Selecting a Column

However, four different moments of inertia for each of the four different materials (hence different moduli of elasticity) are obtained from Equation (7.2). Moreover, knowing the moment of inertia of a column does not teli us the shape or dimensions of a column We usually select a column... 

To select a column for a particular analytical problem, the first step is to make a choice about the pore size(s) to be used for the separation. In general, one cannot expect that a single pore size will fulfill the needs of a separation. In size exclusion chromatography, it is more common that columns of different types are combined with each other to deliver the separation range needed for a particular analysis. Therefore, column banks with different pore sizes are frequently combined with each other to maximize the separation power for... 

Select a column from a data file and calculate mean, median, and standard deviation. Display the original data together with Xmean and mean i ... 

Equation (16-168) shows that the resolution is the result of independent effects of the separation selectivity (a), column efficiency (Np), and capacity (k ). Generally peaks are essentially completely resolved when R, = 1.5 (>99.5 percent separation). In practice, values of fl, 1, corresponding to 98 percent separation, are often considered adequate. 

In reversed-phase HPLC, column temperature is a strong determinant of retention time and also affects column selectivity. A column oven is therefore required for most automated pharmaceutical assays to improve retention time precision, typically at temperatures of 30-50°C. Temperatures >60°C are atypical due to concerns about thermal degradation of the analytes and column lifetimes. Exceptions are found in high-throughput screening where higher temperatures are used to increase flow and efficiency. Ambient or snb-ambient operation is sometimes found in chiral separations to enhance selectivity. Column ovens... 

One of the most important causes of poor resolution in protein and large peptide analysis with conventional phases is slowness of mass transport and of sorption/desorption. When selecting a column for a protein separation, it is important to look for obtaining rapid separations in order to limit protein denaturation. 

There are many factors to consider in selecting a column for a specific separation. Table 3.3 lists a variety of applications along with columns that have been found suitable for each analysis. While each of these columns has been used successfully for the class of compounds shown, they will not separate all compounds of that class. Separation of isomers can be difficult and special columns are often required. However, the columns listed are useful for the first step in the analytical problem. 

Modified cyclodextrins are the most versatile and widely used chiral stationary phases. The most widely used columns contain 10% to 50% cyclodextrin dissolved in either OV-101 or SE-52 polysiloxane. They are thermally stable up to 230°C but require some care in use because cyclodextrins are soluble in many solvents and can be washed off the column if they are exposed to too much solvent. Considerations in selecting a column... 

Selecting a column for an HPLC separation is a matter of asking yourself a series of questions (Fig. 5.4). You first must determine how much material you wish to separate in a single injection (preparative vx. semipreparative vs. analytical). The next question involves the separation mode to be employed (size exclusion vx. ion exchange vx partition). Finally, there is the question of solubility controlling solvent and column selection in all modes. 

Column selection often has to do with which area of a molecule contains the differentiating portion. Two compounds to be separated may vary by substitution on a benzene ring. Another pair may only vary by a polar functional group. Again, like attracts like. The working rule is to select a column in which the variable parts of two molecules point toward the column. 

Several thousands of different columns are commercially available, and when selecting a column for a particular separation the chromatographer should be able to decide whether a packed, capillary, or monolithic column is needed and what the desired characteristics of the base material, bonded phase, and bonding density of selected column is needed. Commercial columns of the same general type (e.g., CIS) could differ widely in their separation power among different suppliers. Basic information regarding the specific column provided by the manufacturer, such as surface area, % carbon, and type of bonded phase, usually does not allow prediction of the separation or for the proper selection of columns with similar separation patterns. 

Columns for a particular laboratory can be chosen based on some set of internal criteria. One of the criteria to select a column should be such that the column is stable for a certain number of column volumes (efficiency, tailing factor, and retention time criteria for predefined probe analytes) at the recommended maximum and minimum pH at a particular maximum temperature. This would allow the chromatographer to employ such phases with a significant degree of confidence and ensure the robustness of the stationary phase during method development and for release and stability testing. 

Excel also provides "context-sensitive" shortcut menus. If you press the right mouse button (Windows) or press COMMAND+OPTION (Macintosh) while you select a worksheet element with the mouse pointer, a menu is displayed containing commands that apply to the selection. For example, if you select a column while holding down the right mouse button, a shortcut menu containing editing and formatting commands appropriate for a column appears. 

Figure 2.6 Effect of selectivity (a), column efficiency (N) and retention (k ) on resolution (adapted from Snyder and Kirkland (1986)). The curves were simulated using equations (2.4), (2.24) and (2.22) and the following parameters (a) k = 3.0

The greatest advantage of Still s flash chromatography technique is its expediency. On average, a column should not take more than 10-15 min. The rule of thiunb is, if it takes more than 30 min for a routine chromatographic separation, it is likely to be unsucessful or the flow rate is too slow. The trick is to select a column of appropriate diameter. The height of the silica gel should not be taller than 6 inches, changing the diameter to accommodate tlie endpoint, where 25 g of silica gel is loaded for each 1 g of the reaction mixture to be separated. For more difficult separations, a ratio of 100 1 to 200 1 silica gel to mass of mixture is not unusual. As a consequence, run times are extended. 

An array is always assumed to have two dimensions rows, and columns. Therefore, when you select a column, and want to read, say, the value in its second cell, you must specify this cell as cell (2,1), even though there is only one column. Likewise, in a single row, the fifth cell is identified as cell (1,5) because this cell has the row index 1, and the column index 5. 

---