Abstract: The invention relates to a method to clean air for separating materials in the form of particles and/or droplets from a gas flow. The gas flow is directed through a collection in which the outer walls are grounded, and in which high voltage is supplied to the ion yield tips arranged in the collection chamber. Thus an ion beam from the ion yield tips towards collection surfaces is established to separate desired material from the gas flow. The electrically conductive collection surfaces are electrically insulated from the outer castings and high voltage is supplied to the collecting surfaces, in which the direct-current voltage has an opposite sign as the high voltage directed to the ion yield tips. The collection surface is totally conveniently and rapidly detached for clean material replacement.