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Sample preparation / retention time shift when analyzing urine with C18 column
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,