On the ionic strength dependence of the electrophoretic mobility: From 2D to 3D slope‐plots

Abstract

Determining the charge and the nature (small ion, nanoparticle, or polyelectrolyte) of an unknown solute from its electrophoretic characteristics remains a challenging issue. In this work, we demonstrate that, if the knowledge of the effective electrophoretic mobility (μ ) at a given ionic strength is not sufficient to characterize a given solute, the combination of this parameter with (i) the relative decrease of the electrophoretic mobility with the ionic strength (S), and (ii) the hydrodynamic radius (R ), is sufficient (in most cases) to deduce the nature of the solute and its charge. These three parameters are experimentally accessible by CZE and Taylor dispersion analysis performed on the same instrumentation. 3D representation of the three aforementioned parameters (μ ; S and R ) is proposed to visualize the differences in the electrophoretic behavior between solutes according to their charge and nature. Surprisingly, such 3D slope plot in the case of small ions and nanoparticles looks like a "whale-tail," while polyelectrolyte contour plot represents a rather simple and monotonous map that is independent of solute size. This work also sets how to estimate the effective charge of a solute from a given experimental (S,Rh,μ ep 5 mM ) triplet, which is not possible to obtain unambiguously with only (Rh,μ ep 5 mM ) or (S,μ ep 5 mM ) doublet, where μ ep 5 mM is the effective electrophoretic mobility at 5 mM ionic strength.

Capillary electrophoresis for aluminum ion speciation: Optimized separation conditions for complex polycation mixtures

“…Whatever the nature of the solute (small ion, oligomers, polyelectrolyte, nanoparticle), it was shown that eq. (5) is verified (within a 5% error margin) between typically 1 and 100 mM ionic strength [34]. Such plots have the advantage to quantify the dependence on ionic strength and to possibly interpolate the effective mobility values at any ionic strength.…”

“…This behavior can explain the inversion in migration order between ACH oligomers and sodium. The impact of ionic strength on ACH oligomers migration will be discussed in more detail in a subsequent section using the recently developed slope plot approach [34,35]. It can be also noticed from Fig.…”

“…The so-called slope plot is given inFig. 5B, where the S value is plotted as a function of µep,corr(5 mM)[34]. Data represented as plain lines are from reference[35], covering a data set of 5000 values that were numerically obtained for different small ions and nanoparticles of various charge and size, using the modified Yoon and Kim modelling[35].…”

Separation and Characterization of Highly Charged Polyelectrolytes Using Free-Solution Capillary Electrophoresis

“…These figures of merit are very useful for estimating the impact of the ionic strength on the effective mobility and for the modeling of the effective and apparent selectivities according to f and I, as discussed in the next section. The S values are also very informative about the characteristics of the solute (charge, nature, or size) [25,26,27]. As a matter of comparison, proteins have typical S values between 0.3 and 0.4 [27], which are very similar to the values obtained for moderately charged PAMAMPS with 30% ≥ f ≥ 10%.…”

Mobility Shift Affinity Capillary Electrophoresis at High Ligand Concentrations: Application to Aluminum Chlorohydrate–Protein Interactions

“…Indeed, it is a very general trend in electrophoresis that for any solutes (small ion, polyelectrolyte, proteins, and nanoparticles), the effective mobility is a decreasing function of the ionic strength. 43,47,48 Figure 5A displays the mobility ratio R as a function of ACH concentration. Clearly, the R values are much higher for BSA (typically between 4.4 and 9.7) than for LSZ (0.7 to 1.6) suggesting much higher interaction of BSA with ACH compared to LSZ.…”