Abstract: A gradient cam is used in an HPLC pump for controlling proportions of HPLC solvents on the low pressure side of the pump. The gradient cam is not concentric, and has an upward or draw slope along 190.degree. of its circumference and a downward or thrust slope along 170.degree. of its circumference. In operation, the cam is designed to be used with two followers, located 180.degree. apart, which follow along the cam's gradient. The unique cut of the cam insures a constant suction on the inlet or suction side of the cam during the entire pump cycle. During the 20.degree. period of the cycle in which both followers are simultaneously on the 190.degree. suction ridge, the slope of that portion of the gradient is one half of that of the remaining portion of the 190.degree. portion. The gradient cam provides both linear draw and output pressure.

Abstract: A spectrophotometer is shown in which a deuterium lamp emits a plurality of frequencies of interest the beam passes through a filter for selecting the wavelength of the light desired to be incident onto the sample and then onto a beam splitting plate which diverts a relatively small friction of the beam to a first reference photocell. The remaining portion of the beam then is incident directly on a sample tube. The sample tube may be so designed that its transparent walls form a lens focusing the beam on the sample to be tested. The beam then passes through the further wall of the sample tube and is detected by a second testing photocell, the output of which may be compared to the output of the reference photocell to provide a signal indicative of the relative amplitude of the testing beam.