Abstract: A liquid chromatograph includes a multi piston reciprocating pump for delivering liquid at a desired flow rate to a separating column. The pistons in the pump have overlapping delivery strokes and when the pump is provided with two pistons their rate of advance is halved during the overlapping delivery periods. The instant when both pistons start delivery is dependent on solvent compressibility and the delivery pressure of the pump and thus will vary according to the operating conditions. The delivery pressure of the pump is monitored and if a pressure increase (or decrease) is found during one pump cycle between the start of stroke of one piston (SST1) and the end of delivery of the other piston (EOD2) the instant when the speed of advance of the pistons is halved (HS1) is advanced (or retarded) in the next pump cycle.

Abstract: A dual-beam ratio recording spectrophotometer has signal processing circuitry for determining the transmittance of a substrate. The signal processing circuitry has a bandwidth which is dependent on the magnitude of the radiation passing through a reference cell and includes control logic circuitry which produces a pulse whose width is proportional to the square of the magnitude of the radiation detected when the beam passes through the reference cell. A sample and hold circuit whose sample time is controlled by this pulse samples the output of an integrator which produces at regular intervals a signal representative of the transmittance of a substance. Thus, the response time of the sample and hold circuit and hence the bandwidth of the signal processing circuitry depends on the magnitude of the radiation passing through the reference cell.

Abstract: A dual beam spectrophotometer includes signal processing circuitry for producing sample-dark and reference-dark signals. The dark signals substracted from the sample and reference signals are the average of the dark signals produced on each side of the sample and reference pulses. The signal processing circuitry may comprise four sample and hold circuits which store the sample signal, reference signal and first and second dark signals, a resistive combining network for the two dark signals and first and second substractor circuits. The first substractor circuit provides the sample-dark signal at its output while the second substractor circuit provides the reference-dark signal at its output.