Following electrophoretic separation of macromolecular samples, quantitative data are most often obtained by scanning the stained band distribution with a microdensitometer in order to obtain a continuous recording of absorbance versus migration distance. In addition to the high resolution of closely spaced bands and the quantitative integration of individual bands, it is helpful to establish band migration distances quite accurately in order to detect changes in mobility (1). Such changes in mobility may indicate alterations in structure or function of the macromolecule (2,3). An accurate measure of migration distance is also needed in electrophoretic methods involving the estimation of the molecular weight of macromolecules (4-7).
The present work arose from the need for accurate measures of migration distances, The optical component of the microdensitometer available to the authors was a microscope. The immediate problem was to record accurately and simultaneously on a strip chart the horizontal distance traversed by the microscope stage and the densitometric trace of the electrophoresis pattern being scanned. Thus a variable-speed stage drive was developed. This device incorporated an electronic means of activating an auxiliary pen on the strip chart at established increments of stage travel. The device also provided for switching between the continuously variable speed control and a locking control in which some convenient and reproducible speed could be preserved.
The cost of the device including materials and fabrication was about $250.