Sample self-stacking

Kfivankova, L., Caslavska, J., Malaskova, H., Gebauer, P., and Thormann, W. (2004). Analysis of ethyl glucuronide in human serum by capillary electrophoresis with sample self-stacking and indirect detection./. Chromatogr. A 1081, 2 — 8. 

One explanation for the sharpening of the later-eluting impurities is a phenomenon called sample self-stacking [42,43], This is a variant of transient isotachophoresis (ITP) [44], In this case, the leading electrolyte is chloride ion, while the main component acts as the terminating electrolyte. The requirement for ITP is fulfilled since the impurity s mobility is bracketed by leader and terminator. While you may not understand all of this, accidental ITP is not rare... 

Gebauer P, Thormann W, Bocek P. Sample self-stacking in zone electrophoresis, theoretical description of the zone electrophoretic separation of minor components in the presence of bulk amounts of a sample component with high mobility and like charge. J Chromatogr 1992 608 47. 

Another version of a discontinuous system is called sample self-stacking. In this system, a co-ion is placed into the sample matrix, which acts like the leading electrolyte in ITP but only during the injection phase. After injection, the sample will continue to stack until the stacking ion leaves the sample zone. In Reference 9, an equation is presented to calculate a stacking ratio known as flcnt-... 

Whether the stacking technique is called field-amplified stacking, transient isotachophoresis, or sample self-stacking, the driving force in on-column sample preconcentration for charges species is the need to satisfy the initial KRF conditions. Under separation and sample matrix conditions where the KRF is difficult to determine, simply calculations such as flcut and a offer a rapid evaluation of conditions for optimal transient ITP. 

Gerbauer, R, Thormann, W., and Bocek, R, Sample self-stacking and sample stacking in zone electrophoresis with major sample components of like charge General model and scheme of possible modes, Electrophoresis, 16, 2039-2050, 1995. 

Gerbauer, R, Krivankova, L., Rantuckova, R, Bocek, R, and Thormann, W., Sample self-stacking in capillary zone electrophoresis Behavior of samples containing multiple major coionic components. Electrophoresis, 21, 2797-2808, 2000. 

C. Tu and H.K. Lee, Determination of nitrate in seawater by capillary zone electrophoresis with chloride-induced sample self-stacking, J. Chromatogr. A, 966, 205—212, 2002. 

As the velocities of the ions are affected by factors such as the charge, pH, concentration, and coions, successful ITP requires careful attention to all these details, which makes it difficult to use in practice. A simpler form of ITP is transient ITP (t-ITP), which is easier to couple to CZE (in the same capillary). Under appropriate conditions, a concentration step due to a brief ITP can occur before the sample enters the separation buffer. In many instances, the t-ITP step occurs accidentally in samples containing high concentrations of salts (self-stacking) or it can be induced by the addition of an appropriate lead-ing/terminating ion to samples with a complex matrix. The method can concentrate both small and large molecules. 

The systems discussed up to now all showed chiral susceptibilities that were of the same order of magnitude or smaller than the achiral susceptibility components. The system that we discuss in this section has chiral susceptibilities that dominate the nonlinear optical response.53 The material is a chiral helicenebisquinone derivative shown in Figure 9.22. In bulk samples, the nonracemic, but not the racemic, form of the material spontaneously organizes into long fibers clearly visible under an optical microscope. These fibers comprise columnar stacks of helicene molecules.54,55 Similar columnar stacks self-assemble in appropriate solvents, such as n-dodecane, when the concentration exceeds 1 mM. This association can be observed by a large increase in the circular dichroism (CD) of the solutions. 

However, one of the most intriguing examples in this field, which also demonstrates the power of diffusion NMR to characterize supramolecular systems obtained by self aggregation, is the assignment of the different species that prevail in solution following the reduction of 2,5,8,ll-tetra-tert-butylcycloocta[l,2,3,4-def-,5,6,7,8]bis-biphenylene (65) to its respective tetraanions [59]. In this sample, different species were observed and only diffusion NMR provided a proof, in conjunction with 2-D NOESY, that the different molecular species are indeed different helically-stacked anionic aggregates of 65. When NMR diffusion measurements were performed on the obtained solution, four different diffusion coefficients were found for the mixture. These coefficients that were assigned to the monomer, dimer, trimer and tetramer of 65 (Fig. 6.24). Based on Eq. (6.16) ... 

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