Retention migration time

The identification of selenimn species by matching retention (migration) times with those of authentic standards demands the assruance that the resolution of the chromatographic technique produces at a given retention time a signal corresponding to this compound only, and that peak shapes are not distorted by sample overload or imacceptable adsorption. Thus, hyphenated molecular characterization by MS or NMR spectroscopy provides valuable confirmatory data for chromatographic speciation of standards and identifications of imknowns. 

In spite of the several advantages mentioned above for CE, some aspects of this technique stiU require to be improved, such as the reproducibility of retention/ migration times and its sensitivity. However, CE constitutes a separation technique complementary to LC. 

Table 33 Flow Rate, Retention (Migration) Times, Retention Factors and Relative Difference Between Corrected and Observed Retention Factors...

In the System Suitability section, different parameters are described which can be applied in order to check the behavior of the CE system. The choice of the appropriate parameters depends on the mode of CE used. The system suitability parameters include retention factor (k) (only for MEKC), apparent number of theoretical plates (N), symmetry factor (Af), resolution (Rs)> Rtea repeatability, migration time repeatability, and signal-to-noise ratio. Practical equations to calculate different system suitability parameters from the electropherograms are presented, which are also included in Table 3. 

When a combination of CE and HPLC systems would be considered, the most dissimilar from the global set could be selected according to the above approach. For the CE methods, a response should be selected and applied with values in the same order of magnitude as the retention factors of the CSs, e.g., the migration times. Another possibility would be to use the so-called normalized migration indexes (see further Section III.C) for both the CE and the HPLC measurements. ... 

The retention factor in MEEKC for uncharged solutes can be determined from migration time data using Eq. (2). The equation is relevant to MEKC (9) ... 

The time required by a given analyte to migrate under the sole influence of the applied electric field across the capillary tube from the injection end of the capillary to the detection windows (migration distance) is defined as the migration time (tj and, similarly as the retention time in HPLC, is used for identification of sample components. It is given by... 

In order to calculate the retention factor k of a compound or the selectivity coefficient between two compounds, the distance migrated along the plate is usually translated into the migration time for the chromatogram. Assuming that the ratio of the migration velocities u/u0 is the same on the plate as on the column (this is only an approximation), then Rf can be related to k ... 

Where K is the retention factor t0, tj, and are migration times of neutral solutes, solutes of interest and micelles, respectively. In this mode of chromatography,... 

Although both reproducible preparation and operation of CEC columns are extremely important issues that will further stimulate the development and the acceptance of this technique, only a few groups have reported data on column-to-col-umn, run-to-run, and day-to-day reproducibility of monolithic capillary columns. Palm and Novotny showed reproducibility data for migration times tr, efficiencies, and retention factors for a number of analytes on acrylamide-based monoliths [35], The relative standard deviations (RSD) were smaller for run-to-run compared to day-to-day measurements. For example, the average run-to-run RSD for 6 analytes was... 

In HPLC the mobile-phase flow and retention by the stationary phase take place in series so that the total migration time, / , is the sum of the times the eluite spends in the stationary and mobile phases [2,3] ... 

Reproducibility during column preparation is a significant problem in CEC. Preparation methods involving pumped slurries were all found to produce generally highly efficient (> 200,000 plates/m for 3-pm ODS particles), but within a batch of columns packed by the same method, the relative standard deviation (RSD) of EOF, and migration time and retention factor of a standard were [23], respectively, 7-14%, 5-22%, and 9-30%. These values are particularly relevant to considerations of the transfer of HPLC methods to CEC. 

A less complex approach can be used for enzyme activity estimation namely, fractions can be collected and enzymatic activity measured in these fractions. The practical realization involves a normal run before fraction collection to detect the migration time of the peak, which should be collected. The time Tc when the peak is going to leave the capillary can be estimated as retention time multiplied by the length of the capillary divided by the capillary length to the detector. When fractions are to be collected, the potential is switched off just before Tc. A new vial is placed at the end of the separation... 

The parameter normally measured in capillary electrophoresis is migration (retention) time, /. In a given CE system this parameter is inversely proportional to the electrophoretic mobility, pi. The pt (cm /V) is a normalized parameter allowing for comparison of data obtained in different CE systems. If a series of analytes are analyzed under the same conditions then the 1/r and pt are equivalent. There are only a few reports on QSRR analysis of CE data. This may suggest the unsuitability of routinely determined mobility parameters as the LEER descriptors of analyte behaviour. Probably the reproducibility of analyte migration times in CE is poor due mainly to the non-reproducible electroosmotic flow velocity 26. ... 

An ionic chiral micelle is used as a pseudo-stationary phase it works as a chiral selector. When a pair of enantiomers is injected to the MEKC system, each enantiomer is incorporated into the chiral micelle at a certain extent determined by the micellar solubilization equilibrium. The equilibrium constant for each enantiomer is expected to be different more or less among the enantiomeric pair that is, the degree of solubilization of each enantiomer into the chiral micelle would be different for each. Thus, the difference in the retention factor would be obtained and different migration times would occur. 

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