About 2 to 3 grams of sediment in the frozen 

 state and 1 gram of thawed plankton, blotted with 

 absorbent paper, were transferred to irradiation 

 vials and weighed. Three ml of concentrated ni- 

 tric acid were added to these samples prior to 

 irradiation. A section of frozen sediment that was 

 in juxtaposition to the sample was chipped off and 

 its water content was determined by drying at 

 110°C for 1 hour. 



Comparator and Nitric Acid Blanks. The 

 comparator consisted of 10 /xg of mercury as the 

 nitrate in 10 ml of concentrated nitric acid. The 

 nitric acid blank comprised three irradiation vials 

 filled with 14 ml of concentrated nitric acid. 



Irradiation. Samples, comparators, and 

 blanks were irradiated for 1 hour in a flux of 

 1.8 X 10^^ neutrons cm"^ sec"^ in a sample con- 

 tainer that rotated at 1 rpm about the core of the 

 Mark I TRIGA Reactor at Gulf Energy and Envi- 

 ronment Services, San Diego, California. The 

 irradiations occurred during the period 3 to 4 pm. 

 The following morning samples were processed to 

 obtain radiochemical purity. 



Radiochemical Purification. Sedi- 

 ments and plankton were transferred to flasks that 

 finally contained 1 ml concentrated phosphoric 

 acid, 1 ml of mercury carrier (10 mg mercury as 

 nitrate), 15 ml of 7 molar sulfuric acid, and 10 ml 

 of concentrated nitric acid. The flask was at- 

 tached to an apparatus described by Sj0strand 

 (1964) and distilled until fumes of sulfur trioxide 

 appeared. The condensed acid was re-admitted to 

 the flask and the distillation was repeated until 

 sediments were white and the plankton digest was 

 colorless. 



Sediment and plankton were prepared further 

 for radiochemical purification by addition of 25 

 ml of concentrated ammonium hyroxide to the 

 digest, and the mixture was filtered. If at this 

 stage pebbles were detected in the residual sedi- 

 ment, they were removed and the sample was 

 corrected for their weight. 



Ten mg of mercury carrier were added to water 

 samples and the nitric acid blanks. The nitric 

 acid blank was reduced in volume to about 10 ml. 

 Water samples and blanks both received 9 ml of 

 concentrated ammonium hydroxide and 10 mg 

 each of potassium and sodium chloride. (The 

 quantities of chlorides were also added prior to 

 the succeeding precipitations.) 



From this stage, with one exception noted 

 later, all samples were treated similarly. To the 

 filtrate were added 2.5 ml of freshly prepared 

 stannous chloride. Mercury metal that precipi- 

 tated was collected by centrifugation. The pre- 

 cipitate was dissolved in 5 ml aqua regia, 5 mg of 

 copper (as nitrate) were added, and the solution 

 was filtered. The reduction of mercury to the 

 metal was repeated and the solid collected by 

 filtration. The precipitate was again dissolved 

 with 5 ml aqua regia (1 to 2 drops of concentrated 

 phosphoric acid was added to the water samples), 

 and the solution was neutralized with 5 ml of 

 concentrated ammonium hydroxide. Hydrogen 

 sulfide gas was passed through the pH-adjusted 

 solution and the precipitate was collected by fil- 

 tration. The mercuric sulfide was of sufficient 

 purity to permit immediate measurement. 



The comparator was neutralized with 9 mi of 

 concentrated ammonium hydroxide after the ad- 

 dition of mercury carrier, and the mercuric sul- 

 fide was precipitated and collected for measure- 

 ment. 



Carrier Yield Determination. The proc- 

 essed mercury samples as well as mercury carrier 

 standards (10 mg mercury) were re-irradiated for 

 5 seconds. Through comparison of the activity 

 level of the samples and standards, the carrier 

 yield was computed and the counting rate in the 

 original irradiation was corrected for this factor. 



Measurement. The radioactive measure- 

 ments were made with a sodium iodide detector 

 coupled to a 400-channel pulse-height analyzer. 

 The counts attributable to the 77-keV radiation of 

 i97Hg were integrated by the method of Covell 

 (1959). 



RESULTS AND DISCUSSION 



I 



The mercury content of the zooplankton sample 

 was 37 parts per billion (ppb), a value which falls 

 within the range of 6 to 47 for zooplankton that 

 were collected 430 km southwest of San Diego 

 (Williams and Weiss, 1973). 



The analysis of snow samples appears in Table 

 I. Their values ranged from 3.5 to 46.2 parts per 

 trillion (ppt), with an average of 13 ppt. This 

 average value is substantially less than that of 



232 



