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Hey guys. Sorry for the late replies to your comments. As the answers to your questions could be quite long, i decided to have it here instead.

To Ting Jie and Lyn

I have found out that the need for separation of the unbound ligand from the bound complexes indeed takes a longer time due to the steps involved.

I will illustrate using the dextran-charcoal method to remove the unbound radiolabelled ligand. After incubating the samples overnight, instead of counting the samples as for SPA, this method requires the addition of charcoal reagent and incubation for 30 min at 4oC. Afterwhich, the samples have to be centrifuged for at least 5 min. Supernatant has to be decant into vials and subsequently the addition of liquid scintillant. However, even after the addition of scintillant, there is still a need for 1h of incubation before counting.

Hence, as compared to the use of SPA, we can say that it actually takes about an addition of 2 more hours to complete the whole RIA process.

Furthermore, it has to be noted that while adding the charcoal reagent to the samples, the solution has to be in suspension. The time taken for the addition of the reagent also has to be rapid so that incubation time between the sample and the charcoal reagent does not differ too much across the assay. Ideally, the speed should be between 2 to 3 min. This however, takes practice and depends on the technican skills.

Hence, because of these factors it can increase the chances of human error.

While with the use of SPA, these steps are eliminated and thus decrease the human errors.

To Lloyd

The principle of scintillation counter is quite physics and lengthy. So not to bore you, I just give you the main principle.

Basically, the photons (electromagnetic waves) of light emitted from the radioisotopes are detected by a light-sensitive device called as the photomultuplier tube (PMT).

Firstly, interaction between the photons and the photocathode occur first, ejecting photoelectrons. The photoelectrons then accelerate to a series of dynodes (metallic element) where each dynode is at a higher position voltage than the previous one. The first strike of the photoelectrons to the first dynode will release secondary electrons which in turn will strike on the second dynode to release more electrons. This will continue for 12 dynode stages, each time the electrons increase at about 10to the power of 7.

The interaction between the photocathode and the photons also provide an electrical pulse where the amplitude of the electrical pulse will be proportionate to the amount of photons that interact with the photocathode and hence will be proportionate to the energy of the beta particle in the sample.

There is also a circuit incorporated into the counter where it can give a total pulse proportional to the total photon yield of the event, for example in 60 sec

Thanks for the comments.

Xin Yi
TG02