A couple of things to check:
1. Clog, wrong flow rate, k-factor is off. Redo the column evaluation
in both backflush and straight modes. In backflush mode, it should be
close to the the chart reading and only a little higher in straight
mode. Also make sure He pressure is high enough at the tank to maintain
flow at temp--depending on column, you could easily need 80 psi when
running at 320. You may just want to clean the inlet anyway before
checking this out (new septa, liner, and check first meter of column for
gunk in it so you can clip it off).
2. Leak at backflush valve inside oven. In straight mode, make sure
there is at least some He flowing out the pigtail capillary. If there
isn't any, you may need to tighten the connections on the valve. Since
the leak in on the bleed out side of a bleed out line (after the split),
you won't see it as an increase in Ar.
Charlotte Lehmann wrote:
> We have a Delta V with GCCIII and Trace GC. Last good runs were at the
> end of November with no use until 1/2/08. On 12/21/07, I turned the
> source off since no one would be around during the Winter Holiday. The
> PI turned on the source and geared up to run some samples, but: "On
> Wednesday last week, things looked fine for the first couple of runs
> of the standard, and then something happened between the third and
> fourth injection -- n-alkane peaks became delayed, broader and much
> smaller. (The CO2 Ref on-offs are fine -- meaning the problem lies up
> stream of the ref inlet.) My initial thought was a leak somewhere.
> Mass 40 was higher when back-flush is not on, suggesting a leak.
> However, I have checked for leaks everywhere using the leak detector
> and snooping with argon (while monitoring mass 40) and there is no
> sign of a leak anywhere. Backflush valve flows are fine. I have
> installed new septa, and re-oxidized the furnace...still no
> I checked, at her request, to see if a mass scan gave an indication
> that the oxidation or reduction furnace was spent. The mass scan, in
> "straight mode" with back-flush and dilution OFF, looks the same as
> with them on, and the masses all appear normal in size, shape, and
> relative proportions. They look very similar to the mass calibration
> scan done on 10/25/07 when the filament was replaced and the
> instrument tuned through the Conflo III (on the EA side).
> I even scanned without REF CO2 in, both "straight" and "not straight"
> modes. Peaks are small, no leaks apparent.
> Any thoughts, suggestions, etc. would be greatly appreciated.