16
General Features of Radioisotopic Methodology
vials. In the last vial with zero nonlabeled hydrolyzate added, the Ro will be
estimated (the nonquenched sample). Then into each vial equal volumes of
liquid radioactive standard solution are added with an automatic pipet, using
a standard solution such as 14C-labeled toluene or a fatty acid dissolved in
toluene. The samples thus charged are then filled with the scintillation cocktail, thoroughly mixed, and counted in the scaler. Then the counting efficiency
K is calculated for each sample (K = R;lRo), and the corresponding curve is
constructed with the dry weight of the material on the abscissa and K on the
ordinate. The use of such quenching curves is convenient for routine counting
samples with similar kinds of radioactive material, containing varying but
roughly estimable mass. With the aid of a quenching curve introduced into the
computer program, the recorded counting rates can be easily corrected for
quenching in accordance with the mass of material contained in the counting
samples.
The most rapid and simple way to control and correct quenching in variously charged individual samples is the use of an internal standard. In this case,
the counting efficiency K is estimated by injection into the sample, after its
counting rate (R;) is also estimated a standard portion of radioactivity emitter
with known relative counting rate (Ro, cpm), or absolute radioactivity (Ra,
dpm). The sample is counted again and then its relative (Kr) or absolute (Ka)
counting efficiency is calculated: Kr = (Rsc - R; - Rb)/Ro, and Ka = (Rsc - R; -
Rb)/Ra, if Rsc is the cpm counting rate in the sample after the injection of the
standard solution, and Rb the background counts. The liquid standard solutions (such as 14C-labeled toluene) or disposable probes in the form of plastic
pellets with known R a , which can be put inside the counting samples, are supplied by specialized companies manufacturing radioisotopic equipment. A
probe to estimate quenching in 14C counting samples in the form of a glass
stick with 14COTIabeled crystals sealed into its end x Ba can be easily prepared in the laboratory. The end of the glass stick is heated to melting point
and inserted in the powder of the labeled Ba-carbonate placed on the bottom
of a narrow test tube. Then it is heated again to a red color, cooled, washed
with toluene, and its surfaces cleaned with paper. The stick thus prepared is
fixed into the cup of the scintillation vial with its radioactive end 0.5-1 cm from
the bottom, and is preserved in the toluene solution. Its cpm and dpm radioactivity is estimated and corrected with the aid of a standard counting sample.
During estimation of counting efficiency in the experimental samples, the
latter are first counted in a scaler, then recounted again with the stick standard inside (Dobbs 1963).
Modern scintillation counters, liquid scintillation spectrometers, also have
devices for automatic quench correction, based on an external standard. The
instrument commanded by the microprocessor first measures the radioactivity of the counting sample (R;) in cpm units. Then behind the scintillation vial
the stick with the y-emitter is shown automatically ( 133 Ba or 137CS). The y-rays
produce Compton electrons in the counting sample during their dissipation,
which react with the scintillation fiuors like the 13-particles, and are thus sub-
General Features of Radioisotopic Methodology
vials. In the last vial with zero nonlabeled hydrolyzate added, the Ro will be
estimated (the nonquenched sample). Then into each vial equal volumes of
liquid radioactive standard solution are added with an automatic pipet, using
a standard solution such as 14C-labeled toluene or a fatty acid dissolved in
toluene. The samples thus charged are then filled with the scintillation cocktail, thoroughly mixed, and counted in the scaler. Then the counting efficiency
K is calculated for each sample (K = R;lRo), and the corresponding curve is
constructed with the dry weight of the material on the abscissa and K on the
ordinate. The use of such quenching curves is convenient for routine counting
samples with similar kinds of radioactive material, containing varying but
roughly estimable mass. With the aid of a quenching curve introduced into the
computer program, the recorded counting rates can be easily corrected for
quenching in accordance with the mass of material contained in the counting
samples.
The most rapid and simple way to control and correct quenching in variously charged individual samples is the use of an internal standard. In this case,
the counting efficiency K is estimated by injection into the sample, after its
counting rate (R;) is also estimated a standard portion of radioactivity emitter
with known relative counting rate (Ro, cpm), or absolute radioactivity (Ra,
dpm). The sample is counted again and then its relative (Kr) or absolute (Ka)
counting efficiency is calculated: Kr = (Rsc - R; - Rb)/Ro, and Ka = (Rsc - R; -
Rb)/Ra, if Rsc is the cpm counting rate in the sample after the injection of the
standard solution, and Rb the background counts. The liquid standard solutions (such as 14C-labeled toluene) or disposable probes in the form of plastic
pellets with known R a , which can be put inside the counting samples, are supplied by specialized companies manufacturing radioisotopic equipment. A
probe to estimate quenching in 14C counting samples in the form of a glass
stick with 14COTIabeled crystals sealed into its end x Ba can be easily prepared in the laboratory. The end of the glass stick is heated to melting point
and inserted in the powder of the labeled Ba-carbonate placed on the bottom
of a narrow test tube. Then it is heated again to a red color, cooled, washed
with toluene, and its surfaces cleaned with paper. The stick thus prepared is
fixed into the cup of the scintillation vial with its radioactive end 0.5-1 cm from
the bottom, and is preserved in the toluene solution. Its cpm and dpm radioactivity is estimated and corrected with the aid of a standard counting sample.
During estimation of counting efficiency in the experimental samples, the
latter are first counted in a scaler, then recounted again with the stick standard inside (Dobbs 1963).
Modern scintillation counters, liquid scintillation spectrometers, also have
devices for automatic quench correction, based on an external standard. The
instrument commanded by the microprocessor first measures the radioactivity of the counting sample (R;) in cpm units. Then behind the scintillation vial
the stick with the y-emitter is shown automatically ( 133 Ba or 137CS). The y-rays
produce Compton electrons in the counting sample during their dissipation,
which react with the scintillation fiuors like the 13-particles, and are thus sub-