Chromatography Forum

I am having a problem with water build-up in my 5975, I think.
This is what is happening:
Last week:
Monday morning: Calibrate
Tuesday: Calibration looks good, Run a sample batch
Wednesday: notice that with every sample ran, internal standards are getting lower and lower. Batch from Tuesday passes, but when starting new QC for Wednesday's batch, the internal standards are still so far off(low), it is causing the QC to fail high. Prep new internal standard. Run new QC-still failing high. Allow to sit overnight.
Thursday- Run new QC-internal standards still low, but higher than Wednesday and QC passes.
Came in Monday morning, started QC and it looked great, since the Mass Spec was able to pump out all of the water all weekend. By the end of Monday's run, internal standards were a little over 1/2 of what they should have been.
Tuesday-same thing over again, QC won't pass, due to low internal standards.
Am I seeing water build-up?
What do I do to fix it?
I have replaced the transfer line, it didn't help.

some more info about your method would help.

What is the temp of you transfer line?
What are you injecting as an internal std and what is the solvent of which it's contained?
What is the column?

I'm assuming you are not doing this manually..
For starters, I would look more toward the inlet for issues.

Good Luck

Will

What kind of samples are you running? Something with a lot of water? If your MS transfer line has any heat on it at all, it will be very unlikely that you are accumulating water there. And, if water were being trapped in the MS transfer line, later eluting compunds in your method would also be a problem.

My first question would be whether you see water in your MS at all--you don't mention that. Do you run an air/water check before, or after, your run? What is the temp of your inlet? And what temperature do you keep the oven on after the run?

Purge and trap correct? If yes, first use a Vocarb 3000 trap and use 6 min of dry purge @ 40cc/min. Then set the gas saver option to go to 100-150 cc/min @ 1/2 your desorb time.
Then time your system so that you can hold the oven at method max. T , and then be ready before desorb. With my set up I can hold the oven for about 10 min and still have it ready before desorb.

Our VOA dept was having the same issue with a 5975C Triple Axis with Standard Turbo. We traded the same model MS from the SemiVOA dept with a Performance Turbo and problem went away. Standard Turbo is working fine for SemiVOA, it just couldn't handle the trace moisture from Purge & Trap.

It isn't the pump. Any of the "73" or "75" can handle doing purge and trap. If the issue is water, steps must be taken to remove as much water prior to the GC.

Your QC is passing high due to low area counts for your IS? That would mean that your MS is seeing the QC components (I assume all of the standard VOA list compounds) at nominal area counts for the concentration? If so, you definitely do not have a water issue - the first thing you'd lose with water issues would be the lights (the gases) and acetone, not the IS compounds.

If you're having issues with IS areas, but not other compounds, then check your mass assignments. Change your liner - you may be exchanging protons for deuterons in the liner, and it would appear as if you were losing IS area counts.

If this scenario is not what you're seeing then please re-state the problem.

When was your last source cleaning? If you have run about 200 samples after cleanning, try cleaning.