Scintillation Counting
Liquid Scintillation Counting
(LS Counting) is a lab based strategy that utilizes a Liquid Scintillation
Counter (LSC) to check the radioactive emanations from a liquid sample. It is
regularly utilized in the organic sciences to quantify the take-up of
radioactive isotopes into biological materials. Radioactive isotopes interact
with matter in two different ways, ionization and excitation.
Excitation
drives an energized molecule or compound (known as a fluor) to emanate photons
of light. The procedure is known as scintillation. when the light is recognized
by a photomultiplier, it frames the premise of scintillation counting. Basically,
a photomultiplier changes over the energy of radiation into an electrical sign,
and the strength of the electrical
signal that outcomes is legitimately corresponding to the energy of the first
radioactive occasion. This implies two, or significantly more, isotopes can be independently identified and estimated in a
similar example, if they have adequately
different energy emitting spectra.
Types of scintillation counting
Solid scintillation counting
In
solid scintillation counting the sample is placed adjacent to a solid fluor
(e.g. sodium iodide). Solid scintillation counting is particularly useful for
gamma emitting isotopes. This is because they can penetrate the fluor. The
counters can be small handheld devices with the fluor attached to the
photomultiplier tube or larger bench-top machines with a well-shaped fluor
designed to automatically count many samples.
Liquid Scintillation Counting
In
liquid scintillation counting, the sample is mixed with a scintillation fluid
containing a solvent and one or more dissolved fluors. This method is
particularly useful in quantifying weak b-emitters such as 3 H, 14C and 35S,
which are frequently used in biological work. Scintillation fluids are called
‘cocktails’ because there are different formulations, made up of a solvent
(such as Toulene) & fluors such as 2,5-diphenyloxazole (PPO), 1,4-bis(5-
phenyloxazol-2-yl)benzene (nicknamed POPOP, or 2-(40
-t-butylphenyl)-5-(400-bi-phenyl)-1,3,4-oxydiazole (butyl-PBD).
Liquid
Scintillation Counting
There are a number of physical processes that may disrupt LSC. These include:
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Process
|
Explanation
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Examples
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Reduce
Problem By
|
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Chemiluminescence
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Generation
of light due to chemical processes.
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Bleaching
agents, dioxane-based scintillators
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Equilibrate
sample for a period of time in the LSC
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Photoluminescence
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Emission
of photons from excited molecular species.
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Vials,
caps, other materials in the LSC. Some samples such as proteinaceous
materials when dissolved in alkaline solubilisers such as hyamine.
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Acidify
samples; avoid exposure to sunlight or fluorescent lighting. Dark adapt
samples for several hours before counting.
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Quenching
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Reduction
in the scintillation count rate.
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Photon
quenching, chemical impurity quenching, colour quenching (see diagram below).
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Use
Internal Standards to account for quenching. A standard with a known CPM/DPM
(Counts per minute/Disintegrations per minute) is added and measured and the reduction
due to quenching adjusted for in the measured samples.
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Examples
of the use of LS Counting
- Viral Proteins: Proteins
produced by viruses when they infect a cell are produced in very small
amounts and are difficult to detect and purify. If virus-infected cells
are fed a radioactive amino acid, then each time that amino acid is linked
to form the growing protein a radioactive ‘label’ is attached to the
protein. This radioactive ‘label’ is then used to monitor the
identification and purification of the viral protein. Amino acids
containing 3H, 14C and 35S
are often used to label proteins. 35S is particularly
useful as sulphur is only found in two amino acids – methionine and
cysteine.
- Environmental Monitoring: Checking
for 3H spills in the laboratory. Tritium is such a weak
emitter that its presence cannot be detected by a Geiger-Mueller counter.
Wipe testing is usually used. This is where suspect surfaces are wiped
with a piece of tissue. The tissue is placed in LS Cocktail in a LS vial
and counted in the LS Counter.
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