Analytica Chimica Acta 722 (2012) 80–86 Contents lists available at SciVerse ScienceDirect Analytica Chimica Acta jo u rn al hom epa ge: www.elsevier.com/locate/aca Determination of triacylglycerol regioisomers using electrospray ionization-quadrupole ion trap mass spectrometry with a kinetic method Nathalie L. Leveque ∗ , Akwasi Acheampong, Sylvie Heron, Alain Tchapla Université de Paris Sud, Groupe de Chimie Analytique Paris Sud, LETIAM (EA 4041), IUT d’Orsay, Plateau de Moulon, F 91400 Orsay, France a r t i c l e i n f o Article history: Received 17 October 2011 Received in revised form 6 February 2012 Accepted 9 February 2012 Available online 21 February 2012 Keywords: Kinetic method Mass spectrometry Regiospecificity Triacylglycerols a b s t r a c t The kinetic method was applied to differentiate and quantify mixtures of regioisomeric triacylglycerols (TAGs) by generating and mass selecting alkali ion bound metal dimeric clusters with a TAG chosen as reference (ref) and examining their competitive dissociations in a quadrupole ion trap mass spectrome- ter. This methodology readily distinguished pairs of regioisomers (AAB/ABA) such as LLO/LOL, OOP/OPO and SSP/SPS and consequently distinguished sn-1/sn-3, sn-2 substituents on the glycerol backbone. The dimeric complex ions [ref, Li, TAG (AAB and/or ABA) ] + generated by electrospray ionization mass spectrometry were subjected to collision induced dissociation causing competitive loss of either the neutral TAG ref- erence (ref) leading to [Li(AAB and/or ABA)] + or the neutral TAG molecule (TAG (AAB and/or ABA) ) leading to [ref, Li] + . The ratio of the two competitive dissociation rates, defined by the product ion branching ratio (R iso ), was related via the kinetic method to the regioisomeric composition of the investigated TAG mix- ture. In this work, a linear correlation was established between composition of the mixture of each TAG regioisomer and the logarithm of the branching ratio for competitive fragmentation. Depending on the availability of at least one TAG regioisomer as standard, the kinetic method and the standard additions method led to the quantitative analysis of natural TAG mixtures. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In the biosynthesis of triacylglycerols (TAGs) of vegetable oils, each fatty acid (FA) occupies a preferred position on the glycerol backbone [1]. The regiospecific positioning of distinct FA has been indicated to be responsible for specific effects on lipoprotein metabolism and atherogenesis which also affect lipid metabolism in humans. For the first time, the regioisomeric distinction was reported by Barber and Merren [2]. Until now, the differentiation between TAG isomers (AAB/ABA) has been based on the ratio of characteristic fragment ions in mass spectrometry. Thus, Mottram and Evershed [3] and Byrdwell [4] demonstrated that the loss of the acyl chain from the sn-1 or sn-3 position was energetically favoured over loss from the sn-2 position. The APCI mass spectra of pure TAG isomers reported by the above authors, demonstrated that the ion abundances [AA] + /[AB] + were expected to change with the regioi- someric composition of a mixture of TAG regioisomers. However, discordant results were published concerning the identification of regioisomers using APCI. Although Mottram and Evershed [3] could identify the positional isomers, Segall et al. [5] showed that analysis of sodium adduct molecular ions did not provide differences in the relative abundance of regioisomeric triacylglycerols ions. Duffin ∗ Corresponding author. E-mail address: nathalie.leveque@u-psud.fr (N.L. Leveque). et al. [6] reported that the relative abundance of product ions resulting from the dissociation of fatty acid chains from TAGs was not significantly different, while Hvattum [7] found a smaller signal due to the neutral loss of the sn-2 fatty acid residue. Both experi- ments of Duffin et al. [6] and Hvattum [7] were conducted using ESI ionization. In a previous work [8] we showed that it was possible to distinguish regiospecifically some pairs of TAGs thanks to silver adducts and MS 5 analysis. However, this MS approach was long and needed much quantity of TAG to allow regiospecific distinction of TAGs by LC–MS 5 . Therefore, a new simple, faster MS and reliable MS approach for regioisomeric ratio determination is desirable. In an attempt to resolve the problem of regioisomeric distinc- tion of TAGs, we explore the applicability of the kinetic method, a procedure for thermochemical determinations which has seen growing use for chiral recognition [9–15] and some use in isomeric differentiation [16]. The kinetic method [10,16,17] uses a transition metal ion to form complex ions involving both an analyte and a reference com- pound, thereby allowing multiple point interactions between these ligands. In the absence of solvent in the mass spectrometer, all interactions are between the ligands and this increases the impor- tance of the reference compound as well as the strength of the metal–ligand interactions. Most of the kinetic method’s literature deals with trimeric complexes [11,15–21] and more rarely it was also proved that the kinetic method was applicable with dimeric clusters [20,22–24]. 0003-2670/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2012.02.016