The determination of the properties of gaseous films at air/water interface requires very special techniques as far as the measurements of the surface pressure are concerned. We have designed an apparatus based upon the characteristics of Langmuir trough. The measuring system consists of a lamp, three optics fibers and two high-gain phototransistors. The surface pressures that have been measured (accurately with great ease) range from 0.0010-0.1500 ± 0.00005 mN m-1 or ±0.1%. One successful application of the apparatus had been the determination of the molecular weight of proteins such as the bovine serum albumin, egg albumin, bovine rhodopsin, and cytochrome c.
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