Diode-array scanner

Unhke visual evaluation of a chromatograms before derivatization, which can only give quahtative or semiquantitative results, direct optical evaluation using instruments enables quantitative results to be obtained. For this, a traditional TLC scanner, diode-array scanner or video equipment, either alone or in combination with a flat-bed scanner, is used. Quantitative evaluation with these instruments is described in more detail in Sections 1.2-1 A. However, the limits of this book would be exceeded if we gave a detailed description of all the commercially available equipment that can be used to quantify substances on TLC plates. Training in the use of TLC scanners can be obtained in company seminars (e.g. CAMAG) and detailed instructions are provided by the manufacturer when the equipment is purchased. 

In contrast, diode-array scanners use white light for illumination pmposes. Diode-array scanners register absorption and fluorescence spectra and densitograms at different wavelengths simultaneously from the plate. This kind of scanner was first introduced in 2000 by Spangenberg [84b]. 

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. 

Fig. 4 J M 2010 TLC Diode Array Scanner (TIDAS TLC 2010). Source Photo courtesy of SpectrAlliance, Inc., St. Louis, Missouri.

To scan an absorption spectrum directly from a sample spot, a mono-wavelength scanner has to illuminate the plate with light of different wavelengths. Using a diode-array scaimer the absorption spectrum of a TLC-plate spot can be directly extracted from the measured scan. 

Arrayed microlenses are widely used in a variety of applications that involve miniaturized optical components.172 For example, they can be found at the heart of optical communication systems, facsimile machines, laser printers, and many other kinds of digital information storage or processing devices. In all these applications, the arrayed microlenses simply serve as diode laser correctors, fiber-optic couplers or connectors, and optical scanners. In a set of recent publications, Whitesides and coworkers have also demonstrated that arrayed microlenses could be used as a new platform for photolithography, through which submicrometer-sized structures could be conveniently fabricated as patterned arrays by reducing mm to cm scale features on a photomask.157... 

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