Origin of peak asymmetry and isotherm nonlinearity in micellar electrokinetic chromatography: Variation of peak shape with buffer concentration

Abstract

The diffuse fronts and sharp rears of peaks of nitrobenzene (nbz) solubilized at high concentrations in 50 mM SDS and 2.5, 25, and 50 mM sodium tetraborate buffers were modeled in MEKC by measurements of, and fits to, concave upward isotherms, and by numerical solution of the continuity equation. The isotherms varied with buffer concentration, with the smallest limiting slope and largest curvature found for the 50 mM tetraborate buffer. The Brunauer, Emmett, and Teller isotherm described the peak profiles in all buffers, with symmetrical peaks observed at sub- and low-mM levels of nbz, anti-Langmuirian peaks observed at 10-20 mM levels, and aquiline peaks resembling curved noses observed at 20-30 mM levels. The variation of the partition coefficient with nbz and buffer concentrations was shown to result from nonideal thermodynamics. High-buffer concentrations salt out nbz from the mobile phase, as quantified by a mobile-phase activity coefficient related to the Setchenov constant of nbz in sodium tetraborate. The activity coefficient of nbz in SDS micelles was shown to resemble that measured by other researchers for benzene in micelles of sodium octyl sulfate, i.e. it decreases with increasing solute concentration and increases with electrolyte concentration. Many examples from the literature are discussed, in which the variation of the intramicellar activity coefficient with solute mole fraction is consistent with peaks having diffuse fronts and sharp rears.

Two‐chiral component microemulsion EKC – chiral surfactant and chiral oil. Part 2: Diethyl tartrate

“…In addition to the above zone-broadening phenomena (i-v) that are common to both CZE and EKC, additional phenomena have been identified for micellar EKC [59][60][61][62][63][64][65][66]: (vi) solute-PSP mass transfer kinetics, (vii) PSP polydispersity, (viii) PSP overload (nonlinear isotherms), and two EKC-specific phenomena resulting from previously described Joule-heating-induced radial thermal gradients. The two EKC thermal phenomena are (ix) PSP migration anisotropy and (x) solute-PSP partition spreading.…”

“…Although MEEKC separation efficiency is often reported, and in several direct comparisons with MEKC it was more frequently observed to be higher [28,39,58] rather than lower [40], zone broadening in MEEKC has received little if any attention from a fundamental perspective, in contrast to several carefully-conducted efficiency studies in micellar EKC [59][60][61][62][63][64][65][66]. The latter studies along with well-known results for CZE facilitate our synopsis below of relevant zone broadening processes for EKC, prior to the discussion of our chiral MEEKC efficiency results.…”

Micellar electrokinetic chromatography: Methodological and instrumental advances focused on practical aspects

“…As in the previous review 1, some current relevant aspects to do with MEKC, such as the use of polymeric pseudo‐stationary phases (PSPs), its implementation on microchip devices and advances in MEEKC (at present, a consolidated extension of MEKC), have been omitted in this review as they are now the subject of dedicated reviews in this issue. On the other hand, some papers related to theoretical aspects of MEKC have also been published in this period, the most relevant being those focusd on studies on the selectivity of single, mixed and modified PSPs 2, on the origin of peak asymmetry and isotherm non‐linearity 3, on the analysis of substances to be used as internal standards 4 and on migration models for acidic solutes 5.…”

Electrokinetic Chromatography and Mass Spectrometric Detection Using Latex Nanoparticles as a Pseudostationary Phase

“…This broadening and tailing is not observed for all compounds or under all buffer conditions. The cause of this tailing may be related to nonlinear Langmuir type isotherms for some compounds, , even at these relatively low concentrations. Reducing solute concentrations to 5 ppm, where signal-to-noise ratios are relatively low, did not improve the peak shapes.…”