Detail
In more detail, when the radio-labelled molecule is attached or is in proximity to bead, light emission is stimulated. However, if the bead does not become bound to the radio-labelled molecule, the bead will not be stimulated to emit light. This is because the energy released from the unbound molecule is not strong enough to excite the SPA bead which is not then stimulated to produce a signal.
The decay of radioactive atom releases electrons as one of the subatomic particles. The energy of these particles influences the distance travelled by the particles itself through a medium such as water. This influence of the energy or the limitation of the water is what this SPA method is dependent upon.
For instance, the decay of a Tritium atom releases a beta particle. Tritium is highly recommended as it suits SPA very well. It is due to the 1.5 µm path length through water, which is very short. So, when the ß-particle is within that particular range of 1.5 µm with the scintillant bead, there is sufficient energy to stimulate the bead to emit light. If the distance between them is greater than 1.5 µm, then the ß-particle is incapable of travelling the required distance to stimulate the bead as it has insufficient energy.
The beads in SPA are formed from the incorporation of scintillant into small beads known as fluomicrospheres. These are specially designed to bind with specific molecules. When the bead is in close proximity to the radioactive molecule, light is stimulated.
The photonmultiplier tube (PMT) can be used to detect the emitted photons. This device is converts the emitted photon energy into electrical energy by a photocathode via a series of other electrodes. Another device is known as CCD Imager, which is composed of a set of cooled digital cameras with sensitive charge coupled device detectors and with some refined telecentric lenses to convert the captured photon energy into high quality images.
There is also an assortment of bead coatings available that allows this method to be applied to a broad range of applications, such as enzyme assays and radio-immuno assays.
Read more about this topic: Scintillation Proximity Assay
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