What are your backgrounds? You could have a bad tank as well. Have any been changed recently? If 30 is increasing throughout your EA sequences it could be that the water scrubbing chemicals are not good, or there is a pathway through the trap. When was the last time the GC was baked? (Do this with the EA vented away from conflo)
What I meant about instability is that is usually a consequence of excessive water. If you have good N2 on-off stability then I would think the water in your system is manageable. Usually, when we see a strong stair step effect in m/z 30 N2 is not reproducible to 0.06 per mil or better. If N2 is continually flowing into the source you will see 30 rise. But with time it should return down.
It is my understanding that if heaters are off, water is still likely hanging around the source but you may not see a large m/z 18. Compared to leaving heaters on all the time, the water background will be much less.
If your system doesn't seem stable, it may be worth it to close the inlet, turn on all heaters and let the system pump for a day or two then try again.
From: Stable Isotope Geochemistry <[log in to unmask]> On Behalf Of Arnold,Thomas E
Sent: Tuesday, September 10, 2019 6:52 AM
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] Mass 30 instability on Delta V
Ben (and others),
Thanks for the quick response, we also assumed water in the system. As a follow-up: why would the m/z 30 signal increase throughout a single run and across multiple runs? You mention instability as a reason. What do you specifically mean by this? That the EA is not properly sealed? It’s also strange because we see this issue when the EA/high flow is disconnected from the IRMS and the on/off runs are through the conflo.
> On Sep 9, 2019, at 5:39 PM, Harlow, Benjamin <[log in to unmask]> wrote:
> Hi Tom,
> 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.
> Regards, Ben
> -----Original Message-----
> 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?
> Elliott Arnold
> Pitt Geology