We present a simple technique for determining the absolute fluorine atom concentration in pure CF4 capacitively coupled radio-frequency discharges. It is based on the measurement, by laser-induced fluorescence, of the loss rate kloss of the CF2 radical in the afterglow of a pulsed plasma. We first demonstrate that in our conditions, CF2 is lost only by gas phase recombination with F atoms (with a known rate constant krec) and by recombination at the reactor walls at a rate kwall, independent of the rf power injected. Hence, the total CF2 loss rate, kloss=kwall+krec[F], varies linearly with [F] when the rf power is increased. By recording kloss and the relative variation of the F atom concentration (by optical emission actinometry) as a function of rf power, kwall and [F] can be determined. These measurements of [F] complement previous quantitative measurements of CF and CF2 radicals [Booth et al., J. Appl. Phys. 85, 3097 (1999); and Cunge and Booth, J. Appl. Phys. 85, 3952 (1999)] made in the same reactor for the same plasma conditions.