The invention relates to a method for chemical analysts, in which a gas flow is ionized, the ionized gas flow is led to a filtering area fitted to the flow channel, the ionized gas flow is filtered using the DMS/FAIMS method, in order to remove at least some of the ions from the gas flow. A parallel mainly non-ionized gas flow, which is on at least one side of the ionized gas flow, is led to the filtering area together with the ionized gas flow. The invention also relates to a corresponding structure.