Wedged-tip technique

Wedged-Tip Chromatography Reindel et al (1953) and Mathias (1954) introduced the wedged-tip technique in TLC which essentially exhibit the following two plus points, namely ... 

Figure 28.5, illustrates TLC of an urine sample by the normal TLC-technique vis-a-vis the wedged-tip technique (Figure 28.5(Z>)). One may clearly visualize the beautiful separated bands in the latter as compared to the several odd and irregular-shaped spots in the former. Both the clarity of separation and the reproducibility of the results are predominant in the latter technique. Figure 28.5 (a) and (b) represent the typical analysis of a urine sample containing glucose, arabinose, lactose and fructose respectively.

Figure 28.5 (a) TLC-Normal technique 28.5 (b) TLC-Wedged-Tip Technique 1 = Glucose (Blue-Violet) 2 = Arabinose (Blue) 3 = Lactose (violet) 4 = Fructose (Red) ... 

Tschesche and co-workers recommend the wedged-tip technique (p. 89) for special separation problems. Should this prove inadequate, descending chromatography can be tried. G5ldel and co-workers [76] have applied this method in cases where hi /-values were too similar Khorlin and co-workers [127,128] and Coleman and Parke [33] have preferred two-dimensional TLC on alumina layers. The last named developed with acetone-chloroform (50 + 50) in the first direction and methanol-33% ammonium hydroxide (80 + 20) in the second. 

Fig. d. Separation and detection of various tocopherols [79]. Solvent chloroform. 7 a- 2 /8- 3 y- 4 d-T (30 pig of each) 5 concentrate of naturally occurring vitamin E (100 M-g). Spray-detection A Dipyridyl-iron reagent B + C SbCh reagent. Technique A + B one-dimensional C wedge-tip... 

Solids are usually ground with a material such as potassium bromide and compressed into a pellet. Moisture must be absent, and the transparent disk is placed in the window of the spectrometer. In general, however, measurement of intensity of absorption in the solid phase is unreliable due to scattering and reflection losses, and a uniform distribution of sample in the pellet is difficult to achieve. One method of handling solutions is to allow them to soak into a KBr wedge, evaporate the solvent, and compress the tip into a microdisk or pellet (C26) alternatively the sample (1 /tl) may be either placed directly on the KBr disk (B13) or mixed with a little KBr powder and subsequently incorporated in the disk. These microdisk techniques can be used for the examination of gas chromatograph effluent. For multiple analyses, an enclosed turntable system loaded with the disks can be used. 

A third group of calibration procedures, the (3) direct (single step) [23-25] techniques avoids (many of) these problems. In particular, the so-called improved wedge-calibration method, in which a tip/cantilever is scanned across a calibration sample with two well-defined slopes (see Fig. 2.31), allows one to calculate the calibration factors with an error of ca. 5% [18]. 

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In RTFs craze zones can develop that are important microscopic features around a crack tip governing strength behavior. Plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction techniques, scanning electron microscope, and transmission electron microscopy. 

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