The electron impact mass spectrometric fragmentation patterns of the per-0-trimethylsilyl (TMS) and alkoxime-TMS derivatives of D-glyCerO-D-UftrO and D-glycero-D-ido-octulo~ were fully analysed using seven specifically labelled 13C analogues. Many of the more intense ions in the spectra of the per&-TMS furanose and pyranose derivatives have more than one origin. On the other hand, the mass spectra of the straight-chain methoxime and ethoxime hepta-0-TMS derivatives contain ions each of which in general originates from a single cleavage process. This makes gas chromatography/mass spectrometry of the alkoxime-TMS derivatives a suitable method for monitoring incorporations at individual carbon atoms in the corresponding octulose phosphates during photosynthesis experiments with "CO, . Synthesis of 3C-labelled sugar phosphatesLabelled hexose &phosphates. [ 1-' 3C]Glucose 6phosphate and [l-' 3C]allose 6-phosphate were prepared as described previou~ly.~ The synthesis of [3-13C]-,

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