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8.2.1 The electron capture detector is selective for the measurement of halogenated hydrocarbons. It is virtually insensitive to other hydrocarbons thus eliminating interferences from non -halogenated hydrocarbons. 8.2.2 Any halogenated hydrocarbons present in the sample having retention times very similar to the compounds of interest under the operating conditions described in this method will interfere. Therefore, proof of chem- ical identity requires confirmation. 8.2.3 Water vapor at normal ambient humidity in the sample does not interfere with the separation and quantifi- cation of halogenated hydrocarbons. 8.2.4 High concentrations of nitrogen oxides (500 ppm) and sulfur oxides (50 ppm) interfere in the determination of methylene chloride in the samples of stack emission sources. 6.3 The procedure described herein has both advantages and disadvantages: This method provides a simple way of air sampling. The concentrations of halogenated hydrocarbons in the range of interest are stable for more than 24 hours in the bag, providing sufficient time for the analysis. 8.3.2 The sample is easily and repeatedly introduced into the instrument by means of a gas sampling valve. 8.3.3 A representative composite sample is readily obtain- ed for any selected time interval because the air sampling flow rate is constant. . 8.3.4 Both the upper and the lower limits of detection can be extended by concentrating a larger volume of the sample with a freeze -out trap or by diluting the sample in a Tedlar bag with nitrogen or by loop injection. 8.3.5 Interferences can be eliminated by selecting*chromato- graphic conditions. 9 References 9.1 U. S. Environmental Protection Agency (19760, "Quality Assurance Handbook for Air Pollution Measurement Systems, Volume I -Principles", EPA -600/9-76-005 Environmental monitoring and Support Laboratory, Research Triangle Park, North Carolina 27711