additional ions for ETD-type fragmentations

[lars-kolbowski]

Additional ions c-1 and z+1 can occur due to hydrogen transfer from c to z ions.
Apparently c+1 and y ions are also possible.

MQ does an iterative search where they use the best score result:
first: only c, z, z+1
second: c, z, z+1 & c-1, c+1
third: c, z, z+1 & y
fourth: c, z, z+1 & c-1, c+1, y

Could be worthwhile to include at least z+1 ions

MaxQuant Summer school presentation slide