Missing peaks

[Nikko]

I have a splitless liquid injection (organic solvent) that looks at a variety of peaks including internal standard. My GC system sat inactive and untouched for two weeks (still hot with FID lit). Upon reusing system, I found that the compounds with the longer retention times would not show up until several runs had been repeated. This is a column on the back injection port. I changed septa, liner, and column looking for leaks. I still see the peaks with the longer retention times slowly buiding in intensity over time as multiple samples are run. In the first four samples run, these peaks don't even show up. The compounds with earlier retention times still show up and have the same peak area as before, being very consistent. I retested the standards on another GC column on the front injection port and the standards showed up in the same consistent peak area concentrations as previously observed. Has anyone seen this happen before? What could change in a system that wasn't touched for two weeks?

[SemiAuto]

Have you verified your column flow on the back column? Can you verify the temp. of your rear inlet somehow?

[Peter Apps]

This sounds like classic absorbtive activity - you have something in the inlet that holding onto the heavier analytes until repeated injections establishes some kind of equilibrium between injection and bleed.

Peter

[Nikko]

Peter,
Thanks for the tip on absorbtive activity. I cleaned the inlet and replaced the gold seal and the problem disappeared. As usage continues I begin to see the problem creeping back. Any suggestions on how to prevent this, short of cleaning the inlet each time?
Nikko

[JackC]

If your sample contains nonvolatile material, you will continue to see the same issue occurring as this material builds up in the inlet and sorbs the lower volatility compounds. The other thing that could be causing this is injecting samples that are very acidic or basic in nature.

For the nonvolatile material, you can try offline sample cleanup methods such as GPC (Gel Permeation Chromatography), which is a size exclusion chromatographic technique. Unfortunately that adds time and complexity and cost to the method.

Alternately, if you have a polarity difference between your analytes of interest and the material that seems to be degrading inlet performance, you could try either cartridge SPE or dispersive SPE (a very easy cleanup method) to remove the problematic material.

An alternate injection techique like PTV might allow increased numbers of injections before you have to maintain the inlet, but that would have to be determined experimentally.