Packed columns selectivity optimization

Obviously, there are many ways to influence the capacity factors. However, the effects described above are predictable (see section 4.2.3) and in a sense trivial. It is worth noticing at this point that certain parameters do not at all affect the capacity factor and therefore do not at all affect chromatographic selectivity. These parameters include column length, flow rate and the diameter of packed columns. This renders these parameters irrelevant to the selectivity optimization process. In some cases they may be considered as parameters... 

A column consisting of a deactivated silica-based stationary phase is used for the packed-column mode. A packed column allows larger volumes of sample solvent to be injected, thus improving sensitivity. Generally, the column dimensions are 1 x 100-250 mm and the particle size is 5 / m. Commercial SFC instruments are also available that will handle the classical 4.6 x 150-mm or 250-mm columns. With the introduction of electronically controlled variable restrictors to control the back pressure, the packed columns are becoming increasingly more popular. This feature allows the independent flow and pressure control of mobile phases, thus helping in rapid optimization of selectivities. Some of the commonly used packed columns are as follows ... 

Martin et al. published a paper on the theoretical limits of HPLC which is well worth reading.They used relatively simple mathematics to calculate pressure-optimized columns for which the length L, particle size and flow rate u of the mobile phase were selected such that a minimum pressure Ap is required to solve a separation problem. It has been shown that these optimized colunms are operated at their van Deemter curve minima. Some astonishing facts have emerged from the study, provided that the chromatography is performed on well packed columns (reduced plate height h = 2-3 see Section 8.5). 

The optimized pressure concept can be extended. Figures 2.29 and 2.30 show nomograms for well packed columns operated at their van Deemter curve minima. The nomograms show the interrelationship between column length, pressure, number of theoretical plates, particle size of the stationary phase and breakthrough time. Two of these five parameters may be selected at random, the other three being geared to the optimum flow rate. 

Series coupling of columns containing the same stationary phase is used to enhance efficiency and with different stationary phases to fine tune selectivity [8]. Series coupling of packed columns became popular after it was demonstrated that the column pressure drop did not limit the total column efficiency to the extent that had been predicted (section 7.4.2). Serial coupling of 2 or 3 standard columns is practical for routine applications and provides a total plate count in excess of 50,000. There is no theory for selectivity optimization for coupled packed columns but suitable conditions often can be estimated from separations on the individual columns. Effective selectivity changes require that the coupled columns have different retention properties. A number of practical examples for the separation of polymer additives, polycyclic aromatic hydrocarbons, phytic acid impurities and enatiomers have been described [137,195,196]. Series coupling of open tubular columns with different stationary phases is less common, but changes in selectivity are predictable, at least when the pressure drop is low [197]. 

Packed columns. The techniques for packed columns for HPLC have been extensively developed during recent decades. Thus very small particles with narrow size distributions are available. With such particles, reasonable plate counts can be obtained in short column lengths. The optimal mobile phase velocities are often in the range of 1-2 mm s in such columns, and thus they need to be short in order to avoid excessive analysis times. Compared with the plate numbers obtained in GC, the plate numbers of HPLC are quite low. The classical remedy in chromatography in such cases is to opt for selectivity, and that is precisely what makes HPLC such a versatile technique. Selectivity profits from intensive partitioning of the analytes between the mobile phase and the stationary phase. Thus the ratio / , the volume of mobile phase, ymobiie the volume of stationary phase, Vstationary should be... 

The lonPac AS16 column is specified in US EPA Method 314.0 [109]. The packing material for this column was optimized in terms of capacity, hydropho-bicity, and selectivity to provide improved performance for the analysis of polarizable anions, and perchlorate in particular, in comparison to lonPac ASH that was previously recommended for this type of analysis. To achieve sufficient sensitivity for the analysis of low microgram/Liter levels of perchlorate, large-volume injections ( 1000pL) are necessary. This, in turn, requires the use of a... 

A third parameter to consider is the column construction. Thus the sample applicator should provide optimal sample application to give the most performance possible out of the packed bed. Constructions should also allow simple, fast, and reproducible packing of the column. Because costs for repacking of columns are a substantial operating cost item in industrial chromatography, the selection of column construction from this point of view is also important. Some novel column constructions allow very simple procedures both for laboratory and for industrial scale (e.g., INdEX columns, see Section V). 

---