Isomeric Overlap

Positive and negative ion mode

Lipid classes	Type of overlap	Example
PE O – LPE	PE O = LPE	[PE O-26:1+H]⁺ = [LPE 26:1+H]⁺
PC O – LPC	PC O = LPC	[PC O-26:1+H]⁺ = [LPC 26:1+H]⁺

DB = Double Bond

Positive ion mode

Lipid classes	Type of overlap	Example
PC-PE	PC = 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 – CerPE	SM = CerPE + 3CH₂	[SM d34:1+H]⁺ = [CerPE d37:1+H]⁺

DB = Double Bond

Negative ion mode

Lipid classes	Type of overlap	Example
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.