Poor repeatability on GC

[Callllii]

I am using an Agilent 6890 w/ FID and DB-1701 column to analyze an impurity of NMI. I'm having an issue with a baseline increase in the area of interest. Initial experiment went fine and the next run showed this problem. I thought it may be column bleed/bad column but it does the same with a new column (looked okay for the first few runs and then increased dramatically). Cannot distinguish a spiked from an unspiked sample at 100 ppm. Not sure where to go from here, I have already tried different ramps to no avail. Ideas??

[chromatographer1]

Please clarify:

What is NMI ? lots of possibilities here, clear questions get clear answers.

What is the matrix into which it is spiked?

Thanks.

Rodney George
consultant

[Callllii]

Sorry was in a bit of a hurry last night. NMI is N-Methylimidazole which is the matrix that the impurity is spiked. The impurity is a triazine compound. So 100ppm Triazine spiked into NMI. Have already tried to use a DB-1 column which doesn't give sufficient resolution. The current column (DB-1701) worked initially and then showed this baseline problem in the area of interest. On column degradation is one theory (but seems unlikely as it worked previously).

[aldehyde]

Have you tried doing a few blank runs? Does the baseline disturbance decrease or stay constant?

[mattj63]

Could your sample be decomposing? In addition to running a blank like was suggested, try injecting something stable like hexanes. These could also wash stuff out of the injection port. In that case, you would see the baseline change get less over repeated injections.

[sassman]

Does the baseline change always happen at the same time? In that case, I would suspect some kind of flow/pressure change. Monitor the inlet pressure and see if it changes when the baseline shifts. There was a discussion of a similar problem a while back, but I couldn't find it by a quick search.

[chromatographer1]

I think you are having chemistry issues.

Putting an imidazole and all its trace impurities with the matrix onto a silica surface coated with cyano propyl groups can give you all sorts of reactions producing functional groups which may be reactive with the triazine.

Once the column surface is coated with residuals that may be reactive with the triazine you will lose or broaden the peak. Of course, the injection liner is another possible source of trouble.

Possibly another phase might improve your results. Or another technique other than GC.

Good luck.

Rodney George

[Callllii]

I tried the blank runs to no avail. The initial thought was degradation of the sample on the column. Thinking you may be right in needing a new stationary phase or a completely different technique to quatitate. Haven't given up completely but it doesn't look good.

[krickos]

Hi

I am inclined to lean in chromatographer1/Rodneys direction. The DB-1701 would not be my first choice of trial for those amines.
Though I have run 2,6-dimethyl aniline a few times on a 1µm phase DB-1701 (column test mix) and I think there is an application note on Triazine pesticides (sterical hindered) with that column.

In any case a DB-5, DB-5ms or RTx-5 amine column (increasing inertness) especially the last two can be worth a try. Those have worked fine for some morpholine structures we have worked with in the past.