retention time shift when analyzing urine with C18 column December 20, 2015, 10:35:13 AM Hello,I am trying to do metabolomics analysis with urine samples using C18 column. However, I found that there were quite big retention time shifts (not in one direction) even by injecting the same sample repeatedly. For sample preparation, I simply diluted the urine samples with water (0.1% formic acid).I am wondering if anyone has encountered the same issue and it'd be great if any suggestions on how to deal with it could be provided here.Thanks a lot, Quote Selected