28
The Radiocarbon Method to Estimate Primary Production
tion is distributed in I-ml sealed ampules. Often the specific radioactivity of
this solution did not fit the necessary parameters. It could have impurities,
which induce a significant background 14C02 uptake in dark controls (Williams
et al. 1972). Many researchers therefore prefer to make the a working solution with the necessary scale of radioactivity and necessary purity themselves
from concentrated Nal 14CO.i batch preparations. Batches with a radioactivity
of 1 to 5 mCi are distributed by radiochemical companies, and such ordinary
radioisotope preparations cost several times less than the ready-for-use
working solution. In any case, the sets of ampules with the ready working solution of 14C-carbonate, if used, should be previously checked, especially if they
have been stored for a long time at room temperature. For this two or three
bottles with water samples (as using in primary production experiments)
should be fixed with iodine (Lugol) solution, and portions of the working solution from the ampule should be injected into them. After some time, they
should be filtered, and the radioactivity of the filters counted under the same
conditions as in the experiments. The radioactivity of these control filters
should not be more than 1O-20cpm at a radioactivity thus injected of some
0.5-1 x 10 6 cpm per bottle.
To prepare a working solution from batches of 14C-Na-carbonate, some
inexperienced researchers used simply to dilute it with water, adjusting the pH
with NaOH to about 9, and using some 1O-20mgl- 1 of Na2C03 as a carrier.
The working solution for use in marine waters should contain also 3-4% NaCl.
After dissolution of the 14C-carbonate batch in this water or in the NaCl solution, the working isotope solution thus obtained has to be filtered first through
a glass fiber filter, and then through a 0.1-0.2-~m membrane filter to remove
suspended matter. Then the working solution is distributed into 1-2-ml
ampules, which are sealed and pasteurized twice in boiling water for lOmin.
This kind of working solution may be used, with some care, for primary production measurements in eutrophic waters; but some curious artifacts can
sometimes occur with its use, when the dark uptake or the background counts
in nonexposed samples rises to 100-200cpm, or the parallel estimations
display an unusually high variability. Perhaps it happens because in such a
working solution some colloidal or dissolved organic impurities remain, having
a very high specific radioactivity (ratio of radioactivity to carbon). During the
experiments, this labeled matter may enter the food web, thus becoming accessible for filtration, increasing the radioactivity of the filters. Therefore, I recommend avoiding this easy way of preparing a working solution.
Much safely is the procedure which includes passing the stock of labeled
carbonate in the batch via the gaseous CO2 phase. This guarantees the absolute
purity of the. working solution and a low and stable background (Doty and
Oguri 1959; Vollenveider 1974). The following simple procedure can be used.
Most convenient for this procedure are batches with 14C Na or Ba carbonate,
which have a radioactivity within 1 to 4mCi. They are supplied by the companies in sealed ampules or closed glass containers. The container is opened
and 1-2 ml of 2 % N a2C03 solution is added into it as carrier by a Pasteur pipet
The Radiocarbon Method to Estimate Primary Production
tion is distributed in I-ml sealed ampules. Often the specific radioactivity of
this solution did not fit the necessary parameters. It could have impurities,
which induce a significant background 14C02 uptake in dark controls (Williams
et al. 1972). Many researchers therefore prefer to make the a working solution with the necessary scale of radioactivity and necessary purity themselves
from concentrated Nal 14CO.i batch preparations. Batches with a radioactivity
of 1 to 5 mCi are distributed by radiochemical companies, and such ordinary
radioisotope preparations cost several times less than the ready-for-use
working solution. In any case, the sets of ampules with the ready working solution of 14C-carbonate, if used, should be previously checked, especially if they
have been stored for a long time at room temperature. For this two or three
bottles with water samples (as using in primary production experiments)
should be fixed with iodine (Lugol) solution, and portions of the working solution from the ampule should be injected into them. After some time, they
should be filtered, and the radioactivity of the filters counted under the same
conditions as in the experiments. The radioactivity of these control filters
should not be more than 1O-20cpm at a radioactivity thus injected of some
0.5-1 x 10 6 cpm per bottle.
To prepare a working solution from batches of 14C-Na-carbonate, some
inexperienced researchers used simply to dilute it with water, adjusting the pH
with NaOH to about 9, and using some 1O-20mgl- 1 of Na2C03 as a carrier.
The working solution for use in marine waters should contain also 3-4% NaCl.
After dissolution of the 14C-carbonate batch in this water or in the NaCl solution, the working isotope solution thus obtained has to be filtered first through
a glass fiber filter, and then through a 0.1-0.2-~m membrane filter to remove
suspended matter. Then the working solution is distributed into 1-2-ml
ampules, which are sealed and pasteurized twice in boiling water for lOmin.
This kind of working solution may be used, with some care, for primary production measurements in eutrophic waters; but some curious artifacts can
sometimes occur with its use, when the dark uptake or the background counts
in nonexposed samples rises to 100-200cpm, or the parallel estimations
display an unusually high variability. Perhaps it happens because in such a
working solution some colloidal or dissolved organic impurities remain, having
a very high specific radioactivity (ratio of radioactivity to carbon). During the
experiments, this labeled matter may enter the food web, thus becoming accessible for filtration, increasing the radioactivity of the filters. Therefore, I recommend avoiding this easy way of preparing a working solution.
Much safely is the procedure which includes passing the stock of labeled
carbonate in the batch via the gaseous CO2 phase. This guarantees the absolute
purity of the. working solution and a low and stable background (Doty and
Oguri 1959; Vollenveider 1974). The following simple procedure can be used.
Most convenient for this procedure are batches with 14C Na or Ba carbonate,
which have a radioactivity within 1 to 4mCi. They are supplied by the companies in sealed ampules or closed glass containers. The container is opened
and 1-2 ml of 2 % N a2C03 solution is added into it as carrier by a Pasteur pipet