Extraction (cup) techniques For some years sebum excretion was measured by the 'cup' method in which an organic solvent is placed, for variable periods of time, in direct contact with an area of skin such as the  forehead, the  solvent being contained in a circular glass cup held firmly in contact with the skin. The solvent is then pipetted from the cup, evaporated and the quantity  of sebum ascertained by weighing. This technique is still in use. For example, a variant of the original methods entails cleaning the skin surface by wiping with a gauze pad moistened with a 1 per cent solution of nonionic detergent Triton X-100. The sampling site is then defatted using another gauze pad saturated with hexane.

After allowing the surface to dry, the  site is protected by a plastic weighing boat taped onto  the skin by  its edges, the roof of this chamber being perforated to allow evaporation of sweat and prevent a rise in skin  surface temperature. After a three-hour period, the protective covering  is removed and the lipids that have reached the surface are collected by pipetting 2 ml of hexane. Using an internal standard the amount and composition of the lipid can also be analysed.  There are certain  reservations  about this method of collection, the  most important of which is that it physically extracts lipids from both  the surface and the duct so it does not measure secretion excretion precisely.  However, on the forehead at least, the epidermal contribution in adolescents is minimal.