We had been analyzing O and N isotope ratios of N2O using Precon-GasBench-DeltaV Advantage well, until I left Precon and GasBench off for a half year. When I turned them back on (without changing any plumbing), there is a large (1000-1200mV) m46 peak before the CO2 peak, when 100ml room air is loaded. The N2O and CO2 peak heights are same with what I had been getting before.
· When blank (helium gas in a 100ml glass container) is loaded, the m46 peak does not appear, indicating the peak is not due to the leakage of the system or the sample container.
· When 200ml of room air is loaded, the size of m46 is ~2000mV.
· When I increase transfer time from the sample container to the first LN trap (called T2) from 400 sec to 1200 sec, the m46 peak height is largely reduced to 60mV.
· Increasing transfer time from T2 to T3 (2nd LN trap) does not change the m46 peak heights.
· When I increase the helium flow rates from 20 psi to 35 psi (maximum), the size of m46 is not changed so much.
So I thought the transfer time (400 sec) is not long enough anymore for all N2 gas in room air sample to exit through T2, and remaining N2 is causing the m46 peak. Possibly somewhere in Precon (between the sample container to T2 exit) is clogged? However, the flow rates at several fittings of Precon are constant and 23-25ml/min when helium is 20psi. The flow rate at the Valco valve exit is also 24ml/min.
Now I don’t know what to do and how to get rid of the m46 peak (except changing the analysis time extremely long). Any comments and suggestions are appreciated.
· The m46 peak still appears in a blank (helium) analysis after one 100ml air analysis.
· Baking GC did not help to remove the m46 peak.
· The m46 peak sizes of 100ml room air, 100ml outside air, and 100ml of 0.3ppm N2O in N2 balance are similar, so it is not due to our lab room air.
· Replacing chemicals in the chemical trap, and sizes of the chemical trap did not help to solve this issue.
· If Valco Valve before T2 is not working well, I would think I don’t get good N2O and CO2 peaks.
· (T1 and Oxi combustion furnace are removed from our system.)
Research Assistant Professor, Ph.D.
Stable Isotopes for Biosphere Science Laboratory
215A Kleberg Building
2126 TAMU, Texas A&M University
College Station, TX 77843
[Office phone: 979-845-1497]