The inlet of a commercial atmospheric pressure chemical ionization—mass spectrometer (APCI/MS/MS) has been modified to transform it into an aerosol mass spectrometer, named TD-API-AMS. The new inlet consists in a charcoal denuder (to trap gas phase VOCs and SVOCs) followed by the thermal-desorption unit of the APCI source. Thermal desorption and APCI were chosen because they avoid sample denaturizing while keeping good time resolution. The objectives of this paper are (1) to describe the simple and reversible modifications of the commercial APCI inlet allowing its use as an aerosol mass spectrometer and (2) to characterize the performances of this modified inlet. These performances are characterized in term of efficiency of (i) gas phase organic compounds removal, (ii) particle transmission, and (iii) particle volatilization in the thermal-desorption unit. The characterization was conduced with secondary organic aerosol (SOA) produced from the ozonolysis of α -pinene and 2-buten-1-ol in a continuous flow reactor. The results show a denuder gas phase trapping efficiency higher than 93 ± 3% while the particle transmission efficiency was nearly 100% in particle number, but decreased to as little as 85% in total particle volume. This result highlights a shift of the particle distribution towards the fine particles occurring through the denuder, due to a modification of the gas-particle equilibrium. The inlets' characterization has also shown a particle volatilization efficiency higher than 90% (in volume).