Isomeric Overlap

Positive and negative ion mode

Lipid classesType of overlapExample
PE O – LPEPE O = LPE[PE O-26:1+H]+ = [LPE 26:1+H]+
PC O – LPCPC O = LPC[PC O-26:1+H]+ = [LPC 26:1+H]+
DB = Double Bond

Positive ion mode

Lipid classesType of overlapExample
PC-PEPC = PE + 3CH2[PC 32:0+H]+ = [PE 35:0+H]+[PC 32:0+Na]+ = [PE 35:0+Na]+
PC-PA[PC+H]+ = [PA+5CH2+DB+NH4]+[PC 32:0+H]+ = [PA 37:1+NH4]+
PE-PA[PE+H]+ =[PA+C2H4+DB+NH4]+[PE 34:1+H]+ = [PA 36:2+NH4]+
PS-PG[PS+H]+ = [PG+2DB+NH4]+[PS 34:1+H]+ = [PG 34:2+NH4]+
SM – CerPESM = CerPE + 3CH2[SM d34:1+H]+ = [CerPE d37:1+H]+
DB = Double Bond

Negative ion mode

Lipid classesType of overlapExample
PC-PS*[PC+HCOO] = [PS+3CH2-DB-H][PC 33:1+HCOO] = [PS 36:0-H]
PC-PS[PC+CH3CO2] = [PS+4CH2-DB-H][PC 32:1+CH3COO] = [PS 36:0-H]
all lipid classes forming acetate/formate adducts*[X+CH2+HCOO] = [X+CH3COO][PC 34:1+HCOO] = [PC 33:1+CH3COO][Cer d35:1+HCOO] = [Cer d34:1+CH3COO]
DB = Double Bond

*Acetate salts of LC-MS grade can be contaminated with trace amounts of formate, which can lead to false-positive identification. Methanol can be oxidized to formic acid during electrospray ionization.