electrokinetic mobility window

An electrokinetic mobility window is the range of electrokinetic mobility values within which charged particles, molecules, or other analytes can be detected, transported, focused, selected, or separated under a defined set of experimental conditions. The term is used in electrophoresis, microfluidics, isotachophoresis, electrokinetic separations, and charge-characterization methods. Its exact meaning is method-dependent, so there is no single universal electrokinetic mobility window that applies to every instrument, electrolyte, or sample.

What is electrokinetic mobility?

Mobility describes the velocity of a species per unit applied electric field and is commonly reported in units derived from m² V⁻¹ s⁻¹. In particle electrophoresis, the experimentally relevant quantity is often electrophoretic mobility, which describes particle motion relative to the surrounding liquid. In microchannels and other electrokinetic systems, however, the observed analyte velocity may also contain a contribution from electroosmotic flow of the liquid itself. In such cases, an effective or net electrokinetic mobility can reflect both electrophoretic and electroosmotic contributions.

This distinction matters because electrokinetic mobility and electrophoretic mobility are not necessarily interchangeable terms. Instrument specifications and experimental papers should therefore be interpreted according to the measurement principle and the mobility definition used by the method.

How is an electrokinetic mobility window defined?

The window is bounded by lower and upper mobility limits that are operationally meaningful for a particular system. Depending on the technique, those limits may define which analytes can reach a focusing position, migrate between leading and terminating electrolytes, remain within a selected fraction, be resolved from neighboring species, or fall within the reliable measurement range of a detector.

The position and width of the window can depend on electric-field strength, pH, ionic strength, electrolyte composition, viscosity, permittivity, electroosmotic flow, channel geometry, temperature, and surface properties. A reported mobility window should therefore always be linked to the conditions under which it was established.

Relationship to zeta potential and particle characterization

For dispersed particles, measured electrophoretic mobility can be converted to zeta potential using an appropriate electrokinetic model. That conversion depends on properties of the dispersing medium and assumptions regarding the electrical double layer. Consequently, a mobility window should not automatically be interpreted as a universal zeta-potential range without considering the conversion model, viscosity, permittivity, ionic environment, and particle system.

Where is the mobility window used?

• defining which analytes can be separated in electromigration methods;
• focusing and selecting analytes in microfluidic systems;
• designing isotachophoresis and capillary electrophoresis methods;
• describing the useful operating range of mobility or surface-charge measurements;
• comparing populations with different electrokinetic behavior.

Interpretation and limitations

An electrokinetic mobility window is not an intrinsic, fixed property of a material. Measured mobility can change when the surrounding medium changes, particularly with pH and electrolyte concentration, because these variables affect surface charge, electrical-double-layer structure, and electroosmotic flow. Comparisons between different methods are also risky unless it is clear whether the reported quantity is electrophoretic mobility, apparent mobility, or net electrokinetic mobility.

Frequently asked questions about electrokinetic mobility window

Is an electrokinetic mobility window the same as a zeta-potential range?

No. A mobility window is defined in terms of mobility. Zeta potential can be derived from mobility in certain measurement methods, but the conversion depends on the electrokinetic model and properties of the medium.

What units are used for electrokinetic mobility?

The commonly used SI unit is m² V⁻¹ s⁻¹, although instruments and publications may use scaled or equivalent units. Units must be checked before comparing numerical values.

Is the mobility window fixed for an instrument?

Not necessarily. An instrument may have a specified measurement range, but the practically useful window can also depend on sample properties, applied field, electrolyte composition, geometry, signal quality, and the required separation or detection criterion.