The International Mussel Watch 



emitted by a radioactive source within the detector ( 63 Ni)* with carrier gas molecules. The 

 thermal electrons can be captured by sample molecules. The resulting reduction in cell current 

 provides the signal. The range of sample concentrations where the response of an ECD is linear 

 with concentrations of electron captive compounds is extended considerably in the constant 

 current, pulsed voltage mode. The constant current results from the modification of the frequency 

 of polarizing pulses to the cell electrodes. The optimum flow for an ECD (30 ml min"*) is much 

 higher than carrier gas flow through the column of one or two ml min'l. Thus, additional detector 

 purge flow is necessary (Ar/CH4 or N2). Once leaving the outlet of the column the compounds 

 have to be taken up into an increased gas flow in order to avoid extra-volume band-broadening 

 within the detector. Thus, the detector purge flow also serves as sweep gas. As the ECD is a 

 concentration dependent detector, the detector flow must compromise between low band 

 broadening and high concentration of compounds within the detector. 



High boiling organic compounds eluting from the column may be deposited in the detector. 

 The contamination results in lower sensitivity. The effects are less serious at higher detector 

 temperatures. Periodic heating to 350°C overnight assists in maintaining detector performance. 

 The 63 Ni ECD can be used routinely at 320°C with relatively limited contamination. Oxygen, 

 even at trace levels, has a detrimental effect on ECD performance. This is especially true at higher 

 temperatures. The standing electrode current must be checked regularly according to the 

 manufacturers manual and performance data should be recorded in a log book to maintain a 

 permanent record. 



The linearity of the ECD must be established at regular intervals by injecting several 

 concentrations of standards. Sample determinations should be made only in the linear range. 



Phthalates are common in the atmosphere and in plastic tubing, they may, therefore, be 

 artifacts when detected in natural samples. Their source should be identified and their presence 

 should be eliminated. 



5.5 Identification, Verification and Confirmation 



The most widely used information for identification of a peak is its retention time, or its 

 relative time (i. e., the adjusted retention time relative to that of a selected reference compound). 



NOTE: The use of a tritium detector is not recommended. If its use cannot be avoided, be sure to ventilate the gas stream 

 to the outside atmosphere. Under no circumstances should a " J Ni or ■'H detector be serviced in the laboratory. It should be 

 serviced by a specialized and authorized laboratory only. 



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