A method of mass spectrometry comprising: mass analysing an eluent from a chromatography device; obtaining a plurality of parent ion data sets and a plurality of corresponding product ion data sets by detecting product ions and parent ions and assigning product ions to parent ions, wherein a first product ion data set is assigned to a first parent ion data set attributed to one or more first parent ions, and one or more second product ion data sets are assigned to one or more second parent ion data sets attributed to one or more second, different parent ions; and determining whether, in the first product ion data set of said plurality of product ions data sets, one or more product ions are present that are related to one or more parent ions in the corresponding first parent ion data set of said plurality of parent ion data sets, based on the masses or mass to charge ratios and intensities of said one or more product ions and said one or more parent ions by comparing the masses or mass to charge ratios and intensities of ions in said first parent ion data set and of ions in said first product ion data set; wherein if it is determined that said one or more product ions are present, the method further comprises removing said one or more product ions from the one or more second product ion data sets of said plurality of product ion data sets to produce one or more second modified product ion data sets, wherein said one or more second product ion data sets comprise product ion data sets acquired at the same time, immediately before or immediately after said first product ion data set, and wherein said one or more second parent ion data sets comprise parent ion data sets acquired at the same time, immediately before or immediately after said first parent ion data set; wherein said one or more product ions are removed from said one or more second product ion data sets to produce said one or more second modified product ion data sets after acquisition of the product ion data sets but before they are submitted to a search engine; and wherein the method comprises submitting the one or more parent ion data sets and the one or more corresponding modified product ion data sets to a search engine to determine the composition of the eluent. mass analysing an eluent from a chromatography device; obtaining a plurality of parent ion data sets and a plurality of corresponding product ion data sets by detecting product ions and parent ions and assigning product ions to parent ions, wherein a first product ion data set is assigned to a first parent ion data set attributed to one or more first parent ions, and one or more second product ion data sets are assigned to one or more second parent ion data sets attributed to one or more second, different parent ions; and determining whether, in the first product ion data set of said plurality of product ions data sets, one or more product ions are present that are related to one or more parent ions in the corresponding first parent ion data set of said plurality of parent ion data sets, based on the masses or mass to charge ratios and intensities of said one or more product ions and said one or more parent ions by comparing the masses or mass to charge ratios and intensities of ions in said first parent ion data set and of ions in said first product ion data set; wherein if it is determined that said one or more product ions are present, the method further comprises removing said one or more product ions from the one or more second product ion data sets of said plurality of product ion data sets to produce one or more second modified product ion data sets, wherein said one or more second product ion data sets comprise product ion data sets acquired at the same time, immediately before or immediately after said first product ion data set, and wherein said one or more second parent ion data sets comprise parent ion data sets acquired at the same time, immediately before or immediately after said first parent ion data set; wherein said one or more product ions are removed from said one or more second product ion data sets to produce said one or more second modified product ion data sets after acquisition of the product ion data sets but before they are submitted to a search engine; and wherein the method comprises submitting the one or more parent ion data sets and the one or more corresponding modified product ion data sets to a search engine to determine the composition of the eluent.