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A double-beam recording spectrophotometer has been developed for rapidly obtaining spectra of labile intermediates in biochemical reactions. Light from a monochromator is split into two beams by a chopping mirror and then the ratio of light intensities in two optical paths is measured. This ratio is expressed as percent absorption or converted electronically into units of optical density. Results are recorded on a linear wavelength scale at a maximum rate of 6 mμ per second. A servo system corrects the nonlinearity of the wavelength scale of the quartz monochromator. The noise level corresponds to a change of optical density of 10−4 at 400 mμ with a spectral interval of 3 mμ. The over-all accuracy on standard solutions (National Bureau of Standards) is about 2 percent. The air-against-air zero absorption line varies only 0.004 in optical density from 210 to 650 mμ.
Yang et al. (Sun,) studied this question.