We have now fractionated the tissue into four portions: the acid-soluble fraction (containing small molecules), RNA, DNA, and proteins. (The cell’s lipids and sugars come out during alcohol rinses between the weak acid and the alkali steps.) Chemical analysis allows us to measure precisely the amount of RNA or DNA in its respective fraction and therefore in the tissue or organ. The amount of radioactivity in the RNA fraction can then be determined by a technique known as liquid scintillation counting.
Liquid scintillation counting is the preferred method for the measurement of low-energy beta-emitting radioisotopes commonly used in cell-fractionation studies (see Figure 18). It is convenient, sensitive, and rapid for routine measurement of radiation in hydrocarbons, other organic compounds, and aqueous solutions containing such isotopes as ³H, ¹⁴C, and ³²P.
Liquid scintillation solutions share with other scintillating materials the property of converting into visible light the energy deposited in them by ionizing radiation. In theory, if a sample of a beta emitter is dissolved in a liquid scintillator solution, every beta particle emitted will be absorbed completely because the range of penetration of beta particles in liquids is quite short (ranging from 0.008 millimicron for ³H to 7.9 millimicrons for ³²P in a medium of unit density). The kinetic energy of the beta particles is largely used up in the ionization and excitation of the most abundant molecular species present, the solvent in which the scintillating material was dissolved. A fraction of the energy thus expended by each beta particle is transferred from excited solvent molecules to scintillator molecules; thus the electrons in the atoms of the scintillator molecules are raised to an excited (higher energy) state. When these electrons return to the ground, or unexcited, state, a fraction of them emit a photon of light. Thus each beta particle produces a burst of photons.
[Illustration: Figure 18 _Technician placing a tray of samples in a liquid scintillation counter. The radioactivity of each sample is recorded as the trays revolve._]
If a vessel containing the liquid scintillator and the radioactive sample is placed near a suitably sensitive instrument known as a photomultiplier tube, each burst of scintillator photons activates this device and causes it to release a burst of photoelectrons. Each burst of photoelectrons is multiplied successively in a series of electronic steps; as a result, there is a suitably large electrical-output pulse to be recorded.