[76] Mass spectrometric measurement of photosynthetic and respiratory oxygen exchange

Publisher Summary This chapter focuses on the mass spectrometric measurement of photosynthetic and respiratory oxygen exchange. When photosynthetic organisms are illuminated, net oxygen exchange generally results from the superposition of evolution and uptake processes. To determine the levels of each unidirectional oxygen flux, oxygen isotopes can be used. In this respect, the stable isotope is the most useful, and, therefore, mass spectrometry is the technique of choice to measure the isotopic content. When placed in the presence of 18 O-enriched oxygen, an illuminated photosynthesizing organism takes up all the isotopic species present without significant discrimination. To correct for variations in gas concentration arising from the mass spectrometer gas consumption and from eventual instabilities caused by the inlet system, argon contained in air is taken as an internal reference. The result of the integration is stored in the computer memory and recorded on a floppy disk at the end of the experiment. After calibration the results are expressed as cumulated moles of oxygen exchanged (consumed and produced) as a function of time.