A method (100) for analyzing chemicals includes fractionating a complex sample into at least two sample portions that each includes potions of two polypeptides though in different concentration ratios, digesting and performing LC/MS on each of the sample portions (110, and associating precursor ions observed via LC/MS with their corresponding polypeptide in response to LC/MS provided intensity data (170). A set of precursor ions that has substantially similar intensity ratios in both sample portions is determined to be associated with the same polypeptide.