It is likely elevated water in your system. N from your N2 pulses combines with O from H2O to give you m/z 30. More time will help. We typically run with source and inlet valve heaters on all the time to keep water out. Sometimes a heat gun or hair dryer can be beneficial to drive water off other components. Monitor m/z 18 while heating various components. Watch for the signal to climb, eventually drop and stabilize.
Every time you replace reagents or open up the EA more m/z 18 can get in your system. So venting the EA or allowing ample stability time is needed. If water is really bad then close the IRMS inlet and allow it to pump down independently.
Usually if m/z 30 does the stair step on std on offs it is not a stable condition for us.
From: Stable Isotope Geochemistry <[log in to unmask]> On Behalf Of Arnold,Thomas E
Sent: Monday, September 09, 2019 2:04 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Mass 30 instability on Delta V
To the great minds of the list serve,
Our system: Delta V coupled to a costech EA via conflo IV
Our issue: Mass 30 is drifting to higher and higher voltages across multiple N2 on/off runs — more than a 1V increase from our first to fourth on/off. I can send pictures to individual emails if needed.
What we've tried thus far: 1)Replaced: the seals on the EA reactors, the copper wires in the reduction reactor, the water trap. 2) We turned on the heater to bake out the source and inlet. I believe this heater operates at a temperature of 50-80C. After the bake out, the amplitude of mass 30 was reduced, but the voltage drift was still there. 3) The problem presents itself in both EA mode and in the conflo only mode.
Any suggestions on what to do? Should this even be considered a problem as our standard deviation looks fine after the 3rd series of on/off runs?