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Problem Session

Problem Session. The radioactive isotope 32 P decays by first-order kinetics and has a half-life of 14.3 days. How long will it take before the radioactivity of the 32 P falls to below 6.25 % of the initial value? A. 58 days B. 38 days C. 29 days D. 14 days.

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Problem Session

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  1. Problem Session

  2. The radioactive isotope 32P decays by first-order kinetics and has a half-life of 14.3 days. How long will it take before the radioactivity of the 32P falls to below 6.25 % of the initial value? • A. 58 days B. 38 days C. 29 days D. 14 days

  3. Carbon -14 (14C) is a radioactive isotope with a half-life (t1/2) of 5730 years Mass number (A) = sum of 146C ⇨ 147N + e- A neutron becomes a proton Atomic number (Z) = The process of radioactive decay follows first order kinetics. What fraction of the original 14C remains in a piece of wood cut from a tree 2200 years ago? When the tree is cut it stops consuming carbon dioxide, so the 14C content begins to decrease.

  4. Fraction remaining = N/N0 N0 = number originally in tree N = number after 2200 years t1/2 = 5730 y

  5. Isotope Dilution Analysis • Requires the preparation or purchase of a radioactive form of the analyte (analyte*). • A known weight of analyte*is then mixed with the sample to be analyzed. After the specified separation process, a weighed portion of the pure product is counted. • The extent of dilution of the active material is calculated, allowing calculation of the original concentration of analyte. • Quantitative recovery is not required

  6. Specific activity = cpm per weightDetermine for final product Wt = weight tracer added Wx = weight unknown Wm = weight overall product • Rt = countrate of tracer added • Rm = countrate overall product

  7. Therefore

  8. To a sample of protein hydrolysate, a chemist added 1.00 mg tryptophan labeled with C-14. It exhibited a cpm of 584 above background. • After this labeled compound was thoroughly mixed with the sample, the mixture was passed through an ion exchange column.

  9. The fraction of effluent containing only tryptophan was collected, and from it an 18.0 mg sample of pure tryptophan was isolated. • This isolated sample had a cpm of 204 cpm in the same counter. • What was the weight of the tryptophan in the original sample?

  10. In an isotope dilution experiment, chloride was determined by adding 5.0 mg NaCl containing 38Cl (t1/2=37.3 min) to a sample. • The specific activity of the added NaCl was 4.0 x 104 cps/mg. • What was the total amount of chloride present in the original sample if 400 mg of pure AgCl was isolated and if this material had a counting rate of 35 cps above background 148 min after the addition of the radiotracer?

  11. Consider: • Decay of tracer • So calculate cpm for chloride added (Rt)after 148 minutes • Final compound is AgCl, initial is NaCl • So use weights of chloride only • Ag:107.9 Na:22.99 Cl:35.45 • Then use equation as previously

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