A method for detecting the concentration of one or more gas species by using infrared radiation emitted from one or more sources into a sample cell which is a hollow waveguide with multiple bends collectively greater than 180 degrees in three dimensions, the infrared radiation being quasi-focused into a beam with an angle of incidence between greater than approximately 0° and approximately 10° relative to a longitudinal axis of a first linear segment of the sample cell proximate the source, then detecting two or more signals in which one of the signals is used to compensate for water vapor. The methods can detect gas concentrations down to 1 ppm or less.