A 0.2056 g chocolate sample was digested (completely dissolved) in nitric acid a

Question

A 0.2056 g chocolate sample was digested (completely dissolved) in nitric acid and the resulting solution diluted to a total volume of 10.00 mL. A series of solutions were then prepared with total constant volumes of 5.00 mL, each containing 1.00 mL of the chocolate solution plus variable volumes of a 125 ppb chromium standard. The constant volume solutions were analyzed using ICP-MS to determine the chromium concentrations (results shown below). *See page 16 in textbook for ppb definition. What is the chromium concentration (in ppb) plusminus 95% confidence interval for the diluted (5 mL) sample? What is the chromium concentration (in ppb) for the original (solid) chocolate sample?

Explanation / Answer

A. The chromium concentration can be derived using the R-squared equation

y = 7177.7x + 9950.5

for 100 µl of sample Is + x = 27661

for 5 ml (5000 µl) of sample is +x = 27661 * 5000 / 100 = 1383050

Substituting the value for y = 1383050 in the equation

1383050 = 7177.7 x + 9950.5

x = 191.30

the chromium concentration was found to be 191.30 ppb

B. The chromium concentration in original choclate sample is = 10 * 125 = 1250 ppb

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