A sensor for detecting concentration of a component in a breath sample including a gas sampling cell having an inlet, an outlet, a light source, a first light detector, a sampling channel configured to receive gas samples from the inlet; the sampling channel defining a light path between said light source and said first light detector; and a volume occupying material located within the sampling channel, the volume occupying material having an absorption coefficient for mid-infra-red light below 0.5 cm−1 at room temperature.