Table 2. Summary of the Fragmentation Rules and Diagnostic m/z Values for Keratan Sulfates.

MSn Stage	Fragmentation Rule	Diagnostic m/z
MS1	Molecular ion ([M-nH]n−) is given without considerable loss of sulfate.
	The number of sulfate and/or carboxylate (n), as well as molecular weight (M) is determinable.
MS2 (on [M-nH]n−)	Dehydration ([M-H2O-nH]n−) and 0,2Ar cleavage occur in most oligosaccharides.
	The dehydration is ascertained to take place between C-2 and C-3 of reducing GlcNAc.
	Desialylation occurs preferentially in sialylated oligosaccharides.	m/z 290
MS3 (on [M-H2O- nH]n−)	[Y1 - H2O - OSO3H]− ion is detected for GlcNAc6S at reducing end.	m/z 184
	B1 ion is detected for Gal6S at nonreducing end.	m/z 241
MS3 (on 0,2Ar)	2,4Ar ion appears intensely (except for L2 and SL1).
	[OCHCH2OSO3]− ion is detected for 6S at reducing end.	m/z 139
	[0,2Ar-H2O]n− ion is detected for Gal-GlcNAc6S (L2) sequence at reducing end.
MS4 (on 2,4Ar)	Extensive fragment ions resulting from glycosidic cleavages are obtained, which give valuable information for sequence determination.
	B1 ion is detected for GlcNAc6S or Ga16S at nonreducing end.	m/z 282 (GlcNAc6S) m/z 241 (Gal6S)
	B2 ion is detected for monosulfated disaccharide sequence at nonreducing end.	m/z 444
	Doubly charged B2 ion is detected for disulfated disaccharide sequence at nonreducing end.	m/z 261.5
	Y2 ion is detected for Gal6S-GlcNAc6S (L4) sequence at reducing end.	m/z 301
	[OSO3H]- and/or [2,4Ar-OSO3H]n− ions are detected for Gal6S at any position in the sequence.	m/z 97