A method is described comprising: providing a first heated volume flow of pressurized gas, a second volume flow of pre-heated dilution gas, and a third volume flow of fluid. A concentrated first aerosol is generated by exhausting at least a part of the first volume flow of pressurized gas and the third volume flow of fluid through a nozzle. A fourth volume flow of gas of opposite direction to the concentrated first aerosol is generated such that the concentrated first aerosol is arrested. Evaporation of the aerosol solvent is augmented with infrared radiation. The arrested first aerosol is then mixed with the second volume flow of dilution gas for generating a second aerosol that has been diluted. This aerosol is then concentrated with a virtual impactor.