Analysis of Hydroxy-Containing Seed Oils Using Atmospheric Pressure Chemical Ionization Mass Spectrometry

Abstract

Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) coupled to reversed-phase high performance liquid chromatography (RP-HPLC) was used for direct analysis of intact triacylglycerols (TAGs) from hydroxy-containing plant oils. Castor bean oil, Lesquerella fendleri and Lesquerella gordonii oils were separated into tri-hydroxy, di-hydroxy, mono-hydroxy, and non-hydroxy TAGs using the RP-HPLC method. The APCI-MS ionization source produced fragments representing loss of zero (protonated molecular ion), one, two, and three hydroxy groups. The primary fragments (base peaks) in the mass spectra resulted from loss of all hydroxy groups from the TAGs. Using the acetonitrile/methylene chloride solvent system, diagnostically important acetonitrile adducts were formed which allowed identification of the molecular weights of the hydroxy TAGs as well as confirmation of the number of hydroxy groups contained therein. A series of four adducts was formed: [M+23]⁺, [M+39]⁺, [M+54]⁺, and [M+59]⁺.

A mass difference of 76, 58, or 40 amu between the base peak and the [M+23]⁺ peak allowed the TAGs to be identified as trihydroxy, dihydroxy, or monohydroxy, respectively. Additional fragmentation of the TAGs occurred with cleavage of the acyl chains next to the hydroxy group giving a net loss of 114 amu. This fragmentation occurred in combination with loss of hydroxy groups from the remaining acyl chains. Diacylglycerol fragments were formed which also exhibited sequential loss of the hydroxy groups, as well as the fragment produced by loss of 114 amu.