Dependence of negative-mode electrospray ionization response factors on mobile phase composition and molecular...

Authentic standards of known concentrations serve as references for accurate absolute quantification of plant metabolites using liquid chromatography/mass spectrometry (LC/MS). However, often such standards are not commercially available or not amenable for custom syntheses. Despite the widespread use of electrospray ionization for metabolite analyses, the fundamentals needed for reliable prediction of molecular response factors have yet to be explored in detail for analytes that lack ionized functional groups. In order to lay a foundation for quantifying unknown neutral plant metabolites in absence of authentic standards, sub-milligram quantities of purified homologous acylsucrose metabolites were authenticated by subjecting each to basic hydrolysis and quantifying the sucrose product using stable-isotope dilution ultrahigh performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) with multiple reaction monitoring (MRM). Once authenticated, molar response factors of [M + formate](-) ions for the acylsucrose metabolites were determined at different mobile phase compositions ranging from 40%-80% acetonitrile, and demonstrated relationships of response factors with mobile phase composition and metabolite structural features including nonpolar surface areas, the length of the longest acyl chain, and the number of hydroxyl groups. This approach was employed to calculate predicted response factors for three authenticated acylsucroses based on mean values for all isomers with a common number of total acyl carbon atoms. Absolute UHPLC-MS quantification was performed on these three metabolites in an extract from leaves of the wild tomato Solanum habrochaites LA1777, yielding deviations of 26%, 6.7%, and 7.3% from values established using compound-specific response factors.