Comparison of Parallel With Transverse Measurement

This measurement has already been mentioned in Section 5.2.3 Direct Optical Evaluation below 400 nm , as the evaluation is performed without derivatization. However, the importance of this example hes in the quantitative results of the measmements of trimethoprim and sulfamethoxazole (both being pharmaceutically active), so that it seems more appropriate to discuss it here. 

The preparation and measmements on the TLC plate and the design of the documents were carried out by Gerda Morlock. Especially for this comparative measme-ment, 16 samples (l-pl each) of the same solution were applied to an HPTLC plate and chromatographed. The plate was then scanned once in the usual way along each lane from left to right (Fig. 96a), and it was then measured a second time in a tranverse direction, i.e. scanning all the trimethoprim samples in the first pass and all the sulfamethoxazole samples in the second (Fig. 96b). The coefficients of variation of the calculated mean values of each of the 16 peak values and area values of both series of measurements are given in Table 20. The printouts of the parallel measmements are shown here in size-reduced form as Document 3. 

Professor Ebel (Wurzburg) often introduces his lectures on quantitative TLC by presenting the outcome of this comparison as follows  

This may be roughly translated as If you measure transversely, you measure rubbish  

Mean value height Parallel measurement Transverse measurement 

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