Why is the retention time increasing?

Hello,

I am using ECD on GC 7890A to measure N2O. I have recently come across a strange occurrence and wanted to ask for your opinion on what may be happening. I ran N2O standards on the GC two days ago and the retention time was maintained at 1.37. However, running samples today the starting retention time was 1.37 and slowly worked its way higher to 1.7. I have restarted the machine and computer system twice and continue to get the same results. I appreciate your time and input to helping us find the error

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