Sensitive Determination of Perfluorocarbon Tracers in Air by Intermediate Trapping and GC-ECD Analysis

TRACER EXPERIMENTS ARE USED INCREASINGLY IN DIFFERENT COUNTRIES TO DESCRIBE TRAJECTORIES OF AIR MASSES,ESPECIALLY FOR POLLUTION CONTROL STUDIES. IN THE LAST YEARS THE NEED TO FOLLOW UP TRACED PLUMES FOR DISTANCES OF SEVERAL HUNDRED KILOMETERS HAS MOTIVATED RESEARCH FOR NEW TRACERS, HAVING A FAVOURABLE BALANCE BETWEEN COST OF THE RELEASED TRACER AND QUANTITY NECESSARY TO PRODUCE A MEASURABLE CONCENTRATION AT THE DESIRED DISTANCE. THE PERFLUORINATED DERIVATIVES OF SOME CYCLIC HYDROCARBONS HAVE PROVED EXCELLENT FEATURES IN THIS RESPECT BECAUSE OF THEIR VERY LOW BACKGROUND CONCENTRATION IN THE ATMOSPHERE AND OF THEIR HIGHLY SENSITIVE GC-ECD MEASUREMENT. THIS PAPER REPORTS ON THE DEVELOPMENT OF A SIMPLE METHOD FOR THE DETERMINATION OF TWO PERFLUOROCARBONS PERFLUOROMETHYLCYCLOHEXANE (C7F14) AND PERFLUORODI- METHYLCYCLOHEXANE (C8F16) DOWN TO 5X 10-14 VOLUME FRACTION IN AIR (0.05 PPT). THE METHOD INVOLVES THE FOLLOWING STEPS: A) SAMPLING OF 5-10 L OF AIR ON A 160X6MM (3MM I.D.) COLUMN, CONTAINING 200 MG OF GRAPHITIZED CARBON BLACK. B) DESORPTION FOR 6 MIN AT 250 C IN A SMALL OVEN. C) CRYOGENIC TRAPPING AT -100 C IN A FUSED SILICA CAPILLARY. D) INJECTION IN THE GC BY FLASH HEATING OF THE CAPILLARY. FOR THE DESORPTION TO INJECTION STEPS A COMMERCIALLY AVAILABLE APPARATUS HAS BEEN USED, WHICH HOWEVER HAD TO BE IMPROVED IN SEVERAL ASPECTS. MORE LIMITED EXPERIENCE ON A SIMILAR INSTRUMENT, PRODUCED BY A DIFFERENT FIRM, BUT NOT YET COMMERCIALLY AVAILABLE IS ALSO REPORTED. THE GC ANALYSES ARE CARRIED OUT USING AN APOLAR (METHYLSILICONE) CAPILLAR COLUMN AT SUBAMBIENT TEMPERATURE. THE FOLLOWING ASPECTS OF THE DETERMINATION HAVE BEEN STUDIED FOR OPTIMIZATION PURPOSES. TWO ADSORBANTS HAVE BEEN TESTED, I.E. CARBON MOLECULAR SIEVE AND GRAPHITIZED CARBON BLACK: THE LATTER HAS BEEN FOUND TO GIVE MORE REPRODUCIBLE RESULTS, TO REQUIRE A SHORTER DESORPTION TIME AND TO ADSORB MUCH LESS WATER VAPOUR WHICH MAY CAUSE CLOGGING OF THE CAPILLARY TRAP. THE EFFECTIVENESS OF PERMEATION DISTILLATION DRYING TUBES IN DRYING AMBIENT AIR BEFORE ADSORPTION ON THE TRAP, THE BREAK THROUGH VOLUME OF THE SAMPLING COLUMN AND THE DESORPTION TIME HAVE ALSO BEEN STUDIED.

DE BORTOLI Maurizio;  PECCHIO Emilio; 

1995-03-15

JRC4005