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RADIOACTIVE LABELING AND MEASUREMENT OF
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.
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
They become stable only by emitting some radiation. The
emission of radiation by them is referred to as radioactivity.
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.
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.
3H 12.3 years
14C 5730.0 years
35S 87.4 days
32P 14.3 days
125I 60.0 days
131I 8.0 days
24Na 15.0 hours
Half-life(T1/2) or Physical half –life (TP) is the time required for the radioactivity to be
reduced to one half of its original value.
UNIT OF RADIOACTIVITY
Measured in terms of disintegrations per second (dps) or disintegrations per
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 × 1012 dpm or 3.7 × 1010 dps
• The international system of units (SI) uses the term Bequrrel (Bq) which is
equivalent to one dps. (1Curie = 3.7 × 1010 Bq)
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.
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
For detection of enzyme-labelled antibodies, the most commonly used
radiolabels are 125I and 35S.
The signal produced by radiolabelled antibodies is typically captured on
autoradiographic film, providing a permanent, hard-copy result.
COMMONLY USED ISOTOPES AND THEIR PRIMARY FORMS
Isotopes Primary form
3H Hydrogen gas
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.
18F - 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
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)
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
It is not a quantitative method
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
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
Solid Scintillation Counting
Liquid Scintillation Counting
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
Source of High Voltage
A Scaler encased in an aluminium casing
PMTs perform two functions:
Conversion of ultraviolet and visible light photons into an
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
It is used to measure soft β- emitters such as 3H, 35S and 14C.
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
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
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.