The International Mussel Watch 



4.6 Determination of Fresh Weight/Dry Weight (FW/DW) Ratio 



The constant weight of a weighing bottle is determined by repeated weighing. A tissue 

 subsample of 1-2 g is introduced into the bottle and heated in an oven at 105°C for 24 hours. The 

 flask is then transferred to a desiccator to cool and then weighed again. The procedure is repeated 

 until a constant weight is reached. The (FW/DW) ratio is calculated. 



5. Gas Chromatographic Determinations 



5.1 Principles 



After clean-up of the extracts of marine samples by adsorption chromatography, fractions 

 are analyzed by high resolution gas liquid chromatography (HRGC) with electron capture detection 

 (ECD). As many PCB congeners and pesticide compounds are separated and quantified as 

 individual peaks within feasible and practical limits. The method relies on splitless or on-column 

 injection of up to 50 ng per compound on a small-bore SE-54 coated, 25 m open tubular fused- 

 silica column under optimum conditions of carrier gas type, flow and temperature conditions. 

 Standards of well defined composition are used for peak identification and quantification and to 

 establish optimum chromatographic performance. Confirmation and quantification of the DDT 

 series is accomplished by a dehydrochlorination procedure where the p,p'-DDE peak is measured 

 before and after oxidation with concentrated H2SO4/SO3 (fuming sulphuric acid). Internal 

 standards are used where required for accuracy assurance (see section 8). 



5.2 Reagents 



All reagents, including distilled water, should be of well defined analytical quality. Their 

 use must result in an adequate signal to noise ratio with the electron capture detector. All reagents 

 must be checked for their ECD response individually and by determining complete procedural 

 blanks. If contaminants are detected, the solid reagents must be cleaned by extracting them with 

 pure solvents and/or by heating overnight at 260°C to 300°C. All solvents should be "distilled in 

 glass" quality and pre-tested for their suitability for organochlorine residue analysis. They may 

 require redistillation in the laboratory on a routine basis. 



a. n-hexane 



b. Fuming sulphuric acid, sufficiently concentrated to remove the p,p'-DDE peak. 



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