As a result of the scaling analysis (33a)-(33b), it is clear that the QSSA should hold on the t s1 timescale if t c2 /t s1 1 . The timescale t c2 has no significant physical interpretation with regards to experimental initial conditions: it is simply occurs as a result of the scaling analysis, and we have chosen to introduce it as a means of writing the left hand side of (35) more compactly. Of course, it can be argued that if t c2 is short, then little change will occur in the concentrations of c 2 or s 2 over this timescale. Numerical results confirm that t c2 /t s1 1 supports the validity of the QSSA (see Figs.