The Sensitivity and Accuracy of Dual-Wavelength Spectrophotometers

The ultimate noise limit of a double-beam spectrophotometer is determined by the shot-noise in the photocurrent of the detector. The magnitude of the photocurrents to be expected is discussed for instruments using grating and prism monochromators. It is shown that a shot-noise level <10−4 can be obtained with both grating and prism monochromators at a bandpass of 1 mμ. The nonlinear relation between a concentration change of the substance under study and the detected signal is examined.