Automated particle electrophoresis: Modeling and control of adverse chamber surface properties

Electrophoretic analysis of colloidal particles is adversely affected by a host of surface phenomena, including electroosmosis, phase wall wetting, and sample or air bubble adsorption. Neutral, hydrophilic polymer coatings control such phenomena on a variety of surfaces. Poly(ethylene glycol)-poly(ethylene imine) (PEG-PEI) conjugates significantly reduce electroosmosis and positively control adsorption and wetting in the glass sample chambers (5 mm x 3 mm x 1 mm i.d.) employed in a representative commercial electrophoresis apparatus (Coulter DELSA 440). The reduction in electroosmosis (e.g., 80% in 7.5 mM solution at pH II) was similar to that exhibited by coated 2-mm-i.d. quartz capillaries in a Rank MK I manual apparatus, PEG-PEI coatings significantly reduce electroosmosis over a wide range of pH (2-11) and ionic strength (1-100 mM) and can be stable for weeks under normal laboratory conditions, They greatly enhance ease of operation and accuracy (sample mean electrophoretic mobility +/- SD) of the DELSA 440. The latter results from reduced electroosmosis now profile gradients near the chamber center-axis stationary levels, where particle mobility is typically measured. Such flow profiles may also be affected by chamber wall surface asymmetries. A hydrodynamic description of electroosmotic fluid now in rectangular chambers was adapted in order to analyze the propagation of errors due to both nonideal focusing and chamber surface asymmetry. The analysis indicated that the accuracy of rectangular chambered devices may be improved by measuring particle mobility at stationary levels different than chamber center axes. As a result, some rectangular chambers may confer accuracy advantages over cylindrical chambers.