Chromatography Forum

I've recently inherited a P&T/GC/MS system for running the 8260 compound list and I'm having difficulty getting reproducible recoveries. I'll generate a good calibration curve (%RSD on responses < 15%) one day and within 1-2 days my standard controls begin to fail recovery limits. Typically the internal standard abundances will drop 20-30% and %recoveries jump up to 130-200%. This isn't a standard issue, because I've ran the standards on two other systems with good performance.

Our group has been running the same method on two identical systems (Tekmar 3000 P&T, Agilent 6890 GC, Agilent 5973 MS) without issues for a few years now. The system I'm trying to get this method to work on is (Tekmar 3100 P&T), Agilent 7890 GC, Agilent 5975C MS).

All three systems have VOCARB 3000 traps and Agilent J&W DB624 columns (60m x 0.25mm ID X 1.4um film). The flow is split 50:1

The one systematic trend I find is that the reproducibility is worse for the light compounds, in particular the gases vinyl chloride, chlorethane and trichlorfluoromethane. I would expect a leak would cause gas problems but I am unable to find one. I also find it hard to believe it would cause an increase in recoveries.

Is there something different about the new instruments that might require a significant change in the method?

Any advice is greatly appreciated!

-John

jpcasey,

I am not that familiar with the 3100 but is it possible to swap traps between the units? That is the first place I would look. The other comment/question, what is the column flow since a 50:1 split at 1.5 mL/min would put about 75 mL's across the trap which might be too high for that trap.

Best regards.

After opening up the 3100, the only difference I see from 3000 is an extra pressure gauge on the sample gas regulator.

Our purge flow is 35mL/min and the carrier flow through the trap is 50mL/min.

Unfortunately our working systems are in constant use, so we can't swap out components to see which may be the problem. There was a new trap installed recently so I'm hesitant to blame it. That being said, I can't get the trap back pressure below 5psi , which is easy on our other P&T (we run at ~2-3psi). Could this higher trap pressure suggest it needs to be replaced already?

jpcasey,

My experience is that traps can be sketchy. I would replace the trap. Order two and ask your vendor to send you separate lots.

Best regards.

Is it possible you are having problems with the internal standard? Take a closer look at your sample introduction system. The low recoveries of your internal standard would explain the higher percentage recoveries of your analytes. My guess is you have a leak in your internal standard container and you are losing compounds with the more volatile ones decreasing more quickly.

So what happens if you recalibrate after the instrument falls outside of range? Is the instrument stable for the calibration and then falls off again after a few days? If you look at the internal standard response over the duration of the calibration is it constant or does it fall off or rise up? Do you calibrate low to high or high to low?