Efficiency and Asymmetry

Common standard compounds for reversed phase columns are toluene and naphthalene, which have retention factors, k, of about 3. The eluent modifier is methanol or acetonitrile at a concentration of 50-80%, depending on the hydrophobicity of the stationary phase material. For other stationary phase materials, corresponding analytes, with k = 3-5, can be used. 

Column Test for Re versed-phase Liquid Chromatography 

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Effect of the Counteranion Type and Concentration on Peak Efficiency and Asymmetry. Theoretically, a column can generate a certain maximum number of theoretical plates at the optimum flow rate. This number should be independent of the type of the analyte and mobile phase. In reality, any secondary processes, energetic surface heterogeneity, or restrictions in sorption-desorption kinetics in the column will result in the specific decrease of the efficiency for a particular compound. 

The aim of any chromatographic system is to resolve a number of components in a sample mixture, i.e. to ensure that individual peaks do not overlap or coincide. To achieve this you need to consider several important factors capacity factor, separation factor or selectivity, column efficiency and asymmetry factor. 

However, this test has several disadvantages. For one, the test typically only checks column efficiency and asymmetry, but does not give you any indication of the state of the surface chemistry. This is especially true for reversed-phase columns. The test compounds commonly used for efficiency tests interact only... 

With this approach, accurate descriptions of peaks showing large asymmetries can be obtained, including those showing deformation either to the right or to the left. Also, eq. 8.48 with a linear or a parabolic function can be applied to refine peak parameters, such as the efficiency and asymmetry factor, estimated by direct measurement of the chromatographic signal. The method of Powell can be used to fit the experimental data to the nonlinear functions [28]. 

The chromatographic parameters (retention factors, efficiencies and asymmetry factors) obtained with the selected SDS micellar and methanol-water mobile phases are shown in Table 10.3. The elution order is the same in both cases, except for celiprolol, metoprolol and timolol, which eluted at close times with the micellar eluents. On the other hand, the peaks obtained with the SDS-propanol eluents had better characteristics than the peaks in the aqueous-organic eluents, with a tenfold increase in efficiency for some P-blockers. The peaks in the aqueous-organic mobile phases were often very asymmetrical. The determination of the whole set of drugs was possible with a unique micellar mobile phase, with retention times of less than 15 min, whereas two different aqueous-organic mobile phases were needed to make the same analyses. 

In a subsequent work [182], it was shown that the photoelectrochemical performance of InSe can be considerably improved by means of selective (photo)electrochemical etching. Interestingly, whereas the cleavage vdW plane showed little improvement, the photocurrent in the face parallel to the c-axis was doubled. Note that, in contrast to InSe crystals cleaved in the plane perpendicular to the c-axis that are almost defect free, the crystals cut in the plane parallel to the c-axis contain a high density of defects on their surface which leads to a high rate of electron-hole recombinations and inferior quantum efficiency. The asymmetry in the role of electrons and holes, as manifested, e.g., in the fact that surface holes carry out the selective corrosion of the semiconductor surface in both cleavage orientations, was discussed. 

One of the most intriguing features of the bacterial reaction centre is the asymmetry of trans-membrane electron transfer. Although the BChl, BPhe and UQjq cofactors are arranged in two approximately symmetrical trans-membrane branches, only the so-called A-branch is used for transmembrane electron transfer. The factors determining this functional asymmetry continue to be the subject of great interest, as the reaction centre presents a chain of cofactors that catalyses electron transfer with great efficiency, and a similar chain of cofactors that is much less effective. This functional asymmetry is due to small but crucial differences in the structure of the proteinxofactor system along the two branches. 

Effect of peak asymmetry on column efficiency and separation... 

The most serious limitations of pure micellar solutions are their weak elution strength and poor efficiencies. As early as 1983, the addition of a small percentage of 1-propanol was found to enhance the efficiencies and decrease the asymmetries of chromatographic peaks. Later, the term hybrid micellar mobile phases was given to the ternary eluents of water/organic solvent/ micelles. Although 1-propanol is still the most frequently used additive, other alcohols (methanol, ethanol, 1-butanol, and 1-pentanol) and organic solvents common... 

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