Chromatography Forum

Have an instrument that has been working good until last night. The run started out with internal standard responses near 100% versus the calibration and low and mid calibration checks all passed. The first 15-20 samples looked great, all internal standards and surrogates passing, then with each run the early eluting peaks began to fall off and were pretty much gone after about 10 injections. Now the first internal standard Fluorobenzene is at 5% area counts versus the calibration curve, 1,2-Dichloroethane-d4 surrogate is at 9%, 1,4-Difluorobenzene internal standard is at 12%, Toluene-d8 surrogate is at 24%, Chlorobenzene-d5 internal standard is at 134%, Bromofluorobenzene surrogate is at 228%, 1,4-Dichlorobenzene-d4 internal standard is at 278% and 1,2-Dichlorobenzene-d4 surrogate is at 279%.

Remade the internal standard/surrogate mix in case the unit was leaking and we lost the lighter compounds but they are still the same recoveries. Checked the purge flow and it is 45ml/minute instead of 40ml/min but that should not cause this much problem. Room temperature is running hot since the A/C is acting up and it is now about 83F in the room, but it was just above the normal 70F this morning when we discovered the problem and should have been cooler last night when it began. The BFB tune is still passing and the high/low mass ratios have not changed so it isn't a MS problem, or doesn't seem to be.

I am going to try a new trap to see if that works, but just wondering if anyone else has seen a similar problem in the past and knows what might cause it?

Did you do a run without changing anything after this sequence finished and the instrument sat idle for a while?
My first impression would be water during desorb as the gasses elute with the water/MeOH and the FlBz just after ( on my system).

I have seen the same symptoms in a different context. I was having a problem getting liquid nitrogen to the cryotrap in my vacuum distiller. The temperature would get down to -140 to start but would drift upwards during the distillation. The symptoms were very much like what you are seeing. The more volatile components, vinyl chloride etc. were very low while toluene and later components were unaffected.
Are you using the trap listed in 524.2?

VOCarb3000 trap being used. No cryo but normally the trap temperature will drop into low 30Cs during purge, now mid 30Cs with hot room temperature.

I did another interesting run yesterday where the area percents for the mid cal check analytes were near 100% but the first internal standard was again almost gone. That is pointing to something in the internal standard, but it was made fresh just before that run. I was thinking a leak in the seal for the internal standard vial on the autosampler which would cause the effect on the long run overnight, but not almost immediately after making new internal standard mix.

I didn't change anything after the last run, which was about a week before this one, and the first 15 or so samples on this run were ok. First 10 had the FLBnz at near 100% then began falling down to about 70% through the next 5 samples and down to below 50% on samples 15-20, then rapidly down to almost nothing after that. Weird thing is the BFB and Dichlorobenzenes are increasing which would be indicative of evaporating methanol in the vial, but with this new internal standard mix it would have lost maybe 100ul out of 5ml so far, not enough to make the shift.

Going to do some leak checking now to see if I can find a leak around the internal standard vial. Still got me scratching my head.

Oh, instrument is EST Centurion W/S with EST Encon P&T and Agilent 5975 GCMS.

On our Tekmar Aquatek 70 autosampler we had some strangeness caused by a faulty check valve. The Fl bz was increasing over time when compared with the 2 surrogates that were in the same mix. When I put MeOH only in the IS solution vial I got Flbz but not the other 2. Called Tekmar and they walked me through a trouble shooting session.
In our old building we did have some high temp days, but our symptoms did not present like yours.

I think we figured it out. I found that the teflon ferrule on the internal standard vial was leaking helium. The small leak combined with the unusually hot room was causing the loss of the more volatile internal standards and concentration of the less volatile ones.

We ran a series of blanks and standards this morning and they are back to normal after replacing the ferrule. Wouldn't have found it without the electronic leak detector, amazing how such a small leak can cause such large headaches.