Measuring a CD spectrum

CD spectroscopy has become relatively straightforward with the modem instrumentation available. In the next section, how to measure a CD spectrum is thus preceded by a description of the instrumentation. A full tmderstanding of CD requires some knowledge of CD theory, this is the subject matter of Section 3, 

Section 4 summarizes the units used for CD. The chapter concludes with a discussion of the CD of biomolecules, with particular reference to spectra/ structure correlations. More detail on most aspects may be found in the alphabetical General references list 

In most commercial CD spectrometers, a photoelastic modulator (in older 

The CD spectrometers described above are single beam instruments. A new double beam instrument from On-Line Instrument Systems (OUS) has recently come on the market, which uses sample and reference beams to enable direct calculation of the CD signal. The instrument utilizes two modulated CPL beams out of phase by 180° which are passed through the sample and detected simultaneously. The absorption of left and right CPL is therefore measured directly. The technique has the advantage that factors such as lamp fluctuations are negated since they affect both beams equally, whereas the CD behaviour is characteristic to each beam. The use of a sample/reference system results in zero drift over time and nearly flat baselines. However, the two beams pass through different parts of the sample and cuvette. This may or may not be important. 

Another requirement of the high energy lamps used in CD machines is that 

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