214
Exercise 14
of 14C is absorbed readily by cellular materials, no more algae should be collected
than is necessary to minimize self-absorption. Therefore, in eutrophic waters, 10-ml
or less may provide sufficient radioactivity while simultaneously minimizing
self-absorption problems.
9. The filters are removed while wet from the filtration apparatus with forceps and
placed onto clean planchets. A ring weight that does not come in contact with
the filtered materials is then placed on the edges of the filters to prevent curling.
The filters are then placed under the desiccation at room temperature for a brief
(6- to 24-h) period. Expose dry filters to fumes of HCl under a hood for 10 min to
remove residual inorganic 14C and carbonates that may have precipitated as a
result of photosynthetic activity [cf., Wetzel (1965)]. The samples then are desiccated again before radioassay.
Radioassay of the Filters
The detection of the random emission of f)-radiation over time follows a Poisson
distribution (see Appendix 2), and hence a minimum of approximately 1500 counts
is necessary for statistical significance at the 5~;;; level. Reasonably 'hot' radioactive
samples, e.g., light bottles from shallow depths, may yield 5000 to 10,000 counts in
a reasonable time. Samples from dark bottles are relatively inactive and may require
longer time periods to yield 500 to 1000 counts. Modern automatic instruments reduce
the time required for the assay, thus eliminating the need to separate active from
inactive samples.
Radioassay by liquid scintillation counting has the advantages of greatly increased
counting efficiencies (50 to 95%), estimations of counting efficiency for each sample
counted, and determination of the activity of stock 14C solutions, membrane filters,
and filtrates by the same instrument.
Filters and other substances to be radioassayed are immersed in organic mixtures
(fluors) consisting of solvents (e.g., toluene, dioxane) and organic scintillators. Development and improvement of fluors are proceeding rapidly. Some of the techniques
that have been used are discussed in detail by Schindler et al. (1974; Schindler, 1966)
see also manufacturer's recommendations. Algal pigments can cause severe
interferences and losses in efficiency of counting; this quenching must be carefully
evaluated. (Caution: Many fluor systems are highly flammable and some are suspected
to be carcinogenic; adequate ventilation must be used at all times.)
Place the filters flat on the bottom of standard 20-ml scintillation vials with the
algal coating on the upper surface, avoiding any contact with the plankton on the
filter surface. Add the appropriate scintillation mixtures. Count each filter twice for
at least 10 min. An internal standardization procedure employing 14C-toluene or
-hexadecane may be used to determine counting efficiency (Schindler and Holmgren,
1971). The amount of radioactivity on each filter then can be expressed as
disintegrations per minute (dpm).
Activity of the 14C Transfer Inoculum Solution
The radioactivity of the inoculum added to the sample bottles must be known with
accuracy. Standardized 14C solutions developed specifically for productivity analyses
are available from the Danish 14C Agency (H~rsholm, Denmark) but are expensive
if purchased in small quantities. When 14C solutions are purchased, diluted, ampoulated, and sterilized [cf., Strickland and Parsons (1968)], their activity must be determined exactly by comparison with National Bureau of Standards samples of the
Exercise 14
of 14C is absorbed readily by cellular materials, no more algae should be collected
than is necessary to minimize self-absorption. Therefore, in eutrophic waters, 10-ml
or less may provide sufficient radioactivity while simultaneously minimizing
self-absorption problems.
9. The filters are removed while wet from the filtration apparatus with forceps and
placed onto clean planchets. A ring weight that does not come in contact with
the filtered materials is then placed on the edges of the filters to prevent curling.
The filters are then placed under the desiccation at room temperature for a brief
(6- to 24-h) period. Expose dry filters to fumes of HCl under a hood for 10 min to
remove residual inorganic 14C and carbonates that may have precipitated as a
result of photosynthetic activity [cf., Wetzel (1965)]. The samples then are desiccated again before radioassay.
Radioassay of the Filters
The detection of the random emission of f)-radiation over time follows a Poisson
distribution (see Appendix 2), and hence a minimum of approximately 1500 counts
is necessary for statistical significance at the 5~;;; level. Reasonably 'hot' radioactive
samples, e.g., light bottles from shallow depths, may yield 5000 to 10,000 counts in
a reasonable time. Samples from dark bottles are relatively inactive and may require
longer time periods to yield 500 to 1000 counts. Modern automatic instruments reduce
the time required for the assay, thus eliminating the need to separate active from
inactive samples.
Radioassay by liquid scintillation counting has the advantages of greatly increased
counting efficiencies (50 to 95%), estimations of counting efficiency for each sample
counted, and determination of the activity of stock 14C solutions, membrane filters,
and filtrates by the same instrument.
Filters and other substances to be radioassayed are immersed in organic mixtures
(fluors) consisting of solvents (e.g., toluene, dioxane) and organic scintillators. Development and improvement of fluors are proceeding rapidly. Some of the techniques
that have been used are discussed in detail by Schindler et al. (1974; Schindler, 1966)
see also manufacturer's recommendations. Algal pigments can cause severe
interferences and losses in efficiency of counting; this quenching must be carefully
evaluated. (Caution: Many fluor systems are highly flammable and some are suspected
to be carcinogenic; adequate ventilation must be used at all times.)
Place the filters flat on the bottom of standard 20-ml scintillation vials with the
algal coating on the upper surface, avoiding any contact with the plankton on the
filter surface. Add the appropriate scintillation mixtures. Count each filter twice for
at least 10 min. An internal standardization procedure employing 14C-toluene or
-hexadecane may be used to determine counting efficiency (Schindler and Holmgren,
1971). The amount of radioactivity on each filter then can be expressed as
disintegrations per minute (dpm).
Activity of the 14C Transfer Inoculum Solution
The radioactivity of the inoculum added to the sample bottles must be known with
accuracy. Standardized 14C solutions developed specifically for productivity analyses
are available from the Danish 14C Agency (H~rsholm, Denmark) but are expensive
if purchased in small quantities. When 14C solutions are purchased, diluted, ampoulated, and sterilized [cf., Strickland and Parsons (1968)], their activity must be determined exactly by comparison with National Bureau of Standards samples of the