In our previous work (Yang et al. Anal. Chem. 2018, 90 (5), 3307-3312), we reported that a large diameter Nafion dryer can be used in the PERCA (PEroxy Radical Chemical Amplification) technique to minimize the impact of water vapor on the chain length (CL). By using a Nafion dryer, the sample was first dried to a low relative humidity (RH) and was then drawn into a FEP (fluorinated ethylene propylene) flow reactor tube to start the amplification cycles for peroxy radical measurement. This method provides a promising and simple method to minimize the sensitivity of CL to water vapor. However, there is a trade-off between inlet radical losses and moisture removal efficiency under high ambient RH conditions. In this paper, we report a further improvement by removing the inlet Nafion dryer and using it directly as an amplifier. The sample was drawn into the Nafion dryer directly without preconditioning to reduce inlet losses; the amplification and HO2 reformation cycles were started in the Nafion dryer. The CL value at 3% RHintel was about 150. A linear relationship between CL and RHintel up to 87% RH was observed. Low loss of the CL under high RHintel (about 10% reduction at 87% RH) was achieved. As a result, the modified system can largely remove the uncertainty of the CL arising from water vapor under most ambient conditions.