Publisher Summary This chapter discusses the simultaneous measurement of photosynthesis and respiration using isotopes. Concomitant measurement of photosynthesis and respiration is accomplished utilizing isotopic oxygen and carbon dioxide. The technique employs 14 C-labeled CO 2 to assay respiratory carbon dioxide evolution during photosynthesis of algal cells. Because satisfactory radioactive isotopes of oxygen are not available, mass spectrometric analysis using 18 O is required for the oxygen determinations. This technique has the advantage of being applicable to both oxygen and carbon dioxide. The technique relies on different isotopic distributions between the sources of uptake and evolution. All mass spectrometers depend on ionization of a gaseous sample and separation of ionized species on the basis of their mass-to-charge ratio. Ionization is usually effected by an electron beam of about 70 V energy emitted thermionically. In magnetic deflection instruments, the ions are accelerated and collimated by a series of slotted plates held at a negative potential. Detection may be made either by an ion multiplier or by measuring the current generated when the ions impinge on a plate connected to ground.

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