evaporate 4-6 h at room temperature to minimum 

 weight. (Since marine oils oxidize, the weight of'oil 

 begins to rise again after a few hours.) Weigh the 

 residue, which contains the oil in 1 ml of extract. 



(,"lcaiuip 



9) Prepare.' a Flurisil (.olumn by filling a 9 mm 

 i.d. X 150 mm glass tube, plugged with glass wool, 

 with ca. 5 cm of Florisil. Wash the column with at 

 least 15 ml of hexane added 1 ml at a time. Allow 

 the hexane level to drop to 1-2 mm, but not to 

 dryness. Pipette 1 ml of the extract onto the col- 

 umn. For samples with very high oil content (refer 

 to step 8 for the amount of oil in 1 ml of the ex- 

 tract), adjust the volume placed on the Florisil so 

 that no more than 0.1 g, and preferably no more 

 than 0.08 g, of oil is placed on the Florisil. Elute 

 with 1-ml portions of hexane; collect the first 12-13 

 ml of the eluate in a 13-ml graduated centrifuge 

 tube. I Note: Once the Florisil has been wetted, it 

 must always have solvent above it.) 



10) If DDT and PCB are not going to be sepa- 

 rated, concentrate (in a tube heater) the eluate to 

 an appropriate volume for gas-liquid chromato- 

 graphic analysis."* If separating DDT and PCB. 

 evaporate the eluate to slightly less than 1 ml. 



Separation of DDE, TDE, and DDT from PCB 



Quantitation of TDE and DDT is often difficult 

 and quantitation of the PCB is usually impossible 

 unless the DDT family is separated from the PCB. 

 Separation is achieved by chromatography on 

 silica gel. The behavior of DDT and PCB during 

 solid-liquid chromatography is very similar, and 

 obtaining optimal separation requires careful con- 

 trol of all the parameters of the procedure. Even 

 so, DDE does not separate entirely from PCB. 

 Therefore, the DDE in the PCB fraction is quanti- 

 tated'' and included with that in the DDT fraction. 



Evaluate the degree of separation of DDE, TDE, 

 and DDT from PCB by chromatographing stan- 

 dard solutions of these compounds according to the 

 procedure described below. Adjust the time and 



"•For a detailed de.scription of gas chromatography of chlori- 

 nated hydrocarbon pollutants, see the Pesticide Analytical 

 Manual il977), available from Management Methods Branch, 

 DMS, ACA, HFA-250, 5600 Fishers Lane, Rockville. MD 20857. 

 or National Technical Information Service iNTIS), Springfield. 

 Va. The manual provides extensive background on residue 

 analysis. 



•'^Although DDE elutes from the gas-liquid chromatograph at 

 the same time as one of the PCB peaks, measurement of the other 

 five intense PCB peaks provides accurate quantitation of PCB. 



temperature of activation, the degree of rehydra- 

 tion, the amount of sil ica gel, and the volume of the 

 pentane fraction to obtain the optimum separation 

 of DDT and TDE from PCB: that is, to maximize 

 the amount of PCB in the pentane fraction and the 

 amount of DDT and TDE in the benzene fraction. 



1 1 ) Activate the silica gel by heating at 215°C 

 for 16 h. (The time and the temperature are ad- 

 justed to obtain an arbitrarily, but consistently 

 activated product with suitable separating 

 characteristics, since complete dehydration occurs 

 over a long period of time.) Cool to room tempera- 

 ture in a desiccator. Rehydrate by placing 98 g 

 silica gel in a glass-stoppered bottle and adding 2 g 

 distilled water. Stopper the bottle and shake and 

 tumble until the water is evenly distributed. 

 Allow the silica gel to equilibrate for 2-4 h before 

 use. 



12) Place a portion of this prepared silica gel in 

 a beaker and cover with pentane. Let stand 5-10 

 min to return to room temperature. (Because the 

 chromatographic columns do not contain stop- 

 cocks, a special technique is required for packing.) 

 Quickly transfer silica gel to a glass-wool- 

 stoppered column, 9 mm i.d. x 250 mm long, wet 

 with pentane. (A disposable transfer pipette with 

 the narrow part of the tip removed works quite 

 well for transferring the silica gel slurry. ) Tap the 

 column gently to facilitate packing. Make sure 

 that there is always enough pentane above the 

 column to allow the silica gel to settle slowly in 

 order to eliminate air bubbles and prevent the top 

 of the column from running dry. Pack the column 

 to a height of ca. 8 cm. (Throughout the whole 

 separation procedure the silica gel must always 

 have .solvent above it and must be free of bubbles 

 and cracks, which interfere with the desired sep- 

 aration. If the column runs dry or cracks, discard 

 it.) Rinse the column with 15-20 ml of pentane. 



13) Allow the pentane level to descend to 1-2 

 mm (not dry) and place the Florisil eluate (ca. 1 

 ml) on the column with a Pasteur capillary 

 pipette. Rinse the Florisil eluate tube with three 

 or four 1-ml portions of pentane, and transfer each 

 rinse successively to the column. After the sample 

 and rinses have been adsorbed, fill the tube with 



Use the amount of PCB in those peaks to determine the size of the 

 peak overlapping the DDE and correct the apparent total DDE 

 I actually DDE plus PCB) to obtain the tme DDE concentration. 

 The electron-capture detector is so much more sensitive to DDE 

 than PCB, that the correction affects the accuracy of DDE de- 

 temiination only to a small extent. Consequently the variability 

 in amount of DDE in the pentane fraction does not markedly 

 affect the accuracy of DDE analysis. 



883 



