I have been experiencing poor injection repeatability with my PE AutoSystem XL GC. For 6 repeat injections of a Grob mix, I am getting RSD's of around 6% (peak area counts) for 4 peaks that I am quantitating. The retention time repeatability is very good (RSD<0.1%). I am injecting 1 uL of the Grob mix, while using a 5 uL syringe and the PE automated injector, I am using a 100:1 split ratio. I even replaced the septum, injector sleeve, and syringe. Nothing seems to help. I would expect an RSD of <2% for 6 replicate injections. Anyone have any other hints on how to improve injection repeatability.

Thanks in advance,

Chris

Since the Grob mix is for column testing, have you tried a new column? Maybe the 'old' one has some problems. Is there a pattern to the response changes? Some peaks staying the same, some up, some down with subsequent injections?

What are the signal-to-noise ratios of the peaks? Are the responses in the normal range for this system? Maybe the split ratio is too high, and you are seeing the column effects the Grob mix is designed to detect at low on-column concentrations.

If this test normally works on this system, and you have replaced all the usual problem parts, there aren't that many things left. Column problems, make sure the detector is tuned properly and not running on the edge. Maybe it needs cleaning because of junk on the jet or collector plates.

If I could get 6% on the systems I use for pesticide work, I'd be delighted!!

I would check the seal at the detector end of the column to be sure the ferrule is sealing correctly. A leak at the injector will cause a variation in retention time are area changes, while a leak at the detector will lead to area variation without retention time variation.

You might want to try a new syringe. Try one with a metal plunger instead of the standard Teflon tipped plunger. What size capillary column are you using?