A method of mass spectrometry comprising the steps of: (a) passing parent or precursor ions to a collision, fragmentation or reaction device; (b) operating said collision, fragmentation or reaction device in a first mode of operation wherein at least some of said parent or precursor ions are collided, fragmented or reacted to produce fragment, product, daughter or adduct ions; (c) recording first mass spectral data relating to ions emerging from or which have been transmitted through said collision, fragmentation or reaction device operating in said first mode of operation; (d) switching, altering or varying said collision, fragmentation or reaction device to operate in a second mode of operation wherein substantially fewer parent or precursor ions are collided, fragmented or reacted; (e) recording second mass spectral data relating to ions emerging from or which have been transmitted through said collision, fragmentation or reaction device operating in said second mode of operation; and (f) repeating steps (b)-(e) a plurality of times; characterised in that said method further comprises: (g) determining the accurate or exact mass or mass to charge ratio of one or more first parent or precursor substances or ions, wherein said accurate or exact mass or mass to charge ratio of said one or more first parent or precursor substances or ions comprise a first integer nominal mass or mass to charge ratio component M 1 and a first decimal mass or mass to charge ratio component m 1; and (h) searching for or determining one or more fragment, product, daughter or adduct substances or ions or one or more second parent or precursor substances or ions in or from said first mass spectral data, wherein said one or more fragment, product, daughter or adduct substances or ions or said one or more second parent or precursor substances or ions comprise a second integer nominal mass or mass to charge ratio component M 2 and a second decimal mass or mass to charge ratio component m 2, wherein said second decimal mass or mass to charge ratio component m 2 is between 0 to x 1 mDa or milli-mass to charge ratio units greater than said first decimal mass or mass to charge ratio component m 1 and/or between 0 to x 2 mDa or milli-mass to charge ratio units less than said first decimal mass or mass to charge ratio component m 1. (a) passing parent or precursor ions to a collision, fragmentation or reaction device; (b) operating said collision, fragmentation or reaction device in a first mode of operation wherein at least some of said parent or precursor ions are collided, fragmented or reacted to produce fragment, product, daughter or adduct ions; (c) recording first mass spectral data relating to ions emerging from or which have been transmitted through said collision, fragmentation or reaction device operating in said first mode of operation; (d) switching, altering or varying said collision, fragmentation or reaction device to operate in a second mode of operation wherein substantially fewer parent or precursor ions are collided, fragmented or reacted; (e) recording second mass spectral data relating to ions emerging from or which have been transmitted through said collision, fragmentation or reaction device operating in said second mode of operation; and (f) repeating steps (b)-(e) a plurality of times; (g) determining the accurate or exact mass or mass to charge ratio of one or more first parent or precursor substances or ions, wherein said accurate or exact mass or mass to charge ratio of said one or more first parent or precursor substances or ions comprise a first integer nominal mass or mass to charge ratio component M 1 and a first decimal mass or mass to charge ratio component m 1; and (h) searching for or determining one or more fragment, product, daughter or adduct substances or ions or one or more second parent or precursor substances or ions in or from said first mass spectral data, wherein said one or more fragment, product, daughter or adduct substances or ions or said one or more second parent or precursor substances or ions comprise a second integer nominal mass or mass to charge ratio component M 2 and a second decimal mass or mass to charge ratio component m 2, wherein said second decimal mass or mass to charge ratio component m 2 is between 0 to x 1 mDa or milli-mass to charge ratio units greater than said first decimal mass or mass to charge ratio component m 1 and/or between 0 to x 2 mDa or milli-mass to charge ratio units less than said first decimal mass or mass to charge ratio component m 1.