Isobaric Overlap

Positive and negative ion mode

Lipid classesType of overlapm/zExamples
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]
DB = Double bond; O = ether-linkage instead of ester-linkage

Positive ion mode

Lipid classesType of overlapm/zExamples
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]+
DB = Double bond

Negative ion mode

Lipid classesType of overlapm/zExamples
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-
DB = Double bond; O = ether-linkage instead of ester-linkage

Isobaric fragment ions

Lipid classesType of overlapm/zExamples
FA fragmentsFA X:Y(+O) ≈ FA X+4:Y+6(-CO)0.021[FA 18:0-H] ≈ [FA 22:6-H-CO2]
FA fragmentsFA X:Y(+O) ≈ FA X+3:Y-1(-CO)0.073[FA 20:4-H] ≈ [FA 23:3-H-CO2]
DB = Double bond; O = ether-linkage instead of ester-linkage