Gas chromatography–mass spectrometry selected ion monitoring and gas chromatography–tandem mass spectrometry selected reaction monitoring analyses of mono-, di- and tri-unsaturated C₂₅ highly branched isoprenoid alkene biomarkers in sea ice and sediment samples: A comparative study

Rationale: The efficiency of selected ion monitoring (SIM) and selected reaction monitoring (SRM) analyses for the quantification of three mono-, di- and tri-unsaturated highly branched isoprenoid (HBI) alkenes (IP₂₅, IPSO₂₅ and HBI III, respectively), often used as proxies for the occurrence of Arctic and Antarctic sea ice or the adjacent open waters, was compared. Methods: Gas chromatography (GC)–mass spectrometry (MS)/SIM and GC/MS/MS/SRM analyses were carried out on dilute solutions made from purified standards of these three HBIs, and then on hydrocarbon fractions of several sediment and sea ice sample extracts. More efficient and specific SRM transitions were selected after collision-induced dissociation of each precursor ion at different collision energies. Results: SRM analysis avoided any overestimation of IP₂₅ resulting from the contribution of the coeluting ¹³C mass isotopomer of IPSO₂₅ (M⁺˙ + 2) to the SIM target ion. In contrast, SRM analysis is less reliable for IPSO₂₅ quantification in cases where several regio-isomers are present, likely due to intense double bond migrations following electron impact. In the case of HBI III, SRM analysis constitutes a potentially suitable alternative to SIM analysis, especially in terms of improving limit of detection. Conclusions: Despite the intense migrations of HBI double bonds under electron ionization, the selected SRM transitions should be more suitable than SIM target ions for IP₂₅ and HBI III quantification in complex hydrocarbon fractions of natural samples. However, the advantage is less evident for IPSO₂₅ due to the presence of numerous regio-isomers.