Synchronous coefficient of drag alteration  Synchronous coefficient of drag alteration (SCODA) is a biotechnology method for purifying, separating and/or concentrating bio-molecules. SCODA has the ability to separate molecules whose mobility (or drag) can be altered in sync with a driving field. This technique has been primarily used for concentrating and purifying DNA, where DNA mobility changes with an applied electrophoretic field. Electrophoretic SCODA has also been demonstrated with RNA and proteins. As shown below, the SCODA principle applies to any particle driven by a force field in which the particle's mobility is altered in sync with the driving field. For explanatory purposes consider an electrophoretic particle moving (driven) in an electric field. Let:  andformula_2  denote an electric field and the velocity of the particle in such a field. If formula_4 is constant the time average of formula_5. If formula_4 is not constant as a function of time and if formula_4 has a frequency component proportional to formula_8 the time average of formula_9 need not be zero. Consider the following example:  Substituting (3) in (2) and computing the time average, formula_11, we obtain:  Thus, it is possible to have the particle experience a non-zero time average velocity, in other words, a net electrophoretic drift, even when the time average of the applied electric field is zero. Consider a particle under a force field that has a velocity parallel to the field direction and a speed proportional to the square of the magnitude of the electric field (any other non-linearity can be employed):  The effective mobility of the particle (the relationship between small changes in drift velocity formula_14 with respect to small changes in electric field formula_15) can be expressed in Cartesian coordinates as:  Combining (5), (6) and (7) we get:  Further consider the field E is applied in a plane and it rotates counter-clockwise at angular frequency formula_20, such that the field components are:  Substituting (10) and (11) in (8) and (9) and simplifying using trigonometric identities results in a sum of constant terms, sine and cosine, at angular frequency formula_23. The next calculations will be performed such that only the cosine terms at angular frequency formula_23 will yield non-zero net drift velocity - therefore we need only evaluate these terms, which will be abbreviated formula_2 and formula_26.