Sample pathlength

The amount of rotation observed in a polarimetry experiment depends on the number of optically active molecules encountered by the light beam. This number, in turn, depends on sample concentration and sample pathlength. If the concentration of sample is doubled, the observed rotation doubles. If the concentration is kept constant but the length of the sample tube is doubled, the observed rotation is doubled. It also happens that the amount of rotation depends on the wavelength of the light used. 

To express optical rotations in a meaningful way so that comparisons can be made, we have to choose standard conditions. The specific rotation, [a ]D, of a compound is defined as the observed rotation when light of 589.6 nanometer (nm 1 nm = 10-9 m) wavelength is used with a sample pathlength / of 1 decimeter (dm 1 dm = 10 cm) and a sample concentration C of 1 g/mL. (Light of 589.6 nm, the so-called sodium d line, is the yellow light emitted from common sodium lamps.)... 

Physical correction, n - a type of post processing where the correction made to the numerical value produced by the multivariate model is based on a separate physical measurement of, for example, sample density, sample pathlength, or particulate... 

Spectrophotometer cell, n - an apparatus which allows a liquid sample or gas to flow between two optical surfaces which are separated by a fixed distance, referred to as the sample pathlength, while simultaneously allowing light to pass through the liquid. There are variations of this including variable-pathlength cells, and multi-pass cells, and so on. 

This pre-treatment method1,23 has been effectively used in many NIR diffuse reflectance applications, and in other applications where there are multiplicative variations between sample response profiles. In spectroscopy, such variations can be caused by differences in sample pathlength (or effective pathlength, in the case of reflectance spectroscopy). It is important to note that multiplicative variations cannot be removed by derivatives, mean-centering, or variable-wise scaling. 

To be effective, the light sources used for chiroptical detection systems must have an intensity that is much greater than those ordinarily used in polarimetry. This comes about because the angular rotations observed for the very low analyte concentrations and very short sample pathlengths typical of an analytical liquid chromatograph, are extremely small (mdeg. and less). Conventional light sources have been replaced with laser illumination but these are not without problems, a major one of which is the instability in the emission [23],... 

The magnitude of the optical rotation a (in degrees) is directly proportional to the refractive index difference and to the sample pathlength d, indicative of the fact that rotation is an extensive property, as shown in Eq. (1) ... 

Figure 23. Comparison of the diphenylcarbazide and UV-visible method for Cr (VI) determination, for water and wastewater samples (pathlength 50 mm).

---