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 with a perforated impaction plate. As a result, some desired fraction of the gas going into, or coming out of the analyzer can be controlled independently from the flow of sample through the ionization chamber. Furthermore, penetration into the ionization chamber of the desired fraction of the gas is minimized by controlling the dimensions of the perforated impaction plate. Ions formed in the ionization chamber are driven partly by electric fields through the perforated impaction plate into the analyzer. As a result, most of the gas sampled into the analyzer carries ionized vapors, even when the sample flow is very small, and even when the analyzer uses counterflow gas.