo cada vez mais para determinações analíticas multielementares, com aplicações em diversas áreas. Possui várias vantagens, como por exemplo, ser uma técnica de análise rápida, minimamente destrutiva e ambientalmente amigável, enquanto que a maior dificuldade está baseada na sua relativa baixa sensibilidade. Muitos estudos têm sido desenvolvidos com a finalidade de melhorar a sensibilidade, porém os aprimoramentos elaborados demandam equipamentos mais sofisticados, aumentando o custo e comprometendo a portabilidade da técnica. O presente trabalho avaliou estratégias simples para melhorar a sensibilidade das medidas em LIBS, permitindo o uso de instrumentos compactos de pulso simples. As estratégias desenvolvidas se baseiam na proposição da introdução de uma etapa simples de pré-tratamento da amostra, capaz de agir na fragilização de estruturas químicas que por consequência levem ao aumento das intensidades de emissão das espécies componentes da amostra. As etapas de pré-tratamentos buscam em sua essencialidade não descaracterizar as vantagens analíticas de LIBS, tais como a simplicidade e a não utilização de reagentes potencialmente tóxicos, além de serem aplicáveis à diversos tipos de amostras. A ação da temperatura sobre a modificação de estruturas químicas foi avaliada utilizando aquecimento em mufla para amostra de material vegetal, solo e liga metálica e em forno de micro-ondas para amostra vegetal. Os tratamentos empregados utilizando aquecimento em mufla resultaram em aumentos nas intensidades de emissão em toda região espectral para todas as amostras avaliadas; para a amostra de folha, material para o qual foram obtidos os melhores resultados, foram verificados aumentos de 47x com relação à área de pico da linha de Ca I em 612,14 nm, além do aparecimento de linhas antes não observadas, como a de Sr II em 407,82 nm. Para a amostra de liga, a área da linha de Fe I em 489,22 nm teve um aumento de 8x e para a amostra de solo, a linha de Na em 588,89 nm, por exemplo, teve seu valor de área aumentado em 4x com a aplicação do tratamento térmico. O aquecimento em micro-ondas forneceu incrementos de intensidade menos pronunciados, comparados ao aquecimento em mufla. Adicionalmente foi avaliada a introdução de fundente à amostra de solo com o intuito de melhorar processos físicos, os quais poderiam culminar em aumento dos processos de emissão dos analitos da amostra, entretanto, esta estratégia não atingiu o objetivo proposto. Por fim, o tratamento térmico em mufla foi aplicado no desenvolvimento de um método para determinação de Sr em material foliar utilizando LIBS. O método desenvolvido apresentou curva analítica linear com coeficiente de correlação igual a 0,997, limite de detecção igual a 4,23 mg kg-1 e limite de quantificação igual a 14,09 mg kg-1. A análise de material de referência certificado foliar pelo método proposto indicou que a concentração de Sr determinada é concordante com o valor certificado a 95% de confiança (test t pareado).
Laser-induced breakdown spectroscopy (LIBS) is an analytical technique in development, increasingly applied for multielemental determinations in several areas. It shows several advantages, such as rapidity, minimally destructive and environmentally friendly, although the greatest difficulty is based on its relatively low sensitivity. Many efforts have been expended to improve LIBS sensitivity, but several proposals require more sophisticated equipment, increasing the cost and compromising the portability of the technique. The present work evaluated simple strategies to improve the sensitivity of LIBS measurements allowing the use of compact single pulse instruments. The proposed strategies are based on a simple sample pretreatment, able to promotes simplification of chemical structures, which could lead to an increase of emission intensities. The proposed pretreatment steps aimed to preserve the interesting LIBS analytical advantages, such as its simplicity and non-use of toxic reagents, and its applicability to several types of samples. Then the temperature action on the simplification of chemical structures was evaluated using conventional muffle oven, for samples of plant leaves, soil and metal alloy. Additionally, microwave oven heating was evaluated for plant leaves. The muffle oven treatments resulted in important increases of the emission intensities for all spectral region of all evaluated samples; for the plant leaves, material for which the best results were observed, the integrated peak area of the Ca I line at 612.14 nm was increased about 47x after the thermic treatment. Moreover, new emission lines, not observed before, such as Sr II line at 407.82 nm, appeared. In the alloy sample, the integrated area of Fe I line at 489.22 nm showed an increase of 8x and in soil sample the Na I line at 588.89 nm, showed an increasing of 4x. The microwave heating provided less pronounced intensity increments compared to the muffle heating. The mixture of the soil sample with chemical modifier was also evaluated in order to improve physical processes, which could lead to an increasing of the emission signals. Nevertheless, this strategy did not reach the expected. Finally, the heat treatment in muffle was applied in the development of a method for determination of Sr in leaf material using LIBS. The developed method presented a linear analytical curve with correlation coefficient of 0.997, limit of detection of 4.23 mg kg -1 and limit of quantification of 14.09 mg kg-1 . Analysis of leaf reference material by the proposed method indicated the determined concentration of Sr is in agreement with the certified value at 95% confidence (paired t-test).
Laser-induced breakdown spectroscopy (LIBS) is an analytical technique in development, increasingly applied for multielemental determinations in several areas. It shows several advantages, such as rapidity, minimally destructive and environmentally friendly, although the greatest difficulty is based on its relatively low sensitivity. Many efforts have been expended to improve LIBS sensitivity, but several proposals require more sophisticated equipment, increasing the cost and compromising the portability of the technique. The present work evaluated simple strategies to improve the sensitivity of LIBS measurements allowing the use of compact single pulse instruments. The proposed strategies are based on a simple sample pretreatment, able to promotes simplification of chemical structures, which could lead to an increase of emission intensities. The proposed pretreatment steps aimed to preserve the interesting LIBS analytical advantages, such as its simplicity and non-use of toxic reagents, and its applicability to several types of samples. Then the temperature action on the simplification of chemical structures was evaluated using conventional muffle oven, for samples of plant leaves, soil and metal alloy. Additionally, microwave oven heating was evaluated for plant leaves. The muffle oven treatments resulted in important increases of the emission intensities for all spectral region of all evaluated samples; for the plant leaves, material for which the best results were observed, the integrated peak area of the Ca I line at 612.14 nm was increased about 47x after the thermic treatment. Moreover, new emission lines, not observed before, such as Sr II line at 407.82 nm, appeared. In the alloy sample, the integrated area of Fe I line at 489.22 nm showed an increase of 8x and in soil sample the Na I line at 588.89 nm, showed an increasing of 4x. The microwave heating provided less pronounced intensity increments compared to the muffle heating. The mixture of the soil sample with chemical modifier was also evaluated in order to improve physical processes, which could lead to an increasing of the emission signals. Nevertheless, this strategy did not reach the expected. Finally, the heat treatment in muffle was applied in the development of a method for determination of Sr in leaf material using LIBS. The developed method presented a linear analytical curve with correlation coefficient of 0.997, limit of detection of 4.23 mg kg -1 and limit of quantification of 14.09 mg kg-1 . Analysis of leaf reference material by the proposed method indicated the determined concentration of Sr is in agreement with the certified value at 95% confidence (paired t-test).
Descrição
Palavras-chave
LIBS, Preparo de amostras, Sensibilidade, Tratamento térmico, Sensitivity, Heat treatment, Sample preparation