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Isomerization of octadecapentaenoic acid (18:5n‐3) in algal lipid samples under derivatization for GC and GC ‐ MS analysis
Author(s) -
Svetashev Vasily I.,
Imbs Andrey B.
Publication year - 2014
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1111/jpy.12162
Subject(s) - derivatization , saponification , methanol , chromatography , base (topology) , gas chromatography , gas chromatography–mass spectrometry , transesterification , fatty acid methyl ester , fatty acid , chemistry , mass spectrometry , organic chemistry , catalysis , biodiesel , mathematical analysis , mathematics
During gas chromatography ( GC ) analysis of fatty acid ( FA ) composition of the dinoflagellate G ymnodinium kowalevskii , we found unex‐pectedly low and irreproducible content of all‐ cis ‐3,6,9,12,15‐octadecapentaenoic acid (18:5n‐3), which is an important chemotaxonomic marker of several classes of microalgae. We compared chromatographic behavior of 18:5n‐3 methyl ester and other GC derivatives obtained using different conventional methods of derivatization. The use of methods based on saponification or base‐catalyzed transesterification resulted in a mixture of double‐bond positional isomers of 18:5. On a SUPELCOWAX 10 column, the equivalent chain length ( ECL ) value for authentic 18:5n‐3 methyl ester was 20.22, whereas the main component after base‐catalyzed methylation had ECL 20.88. Attempts to prepare N ‐acyl pyrrolidides or 4,4‐dimethyloxazoline ( DMOX ) derivatives of 18:5n‐3 also gave inadequate results. These derivatives also showed a main peak corresponding to isomerized 18:5. Mass spectra for both DMOX and pyrrolidide derivatives of this compound showed the base peak at m/z 139, probably corresponding to 2,6,9,12,15‐18:5 acid. Of all methods tested for methylation, only derivatization with 5% HCl or 1% sulphuric acid in methanol gave satisfactory results. Therefore, GC or GC ‐mass spectrometry analyses of algal lipids containing 18:5n‐3 may be inaccurate when base‐catalyzed methods of FA derivatization are applied. The best and simplest way to avoid incorrect GC results is to use standard acid‐catalyzed methylation.