An apparatus may have a first reflector and a second reflector positioned on either side of a sample volume for a gas sample. The configuration of the first reflector may be variable between at least first and second configurations, wherein each of the first and second configurations is arranged such that a beam of optical radiation from an optical beam origin is directed to a detector location via the sample volume. In the second configuration the beam of optical radiation is reflected at least once from each of the first and second reflectors and the path length of the beam of optical radiation through the sample volume is greater than in the first configuration.