Abstract: An electronic circuit is used to continuously determine the nitroglycerin ncentration in a sample solution of an electrochemical monitoring cell by controlling the applied potential to a cathode electrode of a three element cell. The cathode of the electrochemical cell is maintained at a desired potential by supplying a square wave modulated negative potential signal thereto and then measuring the difference in cell current between the cathode and anode elements at the start and finish of the pulse signal with a demodulator. Current from any additional reactions that are constant or that occur above a desired reduction potential within the electrochemical cell are automatically cancelled by a gating circuit which is electrically coupled to the demodulator.

Abstract: An analyzer, for measuring the concentration of a particular material in a ample, has its analog voltage output connected to the analog input of a multiplying digital-to-analog converter (MDAC) whose output is connected to a voltmeter and to one input of a voltage comparator having its other input connected to a potentiometer and battery producing a reference voltage somewhat less than the known output voltage of the sample. The comparator output is connected to two gates which control the transmission of electrical pulses from a generator to the UP or DOWN inputs of a binary UP/DOWN counter, having N digital output channels connected to N digital input channels of the MDAC, in such manner that the counter counts UP, increasing the gain of the MDAC, when the MDAC output voltage is lower than the reference voltage, or counts DOWN, decreasing the MDAC gain, when the MDAC output voltage is higher than the reference voltage.

Abstract: A refractive index detector is described comprising a waveguide, means for contacting said waveguide with a fluid, a light source and means to transmit light into said waveguide, means for detecting light exiting from said waveguide as an indication of the refractive index of said fluid and means to automatically change the angle of incidence of the light entering said waveguide in response to changes of refractive index of said fluid. A preferred embodiment of the detector for use as a detector for gradient elution chromatography has means to indicate the rate of change of refractive index. A preferred means to automatically change the angle of incidence of the light entering the waveguide is a movable prism. It is preferred to use a monochromatic light source in the detector. It is preferred to provide reference means for the detector to compensate for changes in intensity of the light output from said light source.