Increase in absorbance over the course of a gradient analysis injection run.

Question

Hello, 
 I am looking for advice on how to fix what I am observing in my HPLC Agilent 1220 Infinity II. Thank you in advance for any insight. 
 When my HPLC is on the absorbance at 230nm baseline is flat and does not move from 0. However, when I run an injection (either with volume of blank methanol or with an injection volume of 0) the absorbance will shift upwards as the gradient method increases in the % of methanol coming from bottle B. https://www.agilent.com/cs/library/applications/application-dedicated-cannabinoid-potency-testing-5991-9285-en-us-agilent.pdf 
 As seen in the picture there is an upward trend over the course of the run. 
 Bottle A : 0.1 % (v/v) Formic acid in water aqueous phase & Bottle B 0.05 % (v/v) Formic acid in Methanol organic phase. 
 Time (min) % B 
 0 60 
 1.0 60 
 7.0 77 
 8.2 95

Andy Wright · Answer

Hi JacobCoppolino 
 Looking at the specifications for these products, the methanol has a maximum absorbance at 230nm of 110mAU and the formic acid has a maximum absorbance of 550mAU at 230nm, but what the absolute value for your batches of methanol and formic acid are is difficult to tell. 
 The application tries to balance the absorbance across the gradient, by using a lower percentage of formic acid in methanol than in water. In your case, the balance isn't quite right, and you may need to reduce the percentage of formic acid in the methanol a little; maybe t o 0.04% might look better. 
 If you are using OpenLab CDS, you could always do blank subtraction, to flatten the baseline, since you have such a consistent background absorbance pattern across the gradient. 
 /Andy