Ultraviolet densitometry

If no references for the degradation products or impurities are available in the laboratory, the sample should be exposed to stress conditions such as heat (50-80 °C), ultraviolet light (2000 lux), acid and base (0.1-1 M HC1 and NaOH), and oxidant (3% H2O2). After incubation in the allotted time, the purity and identity of the analyte peak/spot should be proved by using DAD or MS detection (for LC), MS (for GC), or in situ UV-Vis measurement using a densitometry or TLC-MALDI MS (for TLC). 

Sherma, J. Incarvito, CD. Analysis of tablets and caplets containing ketoprofen by normal- and reversed-phase HPTLC with ultraviolet absorption densitometry on preadsorbent plates. Acta Chromatogr. 1997, 7, 124—128. 

DiGregorio, D. Sherma, J. Analysis of the active ingredient magnesium salicylate in analgesic tablets by HPTLC with ultraviolet absorption densitometry. J. Planar Chromatogr.-Mod. TLC 1999,12, 230-232. 

The following solution is used for staining 0.5% acetic acid, 0.1% pyronine Y and 1 mM citric acid. The gels are stained overnight and destained by washing in 10% acetic acid. Formamide does not interfere with densitometry in the ultraviolet at 280 nm. 

TLC/scanning densitometry is a useful tool for the identification of the target compounds on a TLC plate, because the combined methods can separate and then directly measure ultraviolet-visible absorption spectra of the compounds without the laborious and time-consuming procedures described above. In this paper, we deal with the identification of synthetic and natural colors in foods using TLC/scanning densitometry. 

Naidu, B. V. K., MaUikarjuna, N. N., and Aminabhavi, T. M. 2004. Blend compatibility studies of polystyrene/poly(methyl methacrylate) and polystyrene-acrylonitrile by densitometry, viscometry, refractometry, ultraviolet absorbance, and fluorescence techniques at 30°C. Journal of Applied Polymer Science 94 2548-2550. 

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. 

The eluates from sample and standard spots can be analyzed by any convenient and appropriate procedure, for example, titration, electroanalysis, gas chromatography, or spectrophotometry. Ultraviolet, visible, or fluorescence spectrophotometry is most often the method of choice however, densitometry can he carried out directly on the plate in these modes, and the compound will usually be more concentrated on the chromatogram than it will be in solution in a cuvette. In addition, the scraping and elution approach is time-consuming and labor-intensive and subject to losses of material—for example, by irreversible adsorption. The extraction method has been shown to have a reproducibihty that is markedly poorer than with the use of a modem scanning densitometer (Matysik et al., 1994). 

DENSITOMETRY. Quantification of a zone directly on the layer with an instrument that measures color, absorption of ultraviolet light, fluorescence, or radioactivity. 

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