1220 infinity II decreasing absorption over course of analysis run


The pressure and the absorption at 230nm seen in the Online Signals view when I am operating the HPLC both decline steadily over the course of an injection runtime. The set up of the analysis is as follows in this literature. https://www.agilent.com/cs/library/applications/application-dedicated-cannabinoid-potency-testing-5991-9285-en-us-agilent.pdf 

Both Methanol Blanks and Standards in methanol end up declining below 0 for absorption at around the midpoint of the runtime, then bottoms out and comes back up to 0 at the end of the runtime. This dip in absorbance coincides with a dip in pressure over the course of a run. The peaks are still identified, however they are now below 0. 

We switched to a different Signal Wavelength and there was no dip in the absorbance. Wondering why we might see a dip in absorbance at one wavelength but not another when running blanks and standards? 

The Image is the first run since turning on the device. However, we ran it 20-30 times and each 9.50 min runtime looks the same. The same MeOH blank at 254 nm has a flat line absorbance for the duration of the run with the same variation in pressure.


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