Question

2. Steel is a construction material widely used for its durability under environmental conditions and its structural rigidity. This material has a main composition of iron, carbon (in variable proportions) and other metals (eg, Mn, Co, Cr, Ni among others) in different proportions. The typical analysis method for the determination of manganese consists of taking a sample of 1,0000 g of steel, oxidizing it through acid digestion with HMnO4 (permanganic acid) and subsequently diluting it in a 500 mL flask up to the volume mark, in order to measure its absorbance at a wavelength of 540 nm. In a routine analysis, three calibration curves were prepared using three different reference materials, and the following was observed: a) If the calibration curve is constructed from KMnO4 standard solutions, it turns out to be a straight line that passes through the origin. b) If the calibration curve is prepared from a steel standard sample containing various amounts of Cr, V, Mo and Mn, a straight line also results, but it does not pass through the origin. In reference to the previous information, investigate and answer the following: i) What is the most probable cause or origin why there is a significant difference between the intercept calculated for both methods? ii) Do you consider that there will be a significant difference between the analytical sensitivity, selectivity or detection and quantification limits for both cases? iii) What calibration method could be used to correct these differences, if present?

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Answer to a math question 2. Steel is a construction material widely used for its durability under environmental conditions and its structural rigidity. This material has a main composition of iron, carbon (in variable proportions) and other metals (eg, Mn, Co, Cr, Ni among others) in different proportions. The typical analysis method for the determination of manganese consists of taking a sample of 1,0000 g of steel, oxidizing it through acid digestion with HMnO4 (permanganic acid) and subsequently diluting it in a 500 mL flask up to the volume mark, in order to measure its absorbance at a wavelength of 540 nm. In a routine analysis, three calibration curves were prepared using three different reference materials, and the following was observed: a) If the calibration curve is constructed from KMnO4 standard solutions, it turns out to be a straight line that passes through the origin. b) If the calibration curve is prepared from a steel standard sample containing various amounts of Cr, V, Mo and Mn, a straight line also results, but it does not pass through the origin. In reference to the previous information, investigate and answer the following: i) What is the most probable cause or origin why there is a significant difference between the intercept calculated for both methods? ii) Do you consider that there will be a significant difference between the analytical sensitivity, selectivity or detection and quantification limits for both cases? iii) What calibration method could be used to correct these differences, if present?

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Brice

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Para determinar la concentración de manganeso en una muestra de acero, se utilizan curvas de calibración construidas a partir de disoluciones patrón. En el caso de los dos métodos mencionados:

i) La diferencia significativa en el intercepto calculado para ambos métodos se debe probablemente a la presencia de otros metales (Cr, V, Mo) en la muestra de acero utilizada en la preparación de la curva de calibración para el segundo método. Estos otros metales pueden interferir en la absorbancia medida, causando que la línea recta no pase por el origen.

ii) Es probable que existan diferencias significativas en la sensibilidad analítica, selectividad y los límites de detección y cuantificación para ambos casos. La presencia de otros metales en la muestra de acero puede afectar la respuesta analítica, disminuyendo la sensibilidad y selectividad del método, así como afectando los límites de detección y cuantificación.

iii) Para subsanar estas diferencias, se podría utilizar un método de calibración multivariante, como la calibración por mínimos cuadrados parciales (PLS), que tiene en cuenta la presencia de interferencias y permite corregir las variaciones debidas a la presencia de otros componentes en la muestra. Este método es útil para casos en los que se presentan interferencias o matrices complejas.

\textbf{Answer:}

i) La presencia de otros metales en la muestra de acero utilizada en el segundo método puede causar la diferencia en el intercepto calculado para ambos métodos.

ii) Es probable que existan diferencias significativas en la sensibilidad analítica, selectividad y los límites de detección y cuantificación para ambos casos.

iii) Para subsanar estas diferencias, se podría utilizar un método de calibración multivariante, como la calibración por mínimos cuadrados parciales (PLS).

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