Quality control of any therapeutic substance is sooner or later bound to become an issue. Certain essential oils are scarce or expensive, so temptation for producers is to add synthetic constituents, or constituents from other plant sources to ‘stretch’ the essential oil and thus increase profits. The cheapest way to assess quality is to train readers' own nose. As well as assessing the odour quality of an essential oil, they can assess two physical measures using simple and inexpensive equipment. A more expensive (but more precise) way of finding out the constituents of a particular essential oil is to have a Gas chromatography–MS analysis done. The most useful type of gas chromatography is where chirality of the constituents is taken into account. Gas chromatography is usually combined with mass spectrometry. Steam, or sometimes a combination of boiling water and steam, is passed through a container of plant material. Cold pressing refers to the process of extracting oils from citrus peel.