Detection of HF is important since it is highly toxic and used in large quantities in industrial environments, in particular in the semiconductor and petroleum refining industries. Electrochemical methods have generally been used up to now for HF sensing; however, a major problem is selectivity, i.e., it is not possible to differentiate HF and HCl with electrochemical sensors. These authors report a new technique for HF detection that utilizes a porous silicon interferometer. The method is cheap and is highly selective for HF relative to common interferants.