retention time shifts at the beggining of injection

Hello all.

Here we have a method for analysing solvents in our GCMS 8860 / 5977B With an ALS G4567A

Our method consists of 27 compounds.

In our recent run we notice a drop in internal standard response Figure 1, in the middle of the run, and after signal return to normal. so it was not a matter of vial running out of IS.

During our investigation we notice that, especially for the samples affected, there was some retention time shifts on the first few minutes of the run. Figure 2 ( all these peaks should be the same RT)

These retention time shifts are visibly happening to around 3.4 min into the run.

Although the rest of the peaks in the 15 min run have the expected retention time, we are wondering if these shift is what is affecting signal for Internal standard.(figure 3)

Does anyone know wha could be causing such retention time shifts in the beggining of the run?

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  • You can run an MMI in cold injection mode, where the inlet is cooled off between every injection.

    Your oven initial temperature is 30° C ?  Do you have oven cryogenic cooling? If you do not have oven cryogenic cooling I'd be surprised if the system can get to ready at 30° C unless your room is 15° C  or colder.  Without cryo, achieving that low temperature and have it be stable is difficult. Any fluctuation may/will affect the retention times of peaks throughout the first half of your run.  How long is your oven temperature equilibration time?  It may need to be 3 minutes or even longer.  

    The 30° C/minute ramp from 30° C to 45° C, hold 1 minute, then slow 3° C/minute ramp to 55° C is unusual.  You are trying to separate early peaks this way but I would be recommending a different column instead. 

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