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 result-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 pub 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)

In the data given for the diluted 5mL sample, making use of the best fit equation, we get the following conc. values for the six solutions in series:

0, 0.573, 1.05, 1.405, 1.956, 2.46

The 95% C.I. is then : -0.52 to 3 ppb

(b)

For the original chocolate sample, we put y = 0 in the best fit equation.

Putting y=0, we get:

x = -1.386

The -ve sign is because of extrapolation into the -ve x axis.

So, [Cr] in chocolate sample = 1.386 ppb

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