Optical absorption spectroscopy with multi-pass cell with adjustable optical path length

An optical absorption spectroscopy apparatus comprises a light source, a detector for detecting an optical absorption spectrum of light transmitted from the source through a sample volume and one or more reflectors for reflecting the transmitted light multiple times through the sample volume. An adjuster device is provided for adjusting at least one optical element so as to vary the path length of the transmitted light by controlling the number of times the light is reflected through the sample volume. Drive means is provided for driving the adjuster device, so enabling the detector to detect the transmitted light at a range of different path lengths.