PreCon: A Fully Automated Interface for the Pre-Gc Concentration of Trace Gases on Air for Isotopic Analysis

Abstract

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°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° (δ-notation) have been achieved for ¹³C/¹²C from CH₄ and for ¹⁵N/¹⁴N and ¹⁸O/¹⁶O from N₂O. The precision is adequate to monitor the isotopic changes in these gases during a day's course.