Lesson 14. Radiotracers

Transcription

1 Lesson 14 Radiotracers

2 Introduction Basic principle: All isotopes of a given element will behave identically in most physical, environmental and biological environments. Ea sier to detect radioactive atoms than stable atoms. Simple Cheap Amazing breadth of applications. Now part of the standard arsenal of analytical techniques

3 Designing a Radiotracer Experiment Radioactive isotopes of a given element behave the same as stable isotopes. Isotope Eff ect--most concern for H, 15% effect for 12 C vs 14 C. Nonisotopic tracers

4 Intermolecular vs Intramolecular Effects Intramolecular--decarboxylation of malonic acid (HOO 14 C- 12 CH 2-12 COOH) Intermolecular--decarboxylation of benzoic acid-7-14 C (C 6 H 5-14 COOH vs C 6 H 5-12 COOH)

5 Basic Principle #2 Radioactivity does not change the chemical and physical properties of the system. Tracer activities, bond strengths, energy of emitted radiation 3 H problem Daughter atom problems

6 Basic Principle #3 No deviation from normal physiological state. Injecting stable as well as radioactive material

7 Basic Principle #4 Chemical and physical form of the labeled material is the same as the unlabeled material. Aqueous chemistry Adsorption, carriers, carrier-tracer exchange Radiochemical purity Position of the label

8 Basic Principle #5 Label stays intact

9 Practical Considerations Availability of tracer t 1/2 Labeling, position of the label Ease of detection Health and safety aspects Waste disposal

10 Calculation of the amount of tracer needed No excuse for turning a tracer experiment into a detection problem. Statistics (> 10 4 cts) Counting efficiency, detector type, geometry factors, chemical yields Safety factor

11 Hazard evaluation Dose rates, RBEs Effects on physiology Waste problems Green chemistry

12 Radiotracer Preparation Commonly Used Tracers 3 H(T) R yr β C R 5730 yr β Na C 2.60 yr β +, γ Na R 15.0 hr γ P R 14.3 d β n-rich vs p-rich PET nuclides

13 Labeling Position: acetic-1-14 C acid = CH 3 14 COOH, acetic-2-14 C acid = 14 CH 3 COOH Terms; Specifically labeled, U, N G

14 Methods of labeling Chemical synthesis,best except for chirality Biological synthesis, tedious Tritium Commercial supplier problems

15 Physical tracers Wear, corrosion Mixing Fluid motion Pollutant tracing Measuring unknown volume V = V 1 ( A 1 A 2 "1)

16 Chemical Applications Test separation procedures K sp measurements Exchange reactions Chemical reaction mechanisms

17 Example Cyclization of ω- phenoxyacetophenone to 2- phenylbenzofuran

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19 Physical Isotope Effects Gaseous diffusion Distillation

20 Chemical Isotope Effects AX + BX*!AX* + BX Table 4-2 Typical Equilibrium Isotope Effects Reacting System K H 2 H(g) + H 2 O(l)!H 2 (g) + 2 HOH(l) CO 2 (g)+ 12 COCl 2 (g)! 14 COCl 2 (g)+ 12 CO 2 (g) [Co(NH 3 ) 4 12 CO 3 ] CO 3 2-! 12 CO [Co(NH 3 ) 4 14 CO 3 ]

21 Kinetic Isotope Effects A + B! AB! products d[b] dt = k[a] a [B] b d[b*] dt = k * [A] a [B*][B] b"1 d[b] d[b*] = k[b] k * [B*] log 10 ( S " S 0 ) = ( k * k #1)log 10(1# " )

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23 Biological applications Autoradiography

24 DNA Analysis

25 Radioimmunoassay

26 Figure 19. The radioimmunoassay technique to measure hormone levels in the body involves mixing natural hormones (from a blood sample) and radioactively prepared hormones together, and adding that mixture to a solution containing artificially produced antibodies. The two hormones compete for binding sites on the antibodies. The hormones attach to the antibodies in amounts that ore proportional to their concentration.

27 Environmental Applications of Radiotracers Used in tracing pollutant motion in atmosphere, hydrosphere, lithosphere Advantages of radiotracers Not influenced by state of system Highly penetrating Good detection sensitivity If t 1/2 short, will disappear after expt. Cheap BIG PROBLEM--HP

28 41 Ar 24 Na 82 Br 140 La Typical Environmental Radiotracers

29 Industrial Uses of Radiotracers Nuclide Application 3 H Self-luminous aircraft and exit signs Luminous dials, gauges, wrist watches Luminous paint 24 Na 46 Sc 55 Fe Location of pipeline leaks Oil well studies Oil exploration tracer Analysis of electroplating solutions Defense power source 60 Co Surgical instrument and medicine sterilization Safety and reliability of oil burners 63 Ni Detection of explosives Voltage regulators and current surge protectors Heat power source

30 Nuclear Medicine Therapy and diagnosis Most important radionuclides are 99 Tc m and the I isotopes. Over 90% of procedures involve these nuclei

31 99 Tc m 143 kev gamma rays 6 hr t 1/2 Use of generators

32 99 Mo $ $ " # % $ $ $ $ % 99 m IT (142.7keV& ) Tc 99 Tc

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34 PET, Positron Emission Tomography

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37 Isotope Dilution Analysis Direct IDA In direct IDA, we are faced with the problem of determining the amount of some inactive material A in a system. Let us define this unknown amount as x grams. To the system containing x grams of inactive A, we add y grams of active material A* of known activity D. Thus we know the specific activity of the added active material, S 1. S 1 = D y After thoroughly mixing the active material A* with the inactive A in the system, one isolates, not necessarily quantitatively, and purifies a sample of the mixture of A and A* and measures its specific activity, S 2. S 2 = D x + y S 2 = D x + D S 1 x = D S 2 " D S 1 = D S 1 ( S 1 S 2 "1) = y( S 1 S 2 "1)