Complexation with iron at sub-ppm levels in HPLC analyses of phospholipids. 
Gradient RP-HPLC analysis of a phospholipid, E5564, utilizing water, methanol and phosphoric acid occasionally results in the appearance of a broad unknown peak in the chromatogram before a well-resolved E5564 peak.  This unknown peak does not elute in a reproducible fashion with regards to peak shape and retention time, and is not present in chromatograms resulting from injections of the diluent alone.  Investigation of this phenomenon revealed that iron ions in sub-ppm levels in the HPLC mobile phase chelated E5564 and the resultant complex(es) comprised the broad peak.  The iron source was identified as phosphoric acid, which was used as a mobile phase modifier.  Further studies were conducted to characterize the nature of the phospholipid-iron association and resultant complex.