Close this window
Study notes icon


Study Notes: LS Counting

Liquid scintillation counting (LS Counting) is a laboratory-based technique that uses a Liquid Scintillation Counter (LSC) to count the radioactive emissions from a liquid sample. It is often used in the biological sciences to measure the uptake of radioactive isotopes into biological materials. The different forms of an element are called isotopes.

For instance, the nucleus of the element phosphorus has 15 protons but it may contain differing numbers of neutrons. If it has 13 neutrons it is called 28P (15 + 13 = 28) and if it has 19 neutrons it is called 34P. In fact phosphorus exists as 7 different isotopes:

28P, 29P, 30P, 31P,32P, 33P, 34P

31P is the most abundant form and is stable whereas the other isotopes may be unstable and emit radioactivity. In the case of 32P, a beta particle is emitted.

LSC is a method of counting radioactive emissions from a limited range of radionuclides. The common isotopes used include:

Isotope
Half-Life
Decay
125I (Iodine)
60 days
γ
3H (Hydrogen also called tritium)
12.3 years
β
14C (Carbon)
5730 years
β
35S (Sulphur)
87.4 days
β
32P (Phosphorus)
14.3 days
β

Notice that all except 125I are beta emitters. Beta emissions are often low or very low energy and although the emission can sometimes be detected by gamma radiation detection equipment such as a Geiger-Mueller Counter, the energy can easily be absorbed by the compound itself, by the surroundings and covers on detection equipment.

Liquid scintillation counting was developed to detect these low energy beta emitters.

The scintillation process

The beta decay electron emitted by the radioactive isotope in the sample excites the solvent molecule which in turn transfers the energy to the solute (called a fluor as it emits light). The fluor emits a photon of light that is detected by the photomultiplier tube and converted into an electric signal and detected by the apparatus.

The sample is dissolved in a liquid scintillation cocktail that is typically composed of the following components:

Component
Reason
Example of a component
Solvent To solubilise the fluor and the sample. Toluene, xylene, pseudodocumene
Emulsifier A detergent to ensure proper mixing of aqueous samples. Triton X-100
Fluor Emits light photon when excited by a beta particle. PPO, POPOP

There are many commercially available LS cocktails that are more environmentally friendly and contain less hazardous solvents than those listed above. Many are formulated to allow up to 40% v/v addition of sample to the cocktail.

Interfering Processes
There are a number of physical processes that may disrupt LSC. These include:

Process
Explanation
Examples
Reduce Problem By
Chemiluminescence Spurious generation of light due to chemical processes. Bleaching agents, dioxane-based scintillators Equilibrate sample for a period of time in the LSC
Photoluminescence Emission of photons from excited molecular species. Vials, caps, other materials in the LSC. Some samples such as proteinaceous materials when dissolved in alkaline solubilisers such as hyamine. Acidify samples; avoid exposure to sunlight or fluorescent lighting. Dark adapt samples for several hours before counting.
Background Radioactivity that does not arise from the sample. Chance coincidence, cosmic rays, Cerenkov radiation, natural radioactivity such as thorium, potassium-40 and uranium. Using appropriate blanks to correct for background.
Quenching Reduction in the scintillation count rate. Photon quenching, chemical impurity quenching, colour quenching (see diagram below). Use Internal Standards to account for quenching. A standard with a known CPM/DPM is added and measured and the reduction due to quenching adjusted for in the measured samples.

The quenching process

The output of the LSC is either in CPM or DPM.

CPM (Counts per minute) - gives a raw value for the number of radioactive events measured per minute. Often the sample is counted for a longer period (5 – 10 minutes) to account for any aberrations in shorter counts due to the radioactive events being a random process.

DPM (Disintegrations per minute) - adjusted CPM value to take into account the efficiency of the LSC. For instance, the LSC does not count all radioactive disintegrations as they occur in all three dimensions around the sample container and the detector is not able to measure them all.

Examples of the use of LS Counting

  1. 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.

  2. 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.


line
Resources and Training Room  >>  Study Notes  >>  LS Counting
Close this window