I'm running SEC HPLC and getting these very long broad tails consistently - the peak doesn't broaden, but it also does not return to baseline except as a very gradual slope. The issue seems to get worse the longer the machine has been running (see below), and switching from steel to PEEK tubing and swapping out different flow cells has not helped. What's most unusual is that the problem only happens on our 1260/1290 series hplcs - the exact same sample, method, buffer, and column on a much older 1100 series does not give any issues.
Blue line is the first run, teal line is the exact same sample after 2 more repeat injections.
Does anyone have any idea what's causing this? And why the older series instrument doesn't have the same issue? We are using an advancebio SEC.