Question

Isotope dilution is a very useful tool for quantitative analysis using MS methods. For instance, there are 3 common isotopes of strontium: ⁸⁶Sr (10% abundance), ⁸⁷Sr (7%), and ⁸⁸Sr (83%). A fourth isotope, ⁸⁴Sr, is also known, but its natural abundance is only 0.56%.

Isotope	Counts
Sr 84	17,210
Sr 86	14,121
Sr 87	9,884
Sr 88	117,201

50.0 mg of a rock containing strontium was dissolved in 100 mL of nitric acid, and 1.00 mg of ⁸⁴Sr was added to the solution before it was injected into the ICP-MS. The table shows the data recorded for the spiked sample.

In a separate experiment, a solution of 1.00 mg/100 mL of ⁸⁴Sr gave a signal of 16,400 counts.

How much mass of Sr was in the ore?

Answer 1

1 mg/ 100mL of 84Sr is giving a signal of 16,400 counts on ICP-MS.

50 mg of rock together with 1mg of 84Sr gave total signal of 158,416 (17210+14121+9884+117201 =158416)

Total Sr .Ore present in the sample = 158,416/16,400 = 9.66 mg

9.66mg of Sr also includes 1mg of 84Sr added to the sample. That mean, the rock contains 9.66-1 = 8.66 mg of Sr.

Please find the table for more details.