Optimum pathlength

When the entire spectmm is used for the calibration, the optimum pathlength is obtained if the mean absorption approximates 0.434 rather than any individual absorbance reading. 

FIGURE 37.10 Optimum pathlength in ideal one-wavelength concentration measurement. 

The optimum thickness for measuring the spectrum of any solute depends on the absorptivity of the solvent It is well known from many textbooks on instrumental analysis that the optimum pathlength of a cell should be such that the transmittance of the solvent should be about He, where e is the base of Naperian logarithms. Thus, the optimum absorbance, Aopt, of the solvent should be logioc, or 0.4... 

The routine use of VCD spectroscopy is limited to liquid solutions. Vibrational circular dichroism is an intrinsically weak phenomenon (g values are very small) and its measurement requires optimum experimental conditions, in addition to state-of-the-art instrumentation. In general, VCD spectra are measured at fairly high concentrations—in the range 0.01-1.0M—in solvents with good mid-IR transmission at fairly short pathlengths (< 1 mm). The accessibility of such conditions depends on the solubilities of the molecules to be studied in available solvents. Compounds only soluble to significant extent in water are generally not easily studied. 

There is an optimum angle, as indicated by wave optics. For example, the effect of spherical aberration of the objective is to distort the wave leaving the specimen from the spherical shape produced by a perfect lens. This vmwanted extra pathlength PD inserted into the wave is given by... 

In order to keep sufficient absorption intensity and to detect VCD corresponding to a difference on the order of 10 " to 10 of the peak intensity, it is known empirically that the optimum absorbance of a band is around 0.4. The absorbance should never exceed 1.0 and not fall below 0.1. To obtain a high-quality VCD spectmm, it is most important to prepare the sample at an appropriate optimum concentration and to use a cell of appropriate pathlength. 

If the polymeric sample being examined is actually a sufficiently thin film, then it may be introduced directly into the sample compartment with no further preparation and examined by infrared transmission techniques, for example. It must be borne in mind that, with films examined by straight transmission, an interference pattern is often observed superimposed on the actual spectrum of the sample. Just as with low molecular weight organic substances, the variation in the band intensities observed for different functional groups means that, in order to obtain the optimum spectrum, the pathlength may differ significantly from sample to sample. The thickness of the film may need to be adjusted in order to obtain the best spectrum. 

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