Wavelengths Used in Photometry of Protein Fractions

The light source. In many cases a rather primitive light source, such as a long automobile lamp is used, but by chance this happens to compensate partially for the uneven distribution of protein on paper (Fig. 30), as the central and darker zone is better illuminated than the lateral zones (P17, S9 Fig. 31). If even illumination of the slit is obtained, this may cause an error analogous to that of a too wide slit whenever there is irregular protein distribution on the strip. 

The power supply must be constant, whether it consists of batteries or is derived via a voltage stabilizer from the mains failure here is the chief cause of erratic readings. 

To evaluate the separate fractions two methods are available. Vertical dividing lines can be dropped from each trough of the curve to the base line, but this ignores the overlapping of the fractions (which varies since not all the fractions are equally widely separated). However, this method gives easily reproducible results, and since its inherent inaccuracy is not out of proportion to the other inaccuracies of the quantitation, it is commonly used (indeed, it is used for all automatic scanners). The alternative (M9) is to construct, within the framework of the complete 

Distribution of light intensity over the slit illuminated with a commercial bulb (P16). Each abscissa division corresponds to 3 mm. 

Integration of the total area under the pattern is done by planimetry (G17), with an adding machine, or by (P6) cutting the areas and weighing the paper pieces. 

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