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Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079 Early Methods Revisited These articles are intended to serve ihe readers O ~ T H I JOURNAL S by calling allention lo new developments i n the lheoy, de+, or auailabilily of chemical laboratory instrumenlalion, or by presenling useful insighls and ezplanations of lopics lhal are of praclical imporlance lo those who use, or teach the use of, modern instrumenlation and inslmmenlal techniques. The editor invites correspondence from prospeclive conlributors.
LXV. Multiplex Spectrophotometry Golen W. Ewing Conventional spectraphotometry is s. highly inefficient process. Continuous radiation, typically from an incandeseept source, is formed into a spectrnm by means of some dispersive element, and swept across an exit slit and detector. At any instant, most of the energy from the source is thrown away. This paper will review some techniques for improving the efficient use of radiant energy. The scanning requirements of a spectrophotometer can he expressed in terms of the nnmber of resolution elements (M). For example, an ahsmptian spectrum extending from 200 t o 700 nm covers s. span of 500 nm. If one is satisfied with data points s t intervals of 0.5 nm, he will need 1000 resolution elements. The same value of M might, of course, represent 200-1200 nm with data. a t 1-nm intervals, or 3003300 cm-' with points a t every 3 em-l, and 8 0 on. M = 1000 is not an unressonable requirement, and will be takeken as B representative value in discussing several forms of spectrophotometry. For convenience we will consider linear wavelength dispersion only. The preferred figure of merit for a spectrophotometric technique (and most other mslytioal tools as well) is the signaltolzoise ratio (SIN). "Noise" in thepresent context can be defined as any portion of a. current, a. voltage, a luminous flux, or other signal-carrying physical quantity which does not carrv information. Hence
Figure 1. Simplest optical scheme for multiplex ,pectrophotometry,
