pentane. Collect 42 ml of pentaneeluateina 50-ml 

 graduated centrifuge tube. (This fraction contains 

 PCB and some DDE.) 



14) After the appropriate volume of pentane 

 eluate has been collected, place a second 50-ml 

 graduated centrifuge tube under the silica gel col- 

 umn. Then fill the tube with benzene. Collect 35 

 ml of benzene eluate. (This fraction contains most 

 of the DDT complex.) (DDE elutes very rapidly 

 with benzene. If benzene is added to the column 

 before the second centrifuge tube is in place, the 

 DDT complex will often be found in the pentane 

 fraction.) 



15) Concentrate each fraction in a boiling 

 water bath to less than the desired final volume 

 and quantitatively transfer with hexane to a vol- 

 umetric flask of the desired final volume. (The 

 50-ml centrifuge tubes are not very accurate vol- 

 umetiic containers.) Proceed with gas-lic|uid 

 chi-omatographic analysis (see footnote 4). 



Notes on DDT/PCB Separation Proccdiirc 



1) Elution with hexane instead of pentane dur- 

 ing the silica gel chromatography fails to provide 

 the necessary separation of DDT from PCB. 

 Hexane is reported to contain variable amounts of 

 benzene, which would obviously affect an already 

 delicate separation. Use of UV-quality pentane or 

 hexane has been recommended by others, and 

 might allow use of hexane in warm weather. 



2) For high residue level samples, evaporation 

 of the Florisil eluate to ca. 1 ml is not necessary; 

 instead an appropriate aliquot is used. However, 

 no more than 1 ml of the eluate should be placed on 

 the silica gel column because the hexane may con- 

 tain benzene. 



Procediirt' Variations 



During the 45-min boildown, scrape the mate- 

 rial on the bottom of the flask. Pile up the solids to 

 leave areas of the flask bottom in direct contact 

 with the solvent to improve boiling action and 

 prevent bumping. Cool. If water separates, add 

 Na.SO,. 



Filter through glass wool and proceed as usual 

 (step 6 under Extraction). 



In oi'der to compensate for the low residue level 

 usually found in plankton, place 2 ml of the extract 

 on the Floi'isil column for cleanup. 



I'islimcals or Dr\ leeds 



If the standard extraction procedure is used for 

 meals and animal feeds, the finely ground meal 

 forms a layer on the bottom of the Virtis flask 

 which causes bumping and loss of solvent during 

 the 45-min boildown. Extraction with hexane pro- 

 vides as good recovery as IPA/benzene. This sub- 

 stitution allows omission of the boildown. 



1) Homogenize sample with 20 ml of hexane. 

 Wash down Virtis blades and Teflon top with min- 

 imal amount of hexane. Add 10 g Na^SO^. 



2) Immediately filter through glass wool 

 tightly wadded to remove as much of the solids as 

 possible. Wash flask and funnel with a minimal 

 amount of hexane so that the volume of the cen- 

 trifuge tube is not exceeded (35-40 ml). 



3) Stir to mix and centrifuge at 1,500 rpm for 

 45-60 min at 10°C. (There should be about 35 ml of 

 clear solution with less than 1 ml of solids.) 



4) Record volume, subtracting 50% of the vol- 

 ume occupied by the solids. (Although this in- 

 volves an approximation, the error involved 

 should be no more than 2^/c.) 



5) Decant the supernatant liquid into a storage 

 bottle containing 1 g Na.,S04. Proceed with 

 Florisil cleanup (step 9 under Cleanup). 



Plankton and Other High-Moistiire Samples 



Plankton do not homogenize well using the 

 standard procedure. They also contain water in 

 excess of the amount that 20 ml of 1: 1 IPA/benzene 

 can absorb. Addition of Na.2S04 before homogeni- 

 zation overcomes both difficulties. Na2S04 not 

 only absorbs water, but also serves as a grinding 

 aid. 



To the weighed sample of plankton add 25 ml of 

 1 : 1 IPA benzene; then add 25 ml hexane and 10-15 

 g Na^SOj. Homogenize 5 min and proceed as 

 usual. 



Fish Oil 



Homogenization and extraction of oil are un- 

 necessary. 



1) On an analytical balance weigh accurately 

 about 2 g of oil into a 50-ml graduated centrifuge 

 tube. 



2) Dilute to about 20 ml with hexane and swirl 

 to dissolve the oil completely. 



3) Record the volume. 



4) Place in a storage bottle, containing 1 g 

 Na., SO4 , and proceed with the usual cleanup ( step 

 9 under Cleanup). 



884 



