Measurement of Radioactivity
17
jected to the quenching. This source of y-rays actually functions as the Ra standard. The computer then corrects the sample for quenching, and, together with
the cpm numbers the instrument prints, also the absolute radioactivity of the
sample corrected for quenching (Ra) in dpm units (De Wachter and Fiers
1967). Another semiautomatic way of quenching correction is the two-channel
method. In this case, the calibration curve is constructed for a range of variously quenched counting samples with the same absolute radioactivity. The
series of these samples is counted at two channels, one the wide channel A,
and the second the narrow channel B, counting only 15 % of the cpm counted
on channel A. With the aid of this curve it is possible to estimate the counting efficiency of any quenched sample after counting in these two channels
(Rogers and Moran 1966). Beckman spectrometers for liquid scintillation
counting are supplied with a microprocessor which commands the automatic
quench compensation based on the accounting of the pulse height changes
after the change of channels. The same instrument makes it possible to count
several isotopes simultaneously in one counting sample.
1.2.2.4. The Effect of Chemoluminescence
The phenomenon of chemoluminescence (and sometimes of bioluminescence)
occurs frequently during liquid scintillation counting of biological and especially of hydrobiological samples. In the counting sample subjected to chemoluminescence, in addition to the secondary photons produced during the
interaction of scintillation fluors with particles (or rays) emitted by the
radioisotope, additional photons are produced by the excited molecules, which
react with the fluors in the same way as the electrons, for example. During
the counting of such samples, their radioactivity may be significantly overestimated. The radioactivity in such samples will be recorded by the liquid scintillation spectrometer even if it does not contain any traces of the radioisotope.
This phenomenon is of a complex nature often unpredictable, and is called
therefore the black cat of the liquid scintillation method. From my own practice, I may give an example. When investigating the feeding of soft corals with
the aid of 14C, I had counted radioactivity in tissues of experimental corals fed
with 14C-labeled food. In one of the species (Lemnalia sp.) this appeared to be
impossible, because of strong chemoluminescence in the tissue preparations
(alkaline hydrolysates), so that samples of even nonradioactive specimens
counted in the scaler as over 2000 cpm. The agent which caused such strong
chemoluminescence obviously was not just chlorophyll, because other soft
corals which also had a lot of it, displayed no chemoluminescence; it was a case
of the black cat.
Most often chemoluminescence occurs in samples which contain photoactive or enzymically active pigments or substances, like chlorophyll or elements of the bioluminescence system. This can be caused by the presence in
the sample of peroxides, oxidizers (or oxygen), strong acids, or bases; but the
17
jected to the quenching. This source of y-rays actually functions as the Ra standard. The computer then corrects the sample for quenching, and, together with
the cpm numbers the instrument prints, also the absolute radioactivity of the
sample corrected for quenching (Ra) in dpm units (De Wachter and Fiers
1967). Another semiautomatic way of quenching correction is the two-channel
method. In this case, the calibration curve is constructed for a range of variously quenched counting samples with the same absolute radioactivity. The
series of these samples is counted at two channels, one the wide channel A,
and the second the narrow channel B, counting only 15 % of the cpm counted
on channel A. With the aid of this curve it is possible to estimate the counting efficiency of any quenched sample after counting in these two channels
(Rogers and Moran 1966). Beckman spectrometers for liquid scintillation
counting are supplied with a microprocessor which commands the automatic
quench compensation based on the accounting of the pulse height changes
after the change of channels. The same instrument makes it possible to count
several isotopes simultaneously in one counting sample.
1.2.2.4. The Effect of Chemoluminescence
The phenomenon of chemoluminescence (and sometimes of bioluminescence)
occurs frequently during liquid scintillation counting of biological and especially of hydrobiological samples. In the counting sample subjected to chemoluminescence, in addition to the secondary photons produced during the
interaction of scintillation fluors with particles (or rays) emitted by the
radioisotope, additional photons are produced by the excited molecules, which
react with the fluors in the same way as the electrons, for example. During
the counting of such samples, their radioactivity may be significantly overestimated. The radioactivity in such samples will be recorded by the liquid scintillation spectrometer even if it does not contain any traces of the radioisotope.
This phenomenon is of a complex nature often unpredictable, and is called
therefore the black cat of the liquid scintillation method. From my own practice, I may give an example. When investigating the feeding of soft corals with
the aid of 14C, I had counted radioactivity in tissues of experimental corals fed
with 14C-labeled food. In one of the species (Lemnalia sp.) this appeared to be
impossible, because of strong chemoluminescence in the tissue preparations
(alkaline hydrolysates), so that samples of even nonradioactive specimens
counted in the scaler as over 2000 cpm. The agent which caused such strong
chemoluminescence obviously was not just chlorophyll, because other soft
corals which also had a lot of it, displayed no chemoluminescence; it was a case
of the black cat.
Most often chemoluminescence occurs in samples which contain photoactive or enzymically active pigments or substances, like chlorophyll or elements of the bioluminescence system. This can be caused by the presence in
the sample of peroxides, oxidizers (or oxygen), strong acids, or bases; but the