All results lower than expected.

Question

HI everyone, I am new to the community and new to working with GC instruments. 
 The annual maintenance was performed on my GC 7890A two weeks, quality control at that time was perfect and running as expected. Last week and this week all of my quality control levels on every volatile tested have shifted downward, all running lower than expected. I have recalibrated the instrument, however results are the same. Any advice and help are greatly appreciated.

alfausett · Answer

Hi Linds, 
 Linds 
 Welcome to the Community. I think this is an excellent question, and something we all face from time to time. Thanks for asking! 
 I'm going to assume that you have a split/splitless inlet. A PM for this inlet usually includes a liner/o-ring, gold seal/washer, septum and split vent trap. I do notice that in your QC charts, there is significantly less 'bounce' after maintenance. This could indicate that the old split vent trap had some restriction in it, which affected the split ratio and ultimately your amount on column. This would affect the high end of your calibration curve more severely, and when corrected, would affect the slope of your curve. Did your calibration curve values change? Specifically the slope value? Are your QCs on the middle to low end of your curve? This might explain why your areas aren't changing as drastically as your found amounts. 
 If you are getting close to the same peak area, and your retention times are consistent, I wouldn't expect any major issues with the GC. However, small leaks can cause a lower response, so it may be worth checking around all inlet and column connections. 
 I realize this post is several months old, but if you already found a remedy, please reply and share! 
 Best, 
 Abbey

All results lower than expected.

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