Phantom peak GC

[larkl]

We're seeing a phantom peak in a GC run of isobutane (gas injection - GSV onto PLOT column, 80:1 split, FID, HP6890). Peak does not appear to be "real" as it is not present in an identical instrument running the same method and sample. The peak is massive (25% of total area). Interestingly enough, changing the inlet liner, o-ring and septa fixed the problem. This is great, but I'd sure like to understand why. Inlet consumables were not more than three months old and we do absolutely no syringe injections here, so the septa wasn't cored.

[AICMM]

Do you have septum purge on your instrument and how much flow does it have if you do? Could be accumulation from the septume. Possibly the gas sampling valve switching but valve peaks are not usually this big.

Best regards.

[Peter Apps]

I'm intrigued - if you never do syringe injections why do you have a septum at all ?

Peter

[larkl]

Peter - We still use the split functionality of the inlet. If we were not, we could just bypass it and run the carrier direct from the sample valves to the column. Or are you suggesting that we remove the septa and replace it with something else?

[larkl]

AICMM- The septum purge is blocked in. We were told by the vendor to leave this as is.

[Peter Apps]

Septa cause all sorts of trouble, so if you could get rid of it and still retain the split it might be worthwhile. A lot will depend on what hardware you have - on the new Varian 450 a blanking nut and metal seal disc come with the instrument spares to replace the septum when baking out the inlet. It's hardly worth buying a whole GC just for that though !!

Peter

[AICMM]

larkl,

I am very puzzled why your vendor told you to blank off the septum purge. I have worked on a number of instruments doing gas sampling or LPG sampling and none of them are blanked off (some split some splitless.) At 80:1 split, you should not have to worry about stuff going out the septum purge. Therefore, I would un-blank and see what that does. I would also recommend routine (6 mos. or yearly) replacement of split vent trap. Additionally, you must have a huge split flow at 80:1 with a PLOT. Seems kind of high.

Along Peter's train of thought, perhaps a Merlin Microseal is appropriate and cheaper than a new GC.

Best regards.