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Advisor(s)
Abstract(s)
A Espectrometria de Massa com Plasma Indutivo Acoplado (ICP-MS) é uma técnica
analítica utilizada para determinações elementares com a capacidade de avaliar
simultaneamente metais e metalóides a partir de amostras. A análise dos metais nas águas
residuais é um passo importante na monitorização da saúde ambiental e na avaliação dos riscos
potenciais para os ecossistemas naturais. Com o objetivo de minimizar a poluição introduzida
nas águas naturais, as descargas de águas residuais são reguladas mas nem sempre dando a
devida importância aos vários poluentes uma vez que nem todos se encontram definidos nos
referidos diplomas legais. O presente trabalho foi desenvolvido no âmbito do estágio do
Mestrado em Tecnologias Ambientais na empresa ALS-Controlvet e incidiu na validação de uma
metodologia capaz de determinar baixos níveis de concentração de 22 elementos. Um problema
que geralmente surge da técnica ICP-MS são as interferências, com origem no gás de arraste
(árgon) ou oxigénio ambiental, mas também de elementos principais de amostras ambientais
como o cloro, o carbono ou o fósforo. Neste estudo, a configuração dos parâmetros da célula de
colisão foi otimizada e um fluxo de gás de reação (hélio) de 5 mL/min foi estabelecido como
condição para o método. A eficiência desta configuração foi testada com soluções de
interferências potenciais (100 e 500 ppm de Cl, P, C, Mg, Ca e Na).
As amostras de águas residuais foram digeridas com ácido nítrico antes da análise, com o
objetivo de oxidação da matéria orgânica presente. Os elementos em estudo foram: Al, As, B, Ba,
Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sb, Se, Sn, V e Zn. Os parâmetros quantificados na
validação foram a linearidade, limites analíticos, brancos, gama de trabalho, sensibilidade,
precisão (precisão intermedia e repetibilidade), exatidão (com testes interlaboratoriais) e
incerteza.
Os limites de quantificação (LQ), estabelecidos como a média de resultados de brancos
acrescido de 10 vezes o respetivo desvio padrão) variaram entre 0,5 ppb (Cd) e 2 ppm (Na). A
maioria dos elementos tem LQ de 1 ppb. A especificidade e a seletividade foram avaliadas por
testes de recuperação através de uma concentração conhecida de elementos principais e de
vestígios, mostrando bons resultados, uma vez que a recuperação foi de cerca de 100% ± 20%. A
precisão analítica foi avaliada como a variabilidade entre medidas repetidas do mesmo analito,
independentemente da presença ou ausência de fontes de erros. Para o estudo da repetibilidade
do método para fins de validação, dez testes independentes foram realizados no mesmo dia na
mesma amostra de água residual. A precisão intermedia foi estimada usando cartas de controlo.
Para a repetibilidade, o desvio padrão relativo foi inferior a 10%, o que é aceitável. Para todos os
elementos, a amplitude média do duplicado nas águas residuais foi inferior a 10%, mostrando-se
adequada. Testes interlaboratoriais foram realizados para avaliar a exatidão através da
determinação do Z-score. Nestes testes, observou-se um desempenho satisfatório em 95% das
determinações e 5% foram avaliados como questionáveis. A incerteza é um parâmetro
importante para estimar amplitude verdadeira onde está inserido o valor do resultado finail. As
incertezas foram calculadas a partir da precisão intermedia de um padrão de controle na faixa
do limite de quantificação de cada elemento e das recuperações (exatidão). As incertezas
variaram entre 10 e 25%.
Abstract Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is an analytical technique used for elemental determinations having the capacity of simultaneously evaluate metals and metalloids from samples. The analysis of metals in wastewaters is an important step in the monitoring of environmental health and evaluation of potential risks to natural ecosystems. With the goal of the pollution introduced into natural waterways wastewaters in different countries and thereby lower attention is given to elements with no maximum permissible levels established. The present work was developed within the scope of the Master's degree in Environmental Technologies in the company ALS-Controlvet and included the validation of a multi-element methodology able to determine low levels of 22 elements. A problem that usually arises from ICP-MS technique is the interferences even from the carrier gas (argon) or environmental oxygen but also from major elements of environmental samples like chlorine, carbon or phosphorous. In this study the configuration of parameters of the collision cell was optimized and a reaction gas flow (helium) of 5mL/min was established as the condition for the method. The efficiency of this configuration was tested with solutions of potential interferences (100 and 500 ppm of Cl, P, C, Mg, Ca and Na). The analytical procedures were done in a ICP-MS 7900 (Agilent) controled by MassHunter software. Wasterwater samples were digested with nitric acid prior to analysis. The elements under study were: Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sb, Se, Sn, V and Zn. The parameters evaluated in the validation were the linearity, analytical limits, blanks, working range, sensitivity, precision (intermediate precision and repeatability), accuracy (with proficiency tests) and uncertainty. The quantification limits (LQ, established as the mean of blanks results plus 10 times its standard deviation) varied between 0,5 ppb (Cd) and 2 ppm (Na). The majority of the elements have LQ of 1 ppb. The specificity and selectivity were evaluated by recovery tests by spiking a known concentration of major and trace elements, showing good results the recovery was around 100% ±20%. Analytical precision is concerned with the variability between repeated measurements of the same analyte, irrespective of the presence or absence of bias. For the study of the repeatability of the method for validation purposes, ten independent tests were carried out on the same day the same wastewater sample. The intermediate precision was estimated using range charts. For the repeatability, the relative standard deviation was below 10%, which is acceptable. For all the elements the average amplitude of duplicate in wastewaters was below 10%, showing a good agreement. Proficiency tests (PTs) were performed to evaluate the accuracy through the determination of the Z-score. In PTs a satisfactory performance was observed for 95% of the determinations and 5% were evaluated as questionable. The uncertainty is an important parameter to estimate the errors in the final results. Uncertainties were calculated from the intermediate precision of a control standard in the range of the quantification limit of each element and from recoveries (accuracy). The uncertainties varied between 10 and 25%.
Abstract Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is an analytical technique used for elemental determinations having the capacity of simultaneously evaluate metals and metalloids from samples. The analysis of metals in wastewaters is an important step in the monitoring of environmental health and evaluation of potential risks to natural ecosystems. With the goal of the pollution introduced into natural waterways wastewaters in different countries and thereby lower attention is given to elements with no maximum permissible levels established. The present work was developed within the scope of the Master's degree in Environmental Technologies in the company ALS-Controlvet and included the validation of a multi-element methodology able to determine low levels of 22 elements. A problem that usually arises from ICP-MS technique is the interferences even from the carrier gas (argon) or environmental oxygen but also from major elements of environmental samples like chlorine, carbon or phosphorous. In this study the configuration of parameters of the collision cell was optimized and a reaction gas flow (helium) of 5mL/min was established as the condition for the method. The efficiency of this configuration was tested with solutions of potential interferences (100 and 500 ppm of Cl, P, C, Mg, Ca and Na). The analytical procedures were done in a ICP-MS 7900 (Agilent) controled by MassHunter software. Wasterwater samples were digested with nitric acid prior to analysis. The elements under study were: Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sb, Se, Sn, V and Zn. The parameters evaluated in the validation were the linearity, analytical limits, blanks, working range, sensitivity, precision (intermediate precision and repeatability), accuracy (with proficiency tests) and uncertainty. The quantification limits (LQ, established as the mean of blanks results plus 10 times its standard deviation) varied between 0,5 ppb (Cd) and 2 ppm (Na). The majority of the elements have LQ of 1 ppb. The specificity and selectivity were evaluated by recovery tests by spiking a known concentration of major and trace elements, showing good results the recovery was around 100% ±20%. Analytical precision is concerned with the variability between repeated measurements of the same analyte, irrespective of the presence or absence of bias. For the study of the repeatability of the method for validation purposes, ten independent tests were carried out on the same day the same wastewater sample. The intermediate precision was estimated using range charts. For the repeatability, the relative standard deviation was below 10%, which is acceptable. For all the elements the average amplitude of duplicate in wastewaters was below 10%, showing a good agreement. Proficiency tests (PTs) were performed to evaluate the accuracy through the determination of the Z-score. In PTs a satisfactory performance was observed for 95% of the determinations and 5% were evaluated as questionable. The uncertainty is an important parameter to estimate the errors in the final results. Uncertainties were calculated from the intermediate precision of a control standard in the range of the quantification limit of each element and from recoveries (accuracy). The uncertainties varied between 10 and 25%.
Description
Keywords
ICP-MS Espectrometria de Massa com Plasma Indutivo Acoplado Metais Acreditação