Ghost peaks from a PTFE filter are almost always one of four chemical families
A 20 µL injection of mobile phase A on a 250 × 4.6 mm C18 column came back with a peak at 8.3 minutes, area 14.2 mAU·s, and a second at 11.9 minutes at 6.8 mAU·s. The method blank had been clean the week before. Nothing on the instrument had changed except the filter lot: we had moved from a 47 mm, 0.45 µm PTFE membrane to a new supplier’s “equivalent,” and the ghost peaks appeared in every blank for the next four injections, then tapered off. That is the classic signature...
