Isobaric Overlap

Positive and negative ion mode

Lipid classes	Type of overlap	∆m/z	Examples
DB containing classes	[M+DB+13C2] ≈ [M] **Overlap resulting from natural 13C abundance of 1.1% was selected as prevailing overlap; other isotopes like two deuterium or 18O may lead to similar isobaric overlaps.	0.009	[PC 32:1+H+13C2]+ ≈ [PC 32:0+H]+
Glycerolipids	[M] ≈ [M+CH2+O]	0.036	[PC 33:1+H]+ ≈  [PC O-34:1+H]+
Sphingolipids	[M] ≈ [M+CH2-DB-OH]	0.036	[SM t42:2] ≈ [SM d43:1]
PI – PS	[PI] ≈ [PS+6CH2+5DB+13C]	0.002	[PI 34:1-H]–  ≈  [PS 40:6-H+13C]–
PC – SM	[PC+13C] ≈ [SM-OH2 +4CH2-DB]	0.065	[PC 38:3+13C] ≈ [SM d42:2]

Positive ion mode

Lipid classes	Type of overlap	∆m/z	Examples
DB containing classes	[M+Na]+ ≈ [M+2CH2+3DB+H]+	0.002	[PC 34:1+Na]+ ≈ [PC 36:4+H]+
PC – PS	[PC+H]+ ≈ [PS+DB+H]+	0.073	[PC 32:0+H]+ ≈ [PS 32:1+H]+
DG – CE	[CE X:Y+NH4]+ ≈ [DG X+30:Y+NH4]+	0.015	[CE 18:1+NH4]+ ≈ [DG 38:1+NH4]+

Negative ion mode

Lipid classes	Type of overlap	∆m/z	Examples
DB containing classes	[M+35Cl]– ≈ [M+DB+37Cl]–	0.019	[PC 34:1+35Cl]– ≈ [PC 34:2+37Cl]–
PA – CL2-	[PA X:Y-H]– ≈ [CL 2X:2Y-2H]2-	0.018	[PA 34:2-H]– ≈ [CL 64:4-2H]2-

Isobaric fragment ions

Lipid classes	Type of overlap	∆m/z	Examples
FA fragments	FA X:Y(+O) ≈ FA X+4:Y+6(-CO)	0.021	[FA 18:0-H]– ≈ [FA 22:6-H-CO2]–
FA fragments	FA X:Y(+O) ≈ FA X+3:Y-1(-CO)	0.073	[FA 20:4-H]– ≈ [FA 23:3-H-CO2]–