INSTRUMENTATION
The two most
vital equipments essentially required for radioimmunoassay (RIA) are, namely :
(i) Centrifuge,
and
(ii)
Radioactive Counters.
These two equipments shall now be discussed briefly as
follows :
A centrifuge which is capable of generating 1200-2500 rpm
using swing-bucket-rotor or 3500 to 4000 rpm using a fixed-angle-head rotor can
be employed effectively. However, the former type is preferred because of the
fact that here the pellet is formed at the bottom of the test tube and the supernatant
layer is more easily removed in comparison to the latter type where the pellet
is formed at an angle. In case, a centrifuge having relatively less
gravitational force is employed then it is absolutely necessary to enhance the
centrifugation time until suitable pellets are formed duly.
In usual practice, two
types of radioactive counters are mainly employed depending on the type of
radioactive substance used, namely :
(a) Gamma
Counters, and
(b)
Scintillation Counters.
These are used invariably for the gamma-energy emitting isotopes, for instance : 125I-the
more com-mon iodine-isotope.
These are mostly used for counting beta-energy-emitting isotopes, such as : tritium 3H and 14C-(Carbon-14)
isotopes.
First and foremost, radioimmunoassays were universally
based on the 3H or 14C isotope labelling tech-nique, but
this has the main disadvantage of using liquid-scintillation counting.
Therefore, the comparatively much simpler technique of gamma-ray counting by
labelling compounds with 124I, 125I, or 131I
is now being increasingly utilized wherever such labelling is practically
feasible.
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