Pathlength variations

Poor reproducibihty of NIR spectra caused by, for example, pathlength variations... 

This study demonstrates the feasibility of the DIT-IR phase studies method using a near-IR microscope apparatus. Reasonable results were obtained on the C oPO/water model system, even using a mld-IR spectrometer which was not optimized for work in the near-IR region. In future work, a near-infrared FT-IR spectrometer with a CaFj beam splitter and a microscope with a small-area InSb detector will be utilized. Precise DIT-NDX measurements of the cell-chamber thickness will also be used to correct all the DIT-IR results for cell pathlength variations along the length of the cell. Such a correction was not performed on the data presented here. Very... 

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. 

The varying distances between two pathlengths result in a sequence of constructive and destructive interferences and hence variations in intensities an inter-ferogram. Fourier transformation converts this interfer-ogram from the time domain into one spectral point on the more familiar form of the frequency domain. 

The original powder SHG method is due to S. Kurtz and T. T. Perry (13). There have been variations and improvements reported since that work(14). In the basic method, a fine powder is loaded into a holder such as a cuvette. The pathlength of the cell should be much larger than the particle size to ensure sampling of many particles. The particles may be immersed in an index matching fluid. Without index matching fluid,... 

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. 

Figure 1.4 Transmittance variation with optical pathlength for three sample concentrations.

Figure 9.3 shows the variation in OD for fluorescein at 495 nm as a function of fluorophore concentration. We obtain a linear plot indicating that the Beer-Lambert-Bouguer law is respected in this range of fluorescein concentrations. Since, in the present measurements optical densities were measured with a pathlength equal to 1 cm, we obtained at 495 nm from the slope of the plot, a value equal to 42 mM-1 cm-1 for s. This result, identical to... 

These two equations are interconvertible and are known as a Fourier-transform pair. The first shows the variation in power density as a function of difference in pathlength, which is an interference pattern. The second shows the variation in intensity as a function of wavenumber. Each can be converted into the other by the mathematical method of Fourier transformation. Do not worry if your knowledge of calculus is not up to these equations. It is not necessary to have a detailed knowledge of the mathematics involved in order to carry out experiments using an FT-IR spectrometer ... 

A pulse ultrasound is measured after transmission dirough a known pathlength of emulsion. The two major variations... 

The geometrical correction term Uji ) accounts for the variation of the scattering volume and the pathlength with detection angle and is given by (Privoznik et al., 1978)... 

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