New column, worse resolution?

[MichaelVW]

Hello

This pertains to a 6890/5975 setup for running 8270. The new column is the same as the old one: a Zebron ZB-Semivolatiles from Phenomenex. 30m x 0.25 mm x 25 um.

This is the first time I've installed a column... and I'm confused. I conditioned it according to some guide I found somewhere: ran helium through it and baked it out and all that. I ran our curve and almost everything looks good. Retention times are about half a minute later, our later internal standards (Chrysene D12 and Perylene D12) are recovering much better, but mostly everything else looks about the same. However, one thing I don't like is that some pairs of compounds seem to be eluting partly or totally on top of each other - worse than before. p-nitrophenol used to be a shoulder on the right side of dibenzofuran; now they pretty well coelute - depending on the concentration. O-toluidine used to be just touching the peak for m and p cresol; now they are basically merged together.

I thought maybe everything was closer together, but it's not really the case: the time from the first internal to the last is the same as it was before. Plus, other pairs that tend not to fully resolve, like benz[a]anthracene and Chrysene, or benzo b&k flouranthene, look about the same as before.

Anyone have any thoughts?






If it's relavent, here's some more info:

Program held at 40 ºC for 4 minutes
Solvent delay of 3.5 minutes
Increased at 10 ºC/min. to 310 ºC and held for 4 minutes
Column Pressure: 7.1 psi
He Flow:
Column: 1.0mL/min
Velocity: 36 mL/min
Purge: 36mL/min
Total: 40 mL/min
Injector Temperature: 250 ºC
Detector Temperature: 280 ºC

[Peter Apps]

As columns age their retention characteristics change - in this case your old column probably had some active silanols that se lectively retained the acid compounds just enough to separate them from their neighbours. Now you have a new column with no silanols, so the acids are eluting a tiny bit earlier and overlapping with the other peaks.

Peter

[Bigbear]

Do you run column profiles regularly? Take the syringe out of the autosampler and make an injection ( no just setting up a run and hitting start doesn't work right)
The chromatogram should not have any peaks and ideally less than 1M counts at the temp ramp max.
I've had new columns bad out of the box. I use Agilent columns and call when I get a bad one. They usually send out a new one.

[MichaelVW]

Sort of bumping up and old topic but I still have the same issue. I tried running an empty injection as suggested but didn't see anything that stuck out too much. Peter suggested that this might be just normal with a new column. I did compare to my old column shortly before I replaced it, and also to that same column when it had first been put in in February. When it was new it still got better seperation than the one I'm using now.

Here are a couple images to show what I mean:

http://i8.photobucket.com/albums/a41/ra ... niline.gif

http://s8.photobucket.com/user/ralphsin ... l.gif.html

Does this seem like something to be concerned with and/or is it anything that I can do something about?

[Bigbear]

Did you re set the column length in the software?
I calibrate my column by timing the air peak in a manual injection. empve the tower and use a syringe to inject about 1 ul. Set the oven to 40C and make note of the injector pressure, you will need it later. Go into manual tune and monitor m/z 28. Using the stopwatch feature on the gc front panel determine the unretained peak time (air). I do this 3X and average. Then go to your method acquisition , column and there is an option for calibrating the column if you know the unretained peak time.

I do this every time I do injector maintenance and rarely have compounds fall out of their RT windows.