12
General Features of Radioisotopic Methodology
scintillation vials. Better solublization or homogenization of tissues samples
can be achieved by employing a combination of the solubilizing agents above,
heating to 50-70°C, and ultrasonic disintegration. The acids nitric and perchloric mentioned above are used with the addition of hydrogen or benzoyl
peroxides, which enhance their solubization effect and also bleach the
samples.
Another common technique for efficient counts of 14C-Iabeled plant
or animal materials by liquid scintillation is the quantitative conversion
of the organic 14C-Iabeled carbon contained in them into CO2 by wet or
dry combustion. The labeled CO2 thus formed is trapped by alkaline solutions prepared from the bases listed above (sodium hydroxide, hyamine)
dissolved in alcohols or toluene. These trapping solutions are easily miscible
with the scintillation cocktails and make highly efficient homogenous counting samples (Cuppy and Crevasse, 1963). Packard offers an automatic
apparatus-oxidizer for combustion of 14C-labeled samples, which can process
60 samples 1 h- 1 •
Examples of the most efficient and reasonable techniques of tissue sample
preparation for liquid scintillation counting and their application in hydrobiology will be described in more detail below in the chapters on primary production and feeding of aquatic animals; also that counts of tissue samples
are used for a minor part of hydro biological studies which need the use of
radioisotopes. The majority of them are based on counting of heterogeneous
counting samples like suspensions, or of materials concentrated on the membrane or glass fiber filters (Gill 1964; Shindler 1966; Pugh 1973). For better
contact with the fluor's liquid, the membrane filters can be dissolved in ethylacetate and the sediment contained at their surface resuspended mechanically
or sonically in the counting sample. In the same way (mechanically or sonically) the glass fiber filters can be disintegrated and resuspended in the liquid
(Johnson and Smith 1963).
The statistical background of liquid scintillation counting is based upon
the assumption that the disintegration of radioactive molecules of a labeled
sample are randomly distributed in time. The number of radioactive transformations in the sample in any particular time interval may vary significantly.
Therefore, in a series of I-min counts, none of them will be acceptable as an
exact record of the real counting rate, which can only be established by numerous counts, computing the averages of I-min counts, or by counting for longer
time intervals. The longer this interval is and the higher the rate of counting,
the closer its approximation to the "true" counting rate. In accordance with
the theory of probability, if the total number of counts is over 100, the error
of average (ea ) from a group of random counts is proportional to the square
root of total counts: ea = K-.JJV. The parameter K corresponds here to the standard deviation which reflects the confidence level in the computed value.
The standard error es of counting rate r, obtained as the ratio of the total
number of counts N and time t, during which these counts are recorded (r =
Nit), could be calculated using the equation: es = W. By mUltiplying es by the
General Features of Radioisotopic Methodology
scintillation vials. Better solublization or homogenization of tissues samples
can be achieved by employing a combination of the solubilizing agents above,
heating to 50-70°C, and ultrasonic disintegration. The acids nitric and perchloric mentioned above are used with the addition of hydrogen or benzoyl
peroxides, which enhance their solubization effect and also bleach the
samples.
Another common technique for efficient counts of 14C-Iabeled plant
or animal materials by liquid scintillation is the quantitative conversion
of the organic 14C-Iabeled carbon contained in them into CO2 by wet or
dry combustion. The labeled CO2 thus formed is trapped by alkaline solutions prepared from the bases listed above (sodium hydroxide, hyamine)
dissolved in alcohols or toluene. These trapping solutions are easily miscible
with the scintillation cocktails and make highly efficient homogenous counting samples (Cuppy and Crevasse, 1963). Packard offers an automatic
apparatus-oxidizer for combustion of 14C-labeled samples, which can process
60 samples 1 h- 1 •
Examples of the most efficient and reasonable techniques of tissue sample
preparation for liquid scintillation counting and their application in hydrobiology will be described in more detail below in the chapters on primary production and feeding of aquatic animals; also that counts of tissue samples
are used for a minor part of hydro biological studies which need the use of
radioisotopes. The majority of them are based on counting of heterogeneous
counting samples like suspensions, or of materials concentrated on the membrane or glass fiber filters (Gill 1964; Shindler 1966; Pugh 1973). For better
contact with the fluor's liquid, the membrane filters can be dissolved in ethylacetate and the sediment contained at their surface resuspended mechanically
or sonically in the counting sample. In the same way (mechanically or sonically) the glass fiber filters can be disintegrated and resuspended in the liquid
(Johnson and Smith 1963).
The statistical background of liquid scintillation counting is based upon
the assumption that the disintegration of radioactive molecules of a labeled
sample are randomly distributed in time. The number of radioactive transformations in the sample in any particular time interval may vary significantly.
Therefore, in a series of I-min counts, none of them will be acceptable as an
exact record of the real counting rate, which can only be established by numerous counts, computing the averages of I-min counts, or by counting for longer
time intervals. The longer this interval is and the higher the rate of counting,
the closer its approximation to the "true" counting rate. In accordance with
the theory of probability, if the total number of counts is over 100, the error
of average (ea ) from a group of random counts is proportional to the square
root of total counts: ea = K-.JJV. The parameter K corresponds here to the standard deviation which reflects the confidence level in the computed value.
The standard error es of counting rate r, obtained as the ratio of the total
number of counts N and time t, during which these counts are recorded (r =
Nit), could be calculated using the equation: es = W. By mUltiplying es by the