I analysed Ulva, Sargassum and Glacilaria for fatty acids, is that impossible to observe C4:O in those sample? I observed it, but literature said it is not common. I run RESTEK FAME mix (cat 35077) in GC_FID. Have you any literature?
the GC_FID system will report the detected fatty acid from C4:O to C24:O but the methylated FAME have very reduced concentration of C4:O, C6:O, C8:O, C10:O. this emanse you will not get concentration or observation of C4:O in GC_FID results. because the C4:O are probability esteriods and the GC will not detect as FAs.