Table 2.

Frequency of observation of different ion types matched in tryptic ETD spectra. Numbers represent the fraction of all matched peaks in a given spectrum that match to each ion type. Included in the list of ions considered is ‘x’ ions, which are not formed in ETD, so represent a measure of the random peak matching frequency. The first column reports average results, which are then split between those from doubly-charged precursors and those of higher charge state in the next two columns.

Ion Type	All	2+ Precursor	3+ or higher precursor
b	0.03	0.02	0.03
c	0.29	0.11	0.31
c-1	0.05	0.09	0.04
x	0.03	0.02	0.03
y	0.11	0.13	0.10
z	0.30	0.30	0.28
z+1	0.19	0.31	0.15