The Column Dimensions

The size of the chiral column - that is, the length and diameter - is also important in optimizing the chiral resolution of environmental pollutants. 

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Assuming the column dimensions are 320 pm I.D. (radius r=0.0160 cm), 30 m long, and it is operated at 120°C using nitrogen as the carrier gas which, at that temperature, has a viscosity of 129 x 10 Poises, then by using equation (3), the change in (y) can be calculated for different flow rates. The relationship between flow... 

The flow rate in SEC significantly affects the resolution. Depending on the selectivity wanted, linear flow rates have to be adapted to the column dimensions. In general, running the column at a low flow rate results in higher resolution, but diffusion may produce diminishing resolution when the flow rate is too low. The flow rates recommended for a particular column diameter should not be increased. In the case of Superformance columns, the best results can be obtained by applying linear flow rates of about 30-80 cm/hr. Of course, linear flow rates below 30 cm/hr can contribute to further increased resolution. 

Individual pore size columns have variable pore volume, and because the column dimensions are fixed, the combination of different columns must result in variable slope of the overall calibration curve and hence variable degrees of resolution as a function of molecular weight. 

Each of the four columns was packed with CPG00120C d = 13.0 nm). The column dimensions and experimental conditions are listed in Table 23.1. The flow rates (solution and solvent) were set to be proportional to the cross section of the column, whenever possible. The number of drops collected in each test tube was almost proportional to the cross section, especially for the initial fractions that might show a shift in M. Figure 23.9 shows chromatograms for some the fractions separated using 2.1-, 3.9-, and 7.8-mm i.d. columns. The result with the 7.8-mm i.d. column is a reproduction of Fig. 23.2 (3). Chromatograms of the fractions obtained from the 1.0-mm i.d. column overlapped with the chromatogram of the injected polymer sample (not shown). 

The adjusted retention time provides a measure of the strength of intermolecular interaction between the analyte and the stationary phase, with stronger interactions giving a longer time. The gas hold-up time is derived from the flow rate and the column dimensions and is often measured by injecting a non-retained compound. The retention factor, which represents a ratio of the mass of analyte dissolved in the stationary phase to the mass in the mobile phase, can be calculated from the adjusted retention time and the gas hold-up time. 

Systems with electronic pneumatic control use pressure transducers at the inlet and outlet, the column dimensions and physical properties of the carrier gas to determine the gas hold-up time and the flow rate. 

For the simulation of SMB-separations efficient software packages,based on the Triangle-Theory, are commercially available. The number of columns, the column dimensions, the theoretical number of plates in the columns, the feed concentration, the bi-Langmuir adsorption isotherm parameters and the number of cycles need to be defined by the user. Then the separation is simulated and values for the flow rate ratios, the flow rates, the switching time and the quality of the separation, purity and yield, are calculated. Based on these values an actual separation can be performed. However, some optimization/further development is usually necessary, since the simulations are based on an ideal model and the derived parameters and results therefore can only be taken as indications for the test runs. 

Thus, the row dimension of A becomes the column dimension of A4, the column dimension of A will be the row dimension of A4. 

While retention time is used for peak identification, it is dependent on the flow rate, the column dimension, and other parameters. A more fundamental term that measures the degree of retention of the analyte is the capacity factor or retention factor (k ), calculated by normalizing the net retention time (% > retention time minus the void time) by the void time. The capacity factor measures how many times the analyte is retained relative to an unretained component. ... 

In contrast to the Kovats relationship, retention indices depend only on the stationary phase and not on the column dimensions or the flow rate of the carrier gas. In practice, compound X is injected with the two bracketing alkanes to ensure that the experimental conditions are uniform. 

Column volume. X. The total volume of the column which contains the stationary phase. (IUPAC recommends the column dimensions be given as the inner diameter and the height or length of the column occupied by the stationary phase under the specific chromatographic conditions. Dimensions should be given in millimeters or centimeters.)... 

Various advantages can be realized by varying the column dimensions from the conventional 4.6 mm ID x 25 cm. One approach has been to use shorter columns of 3-10 cm packed with 3-/ m or 5-fim particles. When operating at high flow rates (2-5 ml/min), this approach has become known as fast HPLC [41]. 

The volume restrictions that apply to sample injection are equally valid for detection. Detectors must be miniaturized relative to the column dimensions and a detection volume in the nanoliter or even subnanoliter level is often required. For... 

FO equals unity this becomes quite impossible. In other words, given the final column for routine analysis, very large values of At are unattractive, since they do not increase the value of FO, but do lead to an increase in analysis time. If, however, we can tailor our column to the result of the optimization procedure (i.e. to the number of plates required), then large values of At leading to very large values of Rs are indeed significant. Hence, in the case where the column dimensions can be chosen after completion of the optimization of selectivity, the use of Rs or S is preferred, because of the clear and simple relationship between these criteria and the required number of theoretical plates. 

Chromatogram c appears to show a much longer analysis time than does chromatogram a. However, if we are free to define the column dimensions after the selectivity optimization process, chromatogram c can be the basis for a very quick separation on a very short column. 

The criteria suggested by 3. imply that the grid search approach will be less useful for optimization processes run on the final analytical column than it will for cases in which the column dimensions are optimized last. 

The required number of plates (Nne) is the most relevant factor for the selection of the type of column and the column dimensions. However, there are various other factors which we need to consider in the selection of the most suitable column for a given analysis ... 

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