Calibration curve linearity issues and loss of sensitivity

[SylvainL]

Good afternoon,

First off great job to all on this site... you guys have helped me alot.

Here's my prob...

What I'm running... 6890 gc with 5973 msd ei, column you're standard db-5, 30m, 0.25mm, 0.25um

performed a full maintenance on the system... changed liner (single tappered with glass wool, new septum, gold seal, cleaned the source, changed the filament.

we also added a guard column.

Leak test good, performed autotune... EM volt went from 2250 to 1775 and my abundance for all ions are whithin range.

I've prepped a 5 pt calibration ranging from 30 ppm to 200ppm... outcome used to be very linear but now it's quadratic.

I've cut a section of the guard colum... no change, I've redid my stds (3 times)... no change.

Here's what I'm thinking... column was installed last summer so most likely it has active sites... if this is the case shouldn't I see some tailing, which i don't.
There was mention on this board about changing the EM... the EM on this machine has never been changed
Could the guard column be a factor?

As for sensitivy... if my ions during autotune look ok but the std output seems low what could this be?

Thanks in advance

Syl

[carl.nott]

It's possible that you're saturating your detector. Is the curve quadratic in that it rises and flattens out or is it curving more upward as the concentration rises? If it's rising and flattening you're probably saturating. Drop the EM voltage by a couple hundred and reshoot the curve.

I suspect it's a chromatographic issue and not a detector issue though. In these sorts of cases I typically reset to what used to be working and go from there. So I would take off the guard column as a first step, see if that changes the response. If no change I would do injection port mainenance (liner, septum, seal) and check again. If no change I would replace the column and check again.

[AICMM]

SylvainL,

I agree with Carl. If the tune is about where it was for 69 last time, I would focus on the inlet. Is the column in the gooseneck, how much wool are you using, and removing the guard column would be the things I would focus on first (in that order.)

Best regards.

[bhahn]

200 ug/ml is pretty high loading, especially for a .25 .25 older column. I suspect that it's a combination of high loading and detector saturation, if it's the high conc dropping off.