Front inlet cooling problem in 7890B GC system


I am experiencing a problem with front inlet (MMI) temperature in our 7890B GC system. The initial front inlet temperature and the oven temperature of my method is 28 C and the first sample run of a sequence happens without any issues. The problem starts with the second sample run. The needle stays frozen in the sample vial until the front inlet temperature is ready.  Although the front inlet cools down to 28 C in between sample runs (I can see it on the display), for some reason temperature automatically increases to 30-31 C within the solvent wash time. Therefore, the needle stays frozen without injecting the sample until the front inlet temperature goes down to 28 C. 

We tried replacing the syringe and increasing the initial front inlet temperature of my method to 33 C. But didn't work. Further we removed front inlet temperature from the readiness list and compared the results. Later eluting peaks of my DBPs standards mix, give lesser abundance in the absence of front inlet temperature in the readiness checklist. Currently, I cannot move forward with my experiments because of this problem.

We were using this method for more than 2 years to analyze DBPs and this is the first time we observed this issue. We use a nitrogen dewar for front inlet cooling. I can hear the cryo gas cooling sound in between sample runs. But I was wondering if there's something wrong with the cooling process.  Appreciate your ideas about this issue.


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