524.2, shifting retention times, coelutions

[Chroman]

Hi everybody:

I have this problem lately that retention times for some peaks are shifting considerably. As a consequence I am having multiple coelutions and not very good CC. Already put a new column but no improvement there. Made manual injections to leave out of the equation XPT and same thing. Any ideas?

GC/MS Clarus from Perkin Elmer.

[nvalentine]

We had a similar issue with shifting RTs on our PE system. Autosampler, P&T, and manual injections all showed same behavior. Something in the injector was causing inconsistent flows. You can check by attaching a flowmeter to the vent and monitoring for a few runs. I think we had to swap out the split vent trap (or whatever PE calls it) to fix it.

[Bigbear]

Co-elutions are to be expected with 524, that's why we use MS detection.
As for the Rt shifts, what is the condition of your regulator. Are you supplying the GC with 10-20 psi over the pressure at the maximum oven temp?

[Bigbear]

Another thought. Are you running constant pressure, or constant flow?