Visual Semiquantitative Evaluation

All limit value tests performed in accordance with the pharmacopoeias include a semiquantitative assessment of the chromatogram. Here, too, in accordance with GMP, a second person should inspect the plate if no pictorial documents are prepared. 

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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. 

L Comparison of semiquantitative visual and quantitative video evaluation of Fig. 72... 

Polyurethane medical devices such as central venous catheters (CVCs) [139-141] and hemodialysis devices [142-144] require goodhemocompatibihty [145], hi Section 2.6, we know that when a device is implanted in the human body protein adsorption occurs in just a few seconds, hi contact with blood, the protein layer interacts with platelets, which leads to thrombus formation and eventually to device failure. There are several tests available to evaluate hemocompatabihty. Visualization by SEM is one semiquantitative method, which is covered in Section 2.5. More accurate quantitative methods are described below. 

Visual spectroscopy can also be semiquantitative. A simple example is the assessment of swimming pool water quality and its chemical content using well-known color reactions and simple sets of colored concentration reference standards. This evaluation of light or color intensity is called photometry. Visual photometry saw one early use in criminal law with the evaluation of breath alcohol concentrations. The test was based on the ability of breath alcohol to chemically reduce the colored chromate ion to a colorless form in aqueous solutions. As more alcohol vapor was bubbled through a chromate solution it became more bleached. The resulting solution was then visually compared to a calibrated, standard set of diluted chromate solutions to determine the subject s breath alcohol content. The test was soon modified to include instrumental measurement of the decolorizing reaction and is referred to as a form of spectrophotometry. 

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