A photoelectric spectrophotometer suitable for the measurement of vitamin A by the antimony-trichloride reaction.

The measurement by visual methods of the fleeting blue colour of vitamin A in the antimony-trichloride reaction presents considerable difficulties which can be largely overcome by the use of photoelectric methods with corresponding increase in accuracy. The photoelectric spectrophotometer now described was constructed in I 941 when no comparable instrument of this type was commercially available here. It has been in continuous use in this laboratory ever since a brief mention at a meeting of the Biochemical Society (Thompson, 1942) no description of the apparatus has so far been published and for this reason it is now reported here. EXPERIMENTAL General The instrument is generally applicable to absorption measurements in the visible spectrum. It is of the direct-reading type and uses a monochromator which, for photo-electric spectrophotometry, has many advantages. If the spectral band isolated is sufficiently narrow, the readings obtained are in terms of universally applicable absolute units, so that results can be expressed without reference to standard solutions or calibration curves. Further, the instrument can be used for identification of coloured substances by measurement of the shape of their absorption curves and of the position of the absorption maxima. The direct reading, instead of the null-point, method was chosen, as the instrument