Plate height selectivity

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

The important parameters to consider are the selectivity (dKJdlogR), the ratio of pore volume, Vp, over void volume, Vq, the plate height, H, and the column length, L. The distribution coefficient, Kq, has a slight effect on resolution (with an optimum at Kp 0.3-0.5). In addition to this, extra column effects, such as sample volume, may also contribute to the resolution. 

To date, insufficient work has been done to ascertain whether the general advantages of very small particle columns for neutral compounds also can be demonstrated for basic compounds, and that no unexpected differences in selectivity or peak shape (other than a reduction in the plate height) occur as a result of the reduction in particle size. 

As with other types of solutes, chromatographers have attempted to improve the speed and efficiency of analysis of bases by fhe use of smaller particle (e.g., sub-2 p,m) or monolithic columns. Small particle columns have not yet been fully evaluated for the analysis of bases, to determine whether they give equivalent selectivity, and reduction in plate height commensurate with the reduction in particle size, as has been demonstrated for neutral compounds. Commercial silica monolith columns give reasonable performance for the analysis of bases at low pH, but show evidence of... 

Another example is the confusion between the terms efficiency, efficacy, and selectivity of a separation. It is not always mentioned how these responses are obtained. Usually, the efficiency is either expressed as the number of theoretical plates or plate number, N, or as the height equivalent to a theoretical plate or plate height, HETP or With efficacy, usually efficiency, thus plate count, is meant, although it is not always clearly specified. The selectivity, in general, is found by calculating the selectivity or separation factor, a, which is a measure for the... 

In pHPLC, there are numerous types of columns used. The comparison and characterization of these columns are often discussed in terms of thermodynamic properties and kinetic characteristics. The retention factor, k, selectivity, a, and the peak asymmetry are believed to be representative parameters for the thermodynamic properties, while the kinetic characteristics are often expressed in dimensionless magnitudes of reduced plate height, h, separation impedance, E, and flow resistance factor, ( ). 3... 

The main argument for making MIP CEC is to combine the selectivity of the MIPs with the high separation efficiency of CEC. This argument appears to fail, however, if the adsorption isotherm of the MIP is nonlinear, which seems to be the rule. In the case of nonlinear isotherms, the peak shapes depend mainly on the isotherm, particularly so if the separation system is otherwise very efficient (has low theoretical plate height, see Fig. 1). In the case of ionized analytes the situation is more complex. If an ionized analyte is not adsorbed at all on the MIP, then it is separated only due to electrophoresis, and its peak will not be widened due to the nonlinear effect. In this case, however, the MIP is merely behaving like an inert porous material. In intermediate cases an ionized analyte may participate in both separation mechanisms and for this case we do not have exact predictions of the peak shape. 

Working on a planet in another galaxy, where laboratory temperatures are 200 K, a scientist discovers a new force field selective for isotopes whose separation promises to fuel a bomb to blow an adjacent planet to smithereens. The maximum force exerted by this field on the two isotopes is 5.00 x 1010 dyn/mol. What is the lowest plate height the scientist can hope to achieve in an Sc-class separation In this field the velocity difference is 2% of the mean (which is remarkably selective for isotopes). What length separation tube is needed to separate the isotopes at unit resolution (Remember, lofty goals are worth the trouble.)... 

Partition coefficient, 9, 10 Partition ratio, 11 time optimization of, 57-58 Peak, definition of, 69 Peak capacity, 18, 19 Pellicular supports, 157 Permeability, 63-64 Phase selection diagrams, 218-219 Phase volume ratio, 11 Pinkerton (ISRP) columns, 225-226 Plate height, 17 Plate number, 14-16 Plate theory, 3, 28 Polarity index, 210, 211 Pore size of LC supports, 157 Porosity, 27 Precision, 99-100 Preparative scale ... 

We have found that the use of 3% n-propanol in the micellar mobile phase and column temperatures of 40° C appear to offer a broadly applicable solution to the low efficiency previously reported for micellar mobile phases. These conditions have resulted in reduced plate heights of 3-4 for SDS, cetyltrimethylammonium bromide (CTAB), and Brij-35 (15). This efficiency optimization scheme then appears to be a broadly-based solution for micellar mobile phases of any surfactant. This means that the surfactant type can be varied to affect separational selectivity with no loss in column efficiency. 

For practical purposes, several terms need to be defined. These are capacity factor (k ), theoretical plate number (N), height equivalent to one theoretical plate (F1ETP), selectivity (a) and peak asymmetry (b/a). As will be discussed later in specific examples (Sects. 9.2.4 and 9.2.5), these parameters are of crucial importance in monitoring and maintaining HPLC column efficiency. 

The resolution consists of two terms, the selectivity term, with the relative difference of the mobilities, and the efficiency term, the square root of the mean plate number. It must be pointed out that the plate height, H, on which this plate number is based consists of all the contributions to peak broadening. 

The change of the selectivity term in the resolution is directly expressible by im. Interestingly, the effect of the EOF on the dispersion effects, expressed by the plate height H, also depends directly on im. For longitudinal diffusion. Joule self-heating, and concentration overload, the variation of the plate height in the presence of the EOF is directly dependent upon this reduced mobility according to... 

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