A method and apparatus are described to increase the efficiency with which a sample vapor is ionized prior to being introduced into an analyzer. Excellent contact between the vapor and the charging agent is achieved in the ionization chamber by separating it from the analyzer by means of a perforated impaction plate. As a result, some desired fraction of the gas going into the analyzer or coming out of the analyzer can be controlled independently from the flow of sample through the ionization chamber. Furthermore, penetration into said ionization chamber of said desired fraction of the gas going into or out of the analyzer is minimized by controlling the dimensions of said perforated impaction plate. Ions formed in the ionization chamber are driven partly by electric fields through said hole in said perforated impaction plate into the inlet to the analyzer. As a result, most of the gas sampled into the analyzer carries ionized vapors, even when the sample flow of vapor is very small, and even when the analyzer uses counterflow gas.