Table 1.

Fragmentation results for α3-derived phosphopeptide of 1620.9 mass

Residue	y ions present		b ions present
	y ion	y - H3PO4
y13 S b1	1620.6	1522.7	—
y12 Q b2	—	—	216.1
y11 P b3	1405.6	(1307.6)	(313.2)
y10 pS b4	1308.5*	(1210.5)	(480.2)
y9 E b5	1141.5	N/A	—
y8 T b6	1012.5	N/A	—

Gated ion of 1620.9, corresponding to phosphopeptide, was subjected to post source decay analysis. Fragment ions obtained are indicated either without parentheses (definitive) or with brackets (less definitive). 

—, fragment peaks that could not be distinguished. 

N/A, not applicable. 

^* The 1308.5 peak is particularly intense. 

None of the fragment masses underlined should appear if the serine at the y13-b1 position was phosphorylated instead of the serine at y10-b4.