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Hello all, can anyone help us diagnose this issue?

We're seeing cyclic noise in the signal coming from our Delta Plus XL. 

The noise is in the form of a voltage drop at regular intervals (every few seconds). The size of the voltage drop increases as the signal intensity increases. All of the cups have this issue, although the size of the voltage drop is different for each one and seems to move from cup to cup. The noise is present even with the Delta closed off from the EA.

Our setup: Carlo Erba NC2500 --> Conflo II --> Delta Plus XL

Here are pictures of the bad signal:
Scan results: http://tinypic.com/r/63vhp1/9
Reference N2 results: http://tinypic.com/r/262wnbo/9
Zoomed in: http://tinypic.com/r/28qwz60/9

The regularity of the noise makes us think this must be an electrical issue. However, we first saw this problem after adding a CO2 trap to the EA. We used soda lime as the absorbent and placed the CO2 trap between two water traps. The coincidence of the trap change and the noise makes us think this could be a contamination issue. The CO2 trap has since been removed, but the issue persists. To make things more confusing, the noise will occasionally disappear completely and the system will be stable for a few days at a time.

We greatly appreciate any suggestions!

-Evan Baker
--
Laboratory Manager
Stable Isotope Laboratory
North Carolina State University