Fig. 2: Relative retention time regularities of lipid subclasses.

a Scatter plots of molecular weight and retention time, where lipid subclasses were distinguished by different colors. b Scatter plots of molecular weight and retention time for free fatty acids, poly-unsaturations were distinguished by different colors, using a polynomial fit for each poly-unsaturation and plotting the fitted curves and 95% confidence intervals, the centerline of the fitted trend is the line connecting the fitted values, some poly-unsaturations could not be fit owing to too few annotations. The red pentagram represents a double bond equivalents (DBEs) of 0, the blue hexagram represents a DBEs of 1, the green heptagram represents a DBEs of 2, the dark blue octagon represents a DBEs of 3, the flesh-colored pentagon represents a DBEs of 4, the light blue-purple hexagon represents a DBEs of 5, the light green heptagon represents a DBEs of 6, the dark red octagon represents a DBEs of 7, the dark brown octagram represents a DBEs of 9, and the light brown flat octagram represents a DBEs of 10. c Scatter plots of molecular weight and retention time for phosphatidylcholines (PCs) with four DBEs, distinguish unsaturated compositions by color, and plot 95% confidence intervals using linear fitting, and the centerline of the fitted trend is the line connecting the fitted values. The red square represents a DBEs composition of 0:4, and the blue square rotated 90° represents a DBEs composition of 1:3. d Statistical results of equivalent carbon number (ECN), intraclass unsaturation parallelism (IUP), and equivalent separated carbon number (ESCN) for positive or negative ionization mode by using >100 published datasets. Blue and red represent positive ionization and negative ionization modes, respectively. e, f Statistical results of ECN, IUP, ESCN in >100 published datasets classified by different lipid category and number of separated chains. Deep purple, red, and green represent Intra-subclass Unsaturation Parallelism (IUP), Equivalent Separated Carbon Number (ESCN), and Equivalent Carbon Number (ECN), respectively.