Analytical equilibrium gradient methods

Abstract

Analytical equilibrium gradient methods are non-linear separation methods in which the separation mechanism involves a force gradient along the separation channel. These methods can be classified into two categories: those in which the gradient is a field gradient applied along the separation channel (i.e., field gradient), and those in which the channel is subjected to a constant field with a gradient formed in some other property (i.e., constant field). Standard deviation of peak width, resolution and peak capacity are important parameters in characterizing equilibrium gradient methods, and general expressions can be obtained from considering both the point of force acting on the analyte and the basic flux equation. Several successful examples, such as density gradient sedimentation, isoelectric focusing and electromobility focusing are discussed. Based on equilibrium gradient methods in the field gradient category, a method to dynamically improve peak capacity is described. An example of such an approach is given using electromobility focusing.

Characterization of voltage degradation in dynamic field gradient focusing

“…Figure 1B illustrates a stair-stepped profile containing stacking regions (steep sections in the electric field) and resolving regions (shallow sections in the electric field). The advantage of using this type of nonlinear electric field gradient was demonstrated by Wang et al [11]. They were able to show an increase in the peak capacity of almost 30 times as compared to using a linear profile.…”

Assessing the scalability of dynamic field gradient focusing by linear modeling

“…A short time later, Greenlee and Ivory [8] published a linear model for focusing in an electric field gradient generated by a salt gradient, disregarding the effect of focused proteins on the electric field. More recently, Wang et al [13] have repeated the derivation of equations that describe focusing in electric field gradients. They also restricted their model to linear systems where the ionic distributions do not affect the electric field.…”

Microfluidic free-flow electrophoresis for proteomics-on-a-chip

“…due to diffusion is continuously counteracted since species leaving the equilibrium zone become charged again and migrate back to the location of zero charge [38].…”

“…where L is the total length of the pH gradient [38]. Also the peak capacity is theoretically independent of the device dimensions as it can be rewritten to…”

“…Here d(pH) / dx is the pH gradient slope and dµ / d(pH) the mobility slope of the analyte [38]. For the separation results shown in Fig.…”