I regularly observe an increasing oxygen-signal in straight mode at higher GC-temperatures (between 250°C and 300°C): Below 500 mV at the beginning, the signal is increasing to more than 20 V within a few hours. The results of the initially desired d13C-values are useless then, probably due to the inconstant conditions in the ion source and/or a bad combustion. Other leak-parameters (Ar, N2) remain constant, the CO2 background is OK (~150 mV at 300°C) and the pneumatic valves are working properly (no increase of oxygen in straight mode at low GC-temperatures). Changing the oxydation furnace did not help.
I am running a Trace-GC coupled to a DeltaV Plus via a customized GC/C III interface (no reduction furnace, acetone-dry ice instead of Nafion). The GC-column is a 30m HP5 column (0.32 mm i.d., 0.25 µm film).
Any hints/recommendations or did anybody recognize something similar?
Best regards, thanks in advance, Moritz
Sports Medicine Research and Testing Laboratory
417 Wakara Way
84108 Salt Lake City