A modular system for gas analysis has a first gas cell that receives and passes at least a portion of an infrared light beam through at least a portion of the first gas cell. A second gas cell disposed proximal to the first gas cell receives and passes at least a portion of the infrared light beam from the first gas cell through at least a portion of the second gas cell. At least a portion of the first gas cell and at least a portion of the second gas cell define a light path having an effective length. The system includes a means for adjusting the effective length of the light path to vary a property of the infrared light beam. Methods of making a variable effective length light path and methods of making a gas detector are also disclosed.