Radio active labeling

Measurement of Radioactivity

1. Radioactive Labeling and Measurement of Radioactivity

2. Atom The smallest part of each element is called the "Atom". It is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons . The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons. Atom is the core of every substance in the universe. Our bodies are made mostly of hydrogen, carbon, oxygen, and calcium atoms. Air contains - Hydrogen, oxygen, nitrogen, and other atoms.

3. Radioactive Atoms There are 92different kinds of atoms present on the earth. The lightest of these is the hydrogen atom and the heaviest is the uranium atom . Generally, the heavier atoms are not so stable as like that of lighter atoms. These unstable atoms are known as radioactive atoms. They become stable only by emitting some radiation. The emission of radiation by them is referred to as radioactivity.

4. Ionising Radiation Uranium ,Thorium etc. are the naturally unstable substance. The atoms of such substances attains stability by releasing energy in the form of electromagnetic radiations or Particles ( α , β , and γ ). These are called Nuclear radiations or Ionising radiations . They are capable of converting the neutral atoms of all substance in to positive and negative ions through interactions.

5. Radioactive isotopes The atoms which has same atomic number but has different mass number are called isotopes . All elements with atomic numbers greater than 83 are radioisotopes meaning that these elements have unstable nuclei and are radioactive. Radioisotope T 1/2 3 H 12.3 years 14 C 5730.0 years 35 S 87.4 days 32 P 14.3 days 125 I 60.0 days 131 I 8.0 days 24 Na 15.0 hours Half-life(T 1/2 ) or Physical half –life (TP) is the time required for the radioactivity to be reduced to one half of its original value.

6. Unit of radioactivity Measured in terms of disintegrations per second ( dps ) or disintegrations per minute ( dpm ) Curie (Ci) is recognized as the unit of radioactivity ( 1 Curie (Ci) = dpm or dps produced by one gram of pure Radium – 226 ). 1 Curie (Ci) = 2.22 × 10 12 dpm or 3.7 × 10 10 dps The international system of units (SI) uses the term Bequrrel ( Bq ) which is equivalent to one dps . (1Curie = 3.7 × 10 10 Bq )

7. Radioactive Labeling Incorporation of a radioactive isotope with in a molecule in order to investigate its metabolism, fate and utilization is called radiolabeling . The compound can be detected using radioactivity detection techniques. A number of different radioactive forms of hydrogen, Carbon, phosphorus, sulfur and iodine are commonly used in applications including biochemical assays, metabolism studies, and medical diagnostics.

8. Radioactive tracing was developed by George de Hevesy, who won the 1943 Nobel Prize for Chemistry for his pioneering work using radioactive tracers to study metabolic processes in plants and animals. Chemiluminescence offers a highly sensitive alternative to radioactivity, but low intensity signal means it is not amenable to automation and quantification is difficult due to the narrow linear range of the signal produced For detection of enzyme-labelled antibodies, the most commonly used radiolabels are 125 I and 35 S . The signal produced by radiolabelled antibodies is typically captured on autoradiographic film, providing a permanent, hard-copy result.

9. Commonly used isotopes and their primary forms Isotopes Primary form 14 C Ba 14 CO 3 3 H Hydrogen gas 35 S H 2 35 SO 4 32 P H 3 32 PO 4 18 F Carbon-14 - Used for radiometric dating ( upto 50,000 years) Tritium (hydrogen-3) - A very low energy emitter that can be used to label Proteins, Nucleic acids, Drugs and Toxins. 18 F - Often used in PET scanning , Tomography can thus be used for diagnosis, staging, and monitoring treatment of Cancers . Sulfur-35 -Used to label Proteins and Nucleic acids . Phosphorus-32 -Widely used for labeling nucleic acids and phosphoproteins . Iodine-125 -Commonly used for labeling Proteins, usually at tyrosine residues

10. Detection of Radiation Radiation cannot be detected by our senses so radiation level can be detect and measure either by using X-ray film method Ionization method- GM counter Scintillation Counting method- Produce light when expose to radiation (The intensity of light emitted by the scintillator is proportional to the radiation level)

11. X-Ray film method Radiations affect the X-ray films This principle used in autoradiography Autoradiography is useful for tracking down various biochemical events, that are taking place in cells using radioisotopes. It is not a quantitative method

12. Ionization method- Geiger-Muller counter GM counter is a gas filled detector consists of a volume of gas between two electrodes, with an electrical potential difference (voltage) applied between the electrodes charged particle in gas ⇒ ionization ⇒ electrons liberated Positive ions (cations) attracted to negative electrode (cathode); electrons or anions attracted to positive electrode (anode) In most detectors, cathode is the wall of the container that holds the gas and anode is a wire inside the container. Gas is usually noble gas (e.g. argon), with some additives e.g. carbon dioxide, methane, isobutane,..) as “quenchers” GM counter is used only for hard β - emitters

13. Geiger-Muller counter

14. Scintillation Counting It is based upon the fact that certain chemical substance emit light on exposure to radiation The measurement of these flashes of light is the basis for Scintillation Counting Types Solid Scintillation Counting Liquid Scintillation Counting

15. Solid Scintillation Counting It is ideal for gamma emitters. It consist of Large crystal of Sodium Iodide Small amount of thallium iodide An assembly of photomultiplier Preamplifier Source of High Voltage A Scaler encased in an aluminium casing

16. Photomultiplier tubes PMTs perform two functions: Conversion of ultraviolet and visible light photons into an electrical signal Signal amplification, on the order of millions to billions Consists of an evacuated glass tube containing a photocathode, typically 10 to 12 electrodes called dynodes, and an anode

17. Photomultiplier tube

18. It is used to measure soft β - emitters such as 3 H, 35 S and 14 C. It consist of two photomultiplier tubes one on either side of a scintillation vial and enclosed in a light tight lead shield. PMT’s connected to a scaler through a high voltage power supply , a coincidence counting circuit and a pulse height analyzer. The test sample to be counted is mixed in a glass vial with scintillation fluid which is composed of 0.5% PPO and 0.01% POPOP in analar , scintillation grade toluene . Liquid Scintillation Counting

19. Solvent Excited Solvent Excited PPO PPO Excited POPOP POPOP PMT Scaler 417 nm 365 nm 315 nm Mechanism of energy Transfer Organic solvent- Toluene PPO (Primary Flour)- 2,5 di-phenyl oxazole POPOP(Secondary Flour)- 1,4 bis(5-phenyloazole-2)

20. Uses of Radioactivity/Radiation Radioactive sources are used to study living organisms, to diagnose and treat diseases, To sterilize medical instruments and food To produce energy for heat and electric power, and to monitor various steps in all types of industrial processes. Tracers - commonly used in the medical field and in the study of plants and animals. Radioactive Iodine-131 -used to study the function of the thyroid gland assisting in detecting disease.