Stable spectrophotometry of small density changes.

Measurement and control circuits for recording small density changes by use of several photo-tube circuits are given. The minimum drift rates obtained are 1 X 10-6 part per minute with a differential photo-tube circuit, 3 X 10-5 part per minute with a single photo-tube circuit, and 4 X 10-4 part per minute with a photo-multiplier circuit. The differential and single photo-tube circuits have been used to measure density increments of roughly 2 X 10-4 and 1 X 10-with an accuracy of about 5 percent. These spectrophotometers are employed in studies of the kinetics of unstable intermediates in enzyme action. Although slow drift has been rendered negligible by suitable control circuits and stable amplifiers, the theoretical limit of performance is not achieved by this apparatus.