RuBP can be measured isotopically via the conversion of  and RuBP into glyceraldehyde 3-phosphate. G3P can then be measured using an enzymatic optical assay. Given the abundance of RuBP in biological samples, an added difficulty is distinguishing particular reservoirs of the substrate, such as the RuBP internal to a chloroplast vs external. One approach to resolving this is by subtractive inference, or measuring the total RuBP of a system, removing a reservoir (e.g. by centrifugation), re-measuring the total RuBP, and using the difference to infer the concentration in the given repository.