Selectivity of a column

The order and the extent of testing a column is not constant, it depends on the task testing a newly bought column for daily work or looking for the best choice for a new polymer type. This list is more a proposal of points worth considering. Often the simple injection of a typical sample gives a lot of hints for the right selection of a column. 

The selectivity of a column determines the degree by which the maxima of the two peaks are separated as in Figure 3.1c. The following factors affect the selectivity of a column ... 

The apparent polarity of the dinonylphthalate column, expressed by a Al of 84, is a measure of retardation of aromatic and olefinic substances. Since the chromatographer is interested in the selectivity of a column for a variety of functional groups, it is important to classify each of the stationary phases by their ability to retard compounds other than benzene. This has been done by Rohrschneider (22) and further developed by McReynolds (23). The system is discussed in detail with many examples by Supina (6). The Rohrschneider/McReynolds (R/M) system involves the measurement of the retention indices for several compounds on a given column to determine the degree to which each is retarded. In each case, the retention indices are compared to... 

The equilibrium constant can be derived from free energy considerations of gas chromatographic data. It may also be obtained directly from the measurement of all concentrations at the specified temperature by selection of a column that can separate each component and by a quantitative method of detection. 

Selection of a column packing that is appropriate for a given analysis does not ensure a successful HPLC separation. A suitable solvent system must also be chosen. Several critical solvent properties will be considered here. 

The selectivity of a column is primarily a function of the packing material, although the chromatographer has some control using the mobile phase or temperature. The value for a can range from unity (1), when the retention times of the two components are identical (t2 = fi), to infinity if the first component of interest is eluted in the void volume. If a is approaching 1,... 

The optimization of the efficiency of the chromatographic system involves the selection of a column with a sufficient but not excessive number of plates. If a column is used with twice the required number of plates, then the observed resolution would exceed the required value by 40%, but both the analysis time and the pressure drop over the column would be double the required value. Moreover, the sensitivity of the detection would be decreased by 40% (see section 7.3). It is clear that we should aim to use a column with the optimum (i.e. the required) number of plates. 

When developing a separation, the most influential force is the chemical interaction of the mobile phase, sample, and stationary phase. This is a three-way interaction (1) mobile phase-sample (2) mobile phase-station-ary phase and (3) sample-stationary phase. The ability to alter the selectivity of a column by changing the composition of the mobile phase is a major reason why LC is such a versatile and, hence, powerful separation tool. When doing retention chromatography, the mobile phase is interactive since it is involved in the three-way chemical interactions. Only in the GPC (size separation) mode is the mobile phase noninteractive. With experience, scientists can easily develop an intuitive knowledge of those chemical interactions that are mainly dependent upon mobile phase and sample properties. At the point when an individual understands the basis for the mechanisms of separation, he/she becomes a chromatographer. 

Generally, the position in the elution order can be quite variable for some cephalosporins it can be seen that 10 moves from the third place to the tenth, while 7 is always found at the ninth or tenth place. It is impossible to see any relationship between the structure of the cephalosporins and their elution order which is different on all the columns. It is clear that it is Impossible to make valid predictions about the selectivity of a column towards cephalosporins. It is also clear that the plate number is of little... 

The selectivity of a column [Eq. (8.11)] depends on the fraction of the column volume occupied by pores s, and the inverse slope of the normalized calibration curve ... 

From Eqs. (12-15), it follows that the separation selectivity of a column series can, for example, be tuned by changing the mobile-phase flows in individual columns. 

Peaks characteristic of different solutes grow further apart linearly in retention time, while the peak width grows as the square root of the retention time. Thus, the selectivity of a column improves with column length. In the analytical mode, individual peaks can be detected in a flame or IR detector. In the preparative mode, different eluted solutes can... 

The most popular column diameters available for fused silica capillaries are 0.18, 0.25 narrow bore), 0.32 and 0.52 mm (magabore or wide bore). Other diameters are also available from various manufacturers. The following guidelines apply to selection of a column diameter. 

Separations in GLC are the resultant of selective solute-stationary-phase interactions and differences in the vapor pressure of solutes. The main forces that are responsible for solute interaction with a stationary phase are dispersion, induction, orientation and donor-acceptor interactions (25-27), the sum of which serves as a measure of the polarity of the stationary toward the solute. Selectivity, on the other hand, may be viewed in terms of the magnitude of the individual energies of interaction. In GLC, the selectivity of a column governs band spacing or the degree to which peak maxima are separated. The following parameters influence selectivity ... 

Another feature of the McReynolds constants is guidance in the selection of a column that will separate compounds with different functional groups, such as ketones from alcohols, ethers from olefins, and esters from nitriles. If an analyst wishes a column to elute an ester after an alcohol, the stationary phase should have a larger Z value with respect to its Y value. In the same fashion, a stationary should exhibit a larger Y value with respect to Z if an ether is to elute before an alcohol. The appendixes in Reference 12 list McReynolds constants in order of increasing A/ for each probe in successive tables that are handy and greatly facilitate the column selection process. 

The selection of a column s physical dimensions determines its fundamental separation characteristics as well as the instrumental and operational requirements for obtaining a desired level of performance. Column physical parameters are not easily changed. Generally it is not convenient or expedient to adjust a column s length once selected and installed, and replacing one column with another that has a different internal diameter is not often practical or economic. Thus, once a column has been selected, further separation optimization can take place within the realm of the operational variables of carrier-gas flow or velocity, and the column temperature or temperature program. 

Another feature of the McReynolds constants is guidance in the selection of a column that will separate compounds with different functional groups, such as ketones from alcohols, ethers from olefins, esters from nitriles, and so on. If an analyst needs a column to elute an ester after an alcohol, the stationary phase... 

A compromise of various factors led to the selection of a column design pressure of 1.3 atmospheres and a recovery in the bottoms of 85 per cent of the HD fed to the column. The pressure selected eliminated the danger of air leaking into the column and provided a satisfactory temperature difference for heat exchange in the condenser and the reboiler. Limitations on column size dictated the recovery value for a fixed bottoms concentration of 3 per cent HD. 

The relative retention a is thus a measure of the selectivity of a column and can be manipirlated by choice of a suitable stationary phase. (In principle, this is also true for the choice of the mobile phase, but in GC-MS helium or hydrogen are, in fact, always used.)... 

A calibration curve is necessary when the molecular weight of a protein is to be determined by SE-HPLC. The calibration curve is also a suitable tool in the selection of a column for a particular separation. A practical example will now be given which allows the comparison of three small SE-HPLC Polyol columns (100 mm X 4.6 mm i.d.) with particle sizes and pore sizes, respectively (a) 5 pan, 50 nm (b) 5 pern, 30 nm (c) 10 pan, 30 nm. At the same time, this example also illustrates the preparation of sodium dodecyl sulphate (SDS)-treated samples for SE-HPLC. To obtain the calibration curve for each column, the procedure detailed in Table 4 was followed. 

Chiral capillary chromatography has increasing utility in flavor, fragrance, pharmaceutical, and industrial chemical applications. The proper selection of a column is essential since chiral stationary phases illustrate different capabilities and applications. The compositions of commercial... 

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