## 2.31.1. General Form for Calculating 13C and 14C Flux[¶](#general-form-for-calculating-13c-and-14c-flux "Permalink to this headline")
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In general, the flux of 13C and corresponding to a given flux of total C (\\({CF}\_{13C}\\) and \\({CF}\_{totC}\\), respectively) is determined by \\({CF}\_{totC}\\), the masses of 13C and total C in the upstream pools (\\({CS}\_{13C\\\_up}\\) and \\({CS}\_{totC\\\_up}\\), respectively, i.e. the pools _from which_ the fluxes of 13C and total C originate), and a fractionation factor, \\({f}\_{frac}\\):

(2.31.1)[¶](#equation-zeqnnum629812 "Permalink to this equation")\\\[\\begin{split}CF\_{13C} =\\left\\{\\begin{array}{l} {CF\_{totC} \\frac{CS\_{13C\\\_ up} }{CS\_{totC\\\_ up} } f\_{frac} \\qquad {\\rm for\\; }CS\_{totC} \\ne 0} \\\\ {0\\qquad {\\rm for\\; }CS\_{totC} =0} \\end{array}\\right\\}\\end{split}\\\]

If the \\({f}\_{frac}\\) = 1.0 (no fractionation), then the fluxes \\({CF}\_{13C}\\) and \\({CF}\_{totC}\\) will be in simple proportion to the masses \\({CS}\_{13C\\\_up}\\) and \\({CS}\_{totC\\\_up}\\). Values of \\({f}\_{frac} < 1.0\\) indicate a discrimination against the heavier isotope (13C) in the flux-generating process, while \\({f}\_{frac}\\) \\(>\\) 1.0 would indicate a preference for the heavier isotope. Currently, in all cases where Eq. is used to calculate a 13C flux, \\({f}\_{frac}\\) is set to 1.0.

For 14C, no fractionation is used in either the initial photosynthetic step, nor in subsequent fluxes from upstream to downstream pools; as discussed below, this is because observations of 14 C are typically described in units that implicitly correct out the fractionation of 14C by referencing them to 13C ratios.

