A Micro-Photoelectric Photometer

Abstract

Photoelectric analytical procedures depend for accuracy upon a depth of color in solution sufficient to produce satisfactory deflection of a galvanometer or other recording device. Ideally the volumes of reagents and sample are adjusted to yield 20-80% absorption of the incident light. Since the necessary intensity of color varies with the length of the path of the light beam in the solution, the dimensions of the absorption cell are limiting factors in adapting procedures to microanalysis. Present instruments on the market do not permit an absorption path longer than 20 mm when the volume of solution is limited to 1 cc. An instrument in use at the Babies Hospital during the past 2 1/2 years demonstrates the feasibility of constructing apparatus in which the light beam traverses a path of 50 mm through a tube-cell of narrow bore and only 1 cc capacity. These specifications have made it possible to apply the photoelectric principle to measuring the low intensities of color in the cholesterol micromethod of Schoenheimer and Sperry and have allowed the development of a technic for determining bilirubin in serum when the sample is only 0.1 cc. The sensitivity is great enough to make obligatory the use of color filters of narrow spectral transparency. Since no basically new features are involved in the electrical or optical systems, the instrument is adequately described by the details in Fig. 1. The light source is a 15-candle-power automobile bulb. The ratchets, not shown in the diagram, for centering the position of the lamp must be capable of fine lateral and horizontal adjustments in order that the beam shall traverse the absorption cell and fall on the dispersing lens, M. Energy for the lamp is furnished by a 6-volt storage battery in series with a sliding-contact rheostat of 0.83 ohm resistance.