Chromatography Forum

I am running PCB in oil via GC with dual ECD's. When ever I run the standard after analysis recovery is low. If I bake the columns and detectors for a wile after the last oil sample or run lots of rinses before the standard the recovery is better but not always passing. When I run standards after the machine sits over night the standard response in back to normal. Has anybody seen this before, or have any incite. My detector temp is 350º C, oven program max is 285ºC.

We've had similar problems with difficult matrices. Running a solvent blank injection with a second method set at max oven temperature for the duration of a typical run every X number of injections (10-12 for us, perhaps more frequently for you) may help bake out the excess oil before it builds up and kills your recovery.

Ah I remember that nightmare - PCB oils .... you may want to lessen the amount of sample (probably a tenth of what you use for your other PCB matrices) used in your extractions and run your analysis at a dilution, then a couple of blanks after your sample. Because you have run many PCB oilis thus far, it may help to clean out your whole injection port - remove the gas lines into the injections port and hexane (or methanol) through and scrubbing with that little brush thingy.

You are likely leaving some of the heavier oil in the PCB either still on the column or in the inlet.

You may not be maintaining your final temperature long enough at the end of the run to allow all of the oil to elute. Though we don't "see" the oil (much) with an ECD, it will impact your calibration so you need to allow enough time for it to elute. Often, in attempting to maximize through-put we don't give enough time for crud (I have a lot of other terms for this stuff, but typically not for polite company) to elute. I would suggest running the column up to the max and hold it for a few minutes. You may get fewer samples per batch, but if your closing CCV fails more often than not, its not helping your situation to push through-put.

Regarding the inlet, I've seen liners after PCB analyses that literally are coated internally with oil. The build-up of material with diminish the volatilization from the inlet to your column causing the closing CCV to fail. Try a higher inlet temperature. Also, this situation may be helped by increasing your split flow. This will tend to increase the clean-out of the inlet at whatever temperature you're running.

Greg