Use of Gas Chromatography-Mass Spectrometry Techniques (GC-MS, GC-MS/MS and GC-QTOF) for the Characterization of Photooxidation and Autoxidation Products of Lipids of Autotrophic Organisms in Environmental Samples

Abstract

This paper reviews applications of gas chromatography-mass spectrometry techniques for the characterization of photooxidation and autoxidation products of lipids of senescent phototrophic organisms. Particular attention is given to: (i) the selection of oxidation products that are sufficiently stable under environmental conditions and specific to each lipid class and degradation route; (ii) the description of electron ionization mass fragmentation of trimethylsilyl derivatives of these compounds; and (iii) the use of specific fragment ions for monitoring the oxidation of the main unsaturated lipid components of phototrophs. The techniques best geared for this task were gas chromatography-quadrupole-time of flight to monitor fragment ions with very high resolution and accuracy, and gas chromatography-tandem mass spectrometry to monitor very selective transitions in multiple reaction monitoring mode. The extent of the degradation processes can only be estimated if the oxidation products are unaffected by fast secondary oxidation reactions, as it is notably the case of ∆⁵-sterols, monounsaturated fatty acids, chlorophyll phytyl side-chain, and di- and triterpenoids. In contrast, the primary degradation products of highly branched isoprenoid alkenes possessing more than one trisubstituted double bond, alkenones, carotenoids and polyunsaturated fatty acids, appear to be too unstable with respect to secondary oxidation or other reactions to serve for quantification in environmental samples.

Keywords: EI fragmentation; TMS derivatives; autoxidation; environment; gas chromatography-mass spectrometry; photooxidation; senescent phototrophs; specific tracers; unsaturated lipids.

Scheme 1

Photooxidation and autoxidation of chlorophyll phytyl side-chain.

Scheme 2

Main EI mass fragmentations of TMS derivatives of phytyldiol, 3,7,11,15-tetramethylhexadec-3-en(Z/E)-1,2-diols and 3,7,11,15-tetramethyl-hexadec-2-en(Z/E)-1,4-diols.

Figure 1

Partial TOF ion chromatograms (m/z 353.3235 and 245.1388) showing the presence of TMS derivatives of phytol and its main photooxidation and autoxidation products in senescent cells of the diatom Thalassiosira sp.

Scheme 3

Photooxidation and autoxidation of ∆⁵-sterols.

Scheme 4

Proposed formation pathways of the fragment ion [M − 143.0887]⁺ in TOF mass spectra of ∆⁴-stera-3β,6β-diol and 3β,5α,6β-trihydroxysterol TMS derivatives.

Figure 2

Partial TOF ion chromatogram (m/z 431.3710 and 486.4260) showing the presence of TMS derivatives of 24-ethylcholest-5-en-3β-ol (sitosterol) ([M]^+• = 486.4260) and its photo- ([M − 143.0887]⁺ = 431.3710) and autoxidation ([M − H₂O − 143.0887]⁺ = 431.3710) products in senescent leaves of Smilax aspera.

Scheme 5

Type II photosensitized oxidation (induced by PAR and UV radiations) and autoxidation of MUFAs.

Scheme 6

Examples of EI fragmentations of TMS derivatives of MUFA oxidation products.

Figure 3

Partial TOF ion chromatograms showing TMS derivatives of MUFA oxidation products in senescent cells of the haptophyte Emiliania huxleyi irradiated by visible light (A), and after aging (B), and of the diatom Thalassiosira sp. irradiated by (visible + UV) light (C).

Figure 4

Low energy collision-induced dissociation (CID)-MS/MS (5 eV) of fragment ions at m/z 227 and 329.

Figure 5

MRM chromatogram (m/z 199 → 129, m/z 213 → 129, m/z 329 → 149 and m/z 343 → 163) showing the presence of TMS derivatives of palmitoleic acid (C_16:1ω9) oxidation products in senescent cells of Thalassiosira sp. irradiated by sunlight.

Scheme 7

Photooxidation and autoxidation of lupanes.

Scheme 8

Main EI fragmentations of the TMS derivatives of lup-20(30)-ene-3β,28,29-triol and lupan-20-one-3β,28-diol.

Figure 6

Partial TOF ion chromatograms (m/z 395.3305, 498.3908 and 500.4053) showing the presence of autoxidation products of lupeol and betulin in higher plant debris collected in the Rhône River.

Scheme 9

Autoxidation of α- and β-amyrins.

Scheme 10

Main EI mass fragmentation of TMS derivatives of 11-oxo-amyrins.

Figure 7

Partial TOF ion chromatogram (m/z 512.4063, 383.3308, 273.2213 and 232.1822) (A) and MRM chromatogram (m/z 273 → 135 and m/z 232 → 217) (B) showing the presence of oxidation products of amyrins in senescent leaves of Quercus ilex.

Scheme 11

Autoxidation of dehydroabietic acid.

Scheme 12

Main EI mass fragmentations of TMS derivatives of 7α/β-hydroxydehydroabietic acids.

Figure 8

Partial MRM chromatogram (m/z 234 → 191 and m/z 252 → 237) showing the presence of autoxidation products of dehydroabietic acid in senescent needles of Pinus halepensis.

Scheme 13

Type II photosensitized oxidation and autoxidation of HBI III.

Scheme 14

Main EI mass fragmentations of TMS derivatives of 9- and 7-alcohols resulting from oxidation of HBI III.

Figure 9

Partial MRM chromatograms (m/z 213 → 123, m/z 213 → 143, m/z 321 → 181 and m/z 295 → 183) showing the presence of oxidation products of HBI III in diatoms collected in Commonwealth Bay (Antarctic).

Scheme 15

Reaction of ¹O₂ with β-carotene.