Input of ratios for isomeric structures

MS analysis of per-methylated glycans by direct infusion presents the challenge of how to handle quantification of isobaric structures. One way of handling this issue is by using ratios of dominant MS/MS fragments that define the presence of each isobar or isobaric group. Here the contribution of each isobar can be calculated by dividing the Full MS signal among the isobars detected based on the ratio of the diagnostic fragments. The fragmentation efficiency of particular glycan bonds is not known however we expect the same glycan to fragment with the same efficiency regardless of the sample from which it came. Therefore, comparison between fragment ratios between biological samples is useful.

For example

The fragment m/z 474.2 (A) indicates the presence of the first isomer whereas m/z 660.3 (B) indicates the presence of the second. It would not be reasonable to say that a ratio of A to B represents the relative amount of structure 1 to 2 since those bonds may not break equally well however the ratio of fragment A/B in sample 1 compared to A/B in sample 2 would reflect a change in the relative abundance of the two structures between the 2 different samples.

Some isomers cannot be easily differentiated by direct infusion MS and would remain as ambiguous, for example differing by only linkage or arm position. However many topologies can be differentiated and their contributions to the total intensity of a mass by this ratio.

I would like to be able to separate distinguishable isomers within samples in GRITS by automatically dividing the total intensity (from either the Full MS or Xtraxt) based on a user defined ratio of selected fragment intensities. In the above example I would choose 660/474. The user must select Which fragments to use for a particular set of isobaric structures. Perhaps the user could store the selected fragments.