You analyze a sample of rice for traces of arsenic. The As content can help you

Question

You analyze a sample of rice for traces of arsenic. The As content can help you find the origin of the rice. You’re given a 155 g sample of rice and you remove 2.50 g, pulverize it with a mortal and pestle, and dissolve it in 10 mL of a 50/50 mixture concentrated nitric acid and sulfuric acids. After dissolution, you take the sample to dryness on a hot plate and then add 10% HNO3 to a volume of 25.0 mL. This sample is stored at 4°C until analysis. The stored sample is removed from the refrigerator and 150 µL are removed and diluted to 10.0 mL, again with 10% HNO3. When that diluted sample is analyzed by AAS the As absorbance is 0.351. If LLS line for your calibration, plotting ppm As versus absorbance, was y = 98.5x + 0.022, what was the concentration of As in the rice in ppb?

Explanation / Answer

From the given data the final absorbance is 0.351 and from the equation given y = 98.5x+0.022

the ppm of As is 0.351 = 98.5 x +0.022

                          x = 0.00334ppm = 3.34 ppb = 3.34µg / L

so 10ml contains 0.0334µg of As.This 10 ml is prepared from 150µL of stored sample

So 150µL of stored sample contains 0.0334µg of As

      25 ml of stored sample contains 5.566µg of As

So 152.5 g of rice contains 5.566µg of As

155 g of rice contains 5.651µg of As

concentration of As in the rice is 36.458ppb

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