The QSSA is valid over the respective depletion timescales of the indicator and non-observable reactions when both λ max and ε are sufficiently small. In addition, simple asymptotic solutions to the mass action equations were derived that are valid when the indicator reaction is very fast (or very slow) in comparison to the non-observable reaction. If the indicator reaction is fast, then the time course of the indicator substrate s 2 is accurately approximated by