The bottom-up solution to the triacylglycerol lipidome using atmospheric pressure chemical ionization mass spectrometry

Presented here is an approach to representing the data from atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) of triacylglycerols (TAG) using a set of one, two, or three Critical Ratios. These Critical Rations may be used directly to provide structural information concerning the regioisomeric composition of the triacylglycerols (TAG), and about the degree of unsaturation in the TAG. An AAA-type, or Typel, TAG has only one Critical Ratio, the ratio of the protonated molecule, M+H]⁺, to the DAG fragment ion, AA]⁺. The Critical Ratio for a Type I TAG is MH]⁺/ΣDAG]⁺, and the mass spectrum of a Type I TAG can be reproduced from only this one ratio. An ABA/AAB/BAA, or Type II, TAG has two Critical Ratios, the MH]⁺/ΣDAG]⁺ ratio and the AA]⁺/AB]⁺ ratio. The AA]⁺/AB]⁺ ratio for a single TAG or TAG mixture can be compared with the AA]⁺/AB]⁺ ratios of pure regioisomeric standards, and the percentage of each regioisomer can be estimated. The abundance of the protonated molecule and the abundances of the two DAG]⁺ fragment ions can be calculated from the two Critical Ratios for a Type II TAG. To calculate the abundances, the Critical Ratios are processed through the Bottom-Up Solution to the TAG lipidome. First, Critical Limits are calculated from the Critical Ratios, and then the Critical Ratios are classified into Cases by comparison with the Critical Limits. Once the Case classification is known, the equation for the abundance of each ion in the mass spectrum is given by the Bottom-Up Solution. A Type III TAG has three different FA and three Critical Ratios. The MH]⁺/Σ/DAG]⁺ ratio is the first Critical Ratio, the AC]⁺/(AB]⁺+BC]⁺) ratio is the second Critical Ratio, and the BC]⁺/AB]⁺ ratio is the third Critical Ratio. The second critical ratio for a Type III TAG can be compared with regioisomeric standards to provide an estimate of the percentage composition of the regioisomers. The three Critical Ratios for a Type III TAG can be processed through the Bottom-Up Solution to calculate the four ion abundances that make up the APCI-MS mass spectrum. The Critical Ratios constitute a reduced data set that provides more information in fewer values than the raw abundances.