The change in H3+ that you see is an improvement, not a degradation of performance. More important is that H3+ is stable: repeat determinations of H3+ agree within say 0.03, day to day determinations within 0.1.
A change in filament sometimes seems to result in a change in H3+; the other factor used to control it is the extraction and I fix this at 90% on our delta, while allowing extraction symmetry to vary as autofocus will have it. I use 3kV HT for our H2 tuning, not sure what the original tuning was from the factory but doing that coincided with a drop in H3+ (now sits around 2.7).
That box/trap ratio is not excellent, but is usable.
Use of the A5 mix implies you have GC-C connected. The range of offsets that you see is due to non-linearity of the GC-C interface and I fooled myself just the other day with that range of offsets when injecting a similar std mix (not the deltaV tho). Once you plot accepted vs measured for your alkanes, you'll get a lovely straight line - your calibration!
> -----Original Message-----
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
> Behalf Of Freimuth, Erika (freimuej)
> Sent: Wednesday, 22 October 2014 7:01 a.m.
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] Low H3 factor
> Dear Isogeochem,
> I wonder if others have experienced low H3 factors on a Delta V. The H3
> factor on this instrument used to range from 5-7, but is now around 3.
> The filament was recently changed, and the box/trap is 0.91/0.59. The
> reactor was just replaced and conditioned and backgrounds don't suggest a
> leak. We see good reproducibility between runs with the alkane standard
> Mix A5, but the difference between expected and measured values ranges
> over ~40 permil.
> If others have insight into the cause of a low H3 factor, any help would
> be much appreciated.
> Thank you,
> Erika Freimuth
> Graduate Assistant
> Department of Geology
> University of Cincinnati