Additional peak in chromatogram
I'm having issues with an additional peak sometimes (not always) appearing in my reference sample chromatogram. The peak is alwasy at ~160 mins and is a sharp disctinct peak. Before the reference sample is injected a waters MassPREP peptide mix vial (186002338 -contains a mix of 10 peptides) reconstituted in 0.05% TFA in water is injected onto the system followed by an injection of 1% solvent B. (Line A1 = 0.05% TFA water and B1 = 0.04% TFA in acetonitrile run over 240 mins using a gradient curve of 6. The column is a BEH C18 with a guard column attached). The reference and samples are prepared in NH4HCO3 solution.The additional peak is never present in the sample chromatograms or contamination check (half sample/half reference). Is this carryover contamination from the MassPREP injection? Any ideas what else this could be?