Problem Session

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Problem Session
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• 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

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Fraction remaining N/N0
N0 number originally in tree N number after
2200 years t1/2 5730 y
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Isotope Dilution Analysis

• Requires the preparation or purchase of a
  radioactive form of the analyte (analyte).
• A known weight of analyteis 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

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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

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• Therefore

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• 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.

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• 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?

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• In an isotope dilution experiment, chloride was
  determined by adding 5.0 mg NaCl containing 38Cl
  (t1/237.3 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?

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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
• Ag107.9 Na22.99 Cl35.45
• Then use equation as previously

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