PRECON: A FULLY AUTOMATED INTERFACE FOR THE PRE-GC CONCENTRATION OF TRACE GASES IN AIR FOR ISOTOPIC ANALYSIS

For the high precision isotope analysis of atmospheric trace gases a computer controlled concentration interface has been developed. From small air samples it collects either N2O or CO2 derived from CH4 at their respective concentrations (0.3 ppm for N2O, 1.7 ppm for CH4) into a small diameter cold trap (-196 degrees C) and interfaces via GC and open split to an isotope ratio mass spectrometer (Finnigan MAT 252) for on-line isotope evaluation. External reproducibilities for repeated measurements of 100 ml air samples from the same source of < 0.2 parts per thousand (delta-notation) have been achieved for C-13/C-12 from CH4 and for N-15/N-14 and O-18/O-16 from N2O. The precision is adequate to monitor the isotopic changes in these gases during a day's course.