Forming reproducible density and solute gradients by computer‐controlled cooperation of stepmotor‐driven burettes

A program has been developed for the computer‐controlled cooperation of 4 commercially available stepmotor‐driven burettes to form density gradients in polyacrylamide gels and solute gradients for other purposes such as centrifugation and low pressure chromatography. The slope of the gradients was defined by entering up to 200 endpoints of straight lines with the percentage of the total volume and the concentration of the gradient‐forming solute as coordinates. The possible minimum increment of the discontinuous gradients is 0.5 % of the total range. They are mixed at variable flow rate by two burettes containing the high and low density acrylamide solutions and two additional burettes for the admixture of two catalysts at variable volume proportions. Within 10 min gradients with total volumes between 4 and 650 ml can be delivered. Gradient volumes smaller than 4 ml are possible if an average increment of > 0.5 % is accepted. The program meets the requirements of reproducibility for high resolution gradient gel electrophoresis and isoelectric focusing in immobilized pH gradients. All input data for the definition of the gradients can be displayed as a hard copy by a matrix printer and stored on tape. Thus the device meets the requirements of proper documentation of a given gradient in a publication and for reproduction in another laboratory. Other types of gradients for volumes of up to 1.4 liters can be produced by a subprogram converting the 4 burettes into a gradient pump with continuous outflow as used for column chromatography. The speed is variable between 0.4 ml and 5.2 liters per hour with minor pulsation only in the very low range.