Densitometry, transmission

The theoretical basis of thin-layer densitometry by transmission is the same as that for absorption in solution. The medium provides the main differences between the two processes. For example, in solution the effects of reflected and scattered light are at a minimum, whereas such effects are significant with samples adsorbed on layers. Also, the distribution of a compound in solution is homogeneous, while that of a spot on an adsorbent is not, neither vertically nor horizontally in the layer. Shibata [42] described the phenomenon of light striking a translucent material as... 

Unlike densitometric determinations where absorption measurements are made by transmission through the surface of the thin layer, measurements of densitometry by reflectance are based on the diffusely reflected light from the surface of the layer. Thus, the light source and phototube are mounted on the same side of the absorbing surface. 

Densitometry can work as transmission measurements, also called forward light measurements. It requires a low coefficient of scatter of the medium. It can also work as reflection measurements also called back scatter determination. 

Densitometry is the mode for quantitative detection optical measurement of a layer is difficult. Three types of measurements are in common use transmission, reflection, and simultaneous transmission/reflection. The reflection mode is most popular. 

Meyers, P.V., Van Sciver, W.J., and Richards, A.E 1974. Theory of nuclear transmission densitometry applied to sedimentology and geotechnology. Journal of Sedimentary Petrology, 44(4) 1010-1015. 

Fast and non-contact methods for the analysis of morphologies evolution during a fast process are highly attractive and, from this point of view, Ught transmission appears the more promising. In contrast to other methods (calorimetry. X-ray diffraction, densitometry), in fact, measurements of light intensity are very fast, economical, and can be applied in situations (rapid cooling, flow) when other methods are not adequate. 

Densitometry was first used to directly measure the amount of light absorbed by radioactive spots on films or stained protein bands on electrophoresis gels in the transmission mode. TLC slit scanners were then developed to directly measure the diffuse reflectance of colored, ultraviolet (UV) absorbing, and fluorescent zones on a layer, followed by videodensitometers for documentation and quantification of layer images. Most recendy, a commercial diode array scanner and office flatbed scanners (as purchased or modified) have been used for quantitative TLC. 

Densitometry is the instrumental measurement of visible or UV absorbance, fluorescence, or fluorescence quenching directly on the thin layer. Measurements are made either through the plate (transmission), by reflection from the plate, or by reflection and transmission simultaneously, and either single-beam, doublebeam, or single-beam-dual wavelength operation of scanning instruments has been used. 

Brain gangliosldes Densitometry Complex sample preparation use of HPTLC plates chloroform-methanol-0.22% CaCl2 (55 45 10) solvent system detection by spraying with resorcinol-hydrochloric acid reagent chromatogram scanned at 580 nm in transmission mode separation and quantification of 4-8 brain gangliosides 133... 

Stained paper and thin-layer electrophoretograms can be evaluated in situ by transmission or reflection densitometry or fluorimetry. Separated substances or the stains they bind can also be eluted and photometry performed in the eluate. 

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