An analyzer with a combustion furnace includes a flow path of byproducts of combustion coupled to a bidirectional ballast chamber by valves which are sequentially actuated for alternately filling and exhausting byproducts of combustion from opposite sides of the chamber during combustion. Alternately, a plurality of low volume ballast chambers are employed. A method of determining the concentration of elements in a sample includes the steps of combusting a sample; and alternately collecting and exhausting the byproduct gases of combustion in opposite sides of a bidirectional ballast. The bidirectional ballast chamber has an outer wall defining a chamber with sealed enclosures at opposite ends of the wall, a movable piston positioned within the chamber, and gas ports associated with the chamber on opposite sides of the piston.