ExD (e.g., EThcD, ETD) fragmentation methods can be used to discriminate between leucine and isoleucine in peptides. Since these two isomeric amino acids have the same molecular weight, they cannot be distinguished in MS/MS spectra obtained from CID. In ExD data, the w-ions that lost the Leu and Ile side chains are observed as Leu z -29 (z ion with Leu on its N-terminus loses ethyl (-29)) and Ile z-43 (z ion with Ile on its N-terminus loses isopropyl (-43)), respectively. Byonic can annotate those ions in the MS/MS spectra.
Byonic can also annotate Ile-26 as d-ions for EThcD and UVPD data, but not for ETD.