using a nitrogen/hydrogen flame, required an im- 

 provisation of two simple interconnecting adapt- 

 ers. The first, attached directly to the instrument, 

 consisted of the female portion of a polyethylene 

 quick disconnect (Nalgene 6150) and a nylon 

 elbow hose connector threaded to fit the auxiliary 

 inlet of the burner assembly. The second consisted 

 of the male portion of the quick disconnect and a 

 gas outlet adapter ( Rentes K- 183000). Both 

 adapters were assembled with minimum length 

 and bore of Nalgene tubing. The following proce- 

 dure was used: A 5-g sample of tissue was placed 

 into a 250-ml beaker to which 10 ml 

 Mg(N03)2 • 6H2O (200 g/1) and 10 ml concentrated 

 HNO3 (Baker 9603) were added. The mixture was 

 covered with a watchglass and evaporated to dry- 

 ness on a hot plate (130°-140°C). It was then placed 

 into a cool mufflle furnace and the temperature 

 raised in steps, first to 250°C for 3 h, then to 400''C 

 for 3 h, and finally to550°C for approximately 15 h. 

 After the beaker was completely cool, 15 ml of 

 concentrated HCL ( reagent grade) were added and 

 the resulting solution transferred to a 25-ml vol- 

 umetric container and brought to volume with 

 distilled water. A 10-ml aliquot of this solution 

 was placed into a 24/40 jointed, 50-ml Erlenmeyer 

 reaction flask and 2 ml of 15% (wt/vol) freshly 

 made KI solution and 2 ml of freshly made StClj 

 solution (20% [wt/vol] in 1:1 concentrated 

 HCL:water) were added, waiting 2 min after each 

 addition. Then 10 ml ofdistilled water were added. 

 Five (5.00) grams of granular (20 mesh, no fines) 

 low arsenic zinc (Fisher Z-15) were placed into the 

 elbow of the second adapter, as noted above, and 

 attached to the first adapter. This assembly was 

 quickly inverted while attaching it to the reaction 

 flask. The arsine generated was then analyzed at 

 the instrument, which was equipped with a 3-slot 

 burner and background corrector. Use of a record- 

 er combined with full noise filtration and slow gas 

 evolution contributed to a smooth and reproduc- 

 ible peak upon which calculations were based. The 

 reaction flask and the first adapter can be quickly 

 removed for cleaning and reuse. 



Analysis of the remaining metals, also per- 

 formed on the Perkin-Elmer Model 403, resembled 

 thatof Middleton and Stuckey (1954): A sample of 

 tissue ( 10 g wet weight) was weighed into a 250-ml 

 beaker and 10 ml of concentrated HNO3 (Baker 

 9603) were added. The beaker was covered with a 

 watchglass and heated to approximately 130°- 

 140°C on a hot plate until the liquid evaporated. 

 One to two milliliters of concentrated HNO3 was 



added and the evaporation repeated. Again, 1-2 ml 

 of acid was added but evaporated at 350°C or more. 

 The hot plate was cooled and the latter acid addi- 

 tion and evaporation was repeated until ashing 

 was complete. The residue was dissolved in and 

 taken up to 25 ml with 10% (wt/vol) reagent grade 

 HNO3 after filtration through Whatman No. 2 

 paper. The solution was then analyzed directly in 

 an air/acetylene flame by conventional atomic ab- 

 sorption spectrophotometry. 



Results 



Greater average concentrations of silver, arse- 

 nic, cadmium, copper, and zinc (122, 44.5, >230, 

 56.0, and 10.9% greater, respectively) were found 

 in ocean quahogs than in surf clams for the entire 

 survey (Table 1). Concentrations of several metals 

 in both clams decreased southward. Concentra- 

 tions of silver decreased steadily from 2.62 to 0.58 

 ppm in ocean quahogs and 1.63 to 0.19 ppm in surf 

 clams. This is a 4.5- and 8.6-fold decrease, respec- 

 tively, from the northernmost range of latitude to 

 the southernmost. Concentrations of arsenic also 

 decreased steadily, 1.6-fold, from 3.90 to 2.41 ppm 

 in ocean quahogs. Although a steady decrease in 

 arsenic concentrations was noted for a full 2.5° of 

 latitude, a distinctive trend for the entire range of 

 latitude was not evidenced. Copper concentrations 

 in ocean quahogs decreased 2.5-fold from 7.16 to 

 2.84 ppm and zinc concentrations in surf clams 

 decreased 2.0-fold from 18.5 to 9.1 ppm. Concen- 

 trations of cadmium and zinc in the ocean quahog 

 and copper in the surf clam did not exhibit any 

 statistically significant trends, while the data for 

 the remaining metal-clam combinations were in- 

 sufficient for statistical analysis (Table 2). 



The results of Pringle and Shuster (see footnote 

 3) for cadmium and zinc (<0.20, 12.39 ppm, wet 

 weight, respectively) in surf clams are in general 

 agi'eement with the mean results of our study. 

 Their result for copper (2. 39 ppm) was lower, while 

 chromium and nickel (2.57, 1.22 ppm, respec- 

 tively) were higher. The collection area of the 

 former study was defined only as Maine through 

 North Carolina; hence, geographic variations 

 might be expected. In addition, neither the 

 number of stations nor of surf clams analyzed was 

 stated. 



Conclusions 



While the Food and Drug Administration (FDA) 



283 



