Packed column dimensions, selection

Based on the requirements of the separation, media of suitable pore size, particle size, and surface properties are selected as well as column dimensions and column material. In some cases a suitable combination of media type and column dimensions may be available as a prepacked column. In most cases, this is a more expensive alternative to preparing the column yourself but will provide a consistent quality as assured by the manufacturing and testing procedures of the vendor. The consistent quality may be critical in obtaining reproducible results and may thus be a cost-effective solution. Also, the fact that smaller particle-sized media are more difficult to pack and require special, and expensive, equipment has resulted in that gel filtration media of small particle size, e.g. smaller than 15 /zm, are predominantly supplied as prepacked columns. 

The factors that control separation and dispersion are quite different. The relative separation of two solutes is solely dependent on the nature and magnitude of the Interactions between each solute and the two phases. Thus, the relative movement of each solute band would appear to be Independent of column dimensions or particle geometry and be determined only by the choice of the stationary phase and the mobile phase. However, there is a caveat to this statement. It assumes that any exclusion properties of the stationary phase are not included in the term particle geometry. The pore size of the packing material can control retention directly and exclusively, as in exclusion chromatography or, indirectly, by controlling the access of the solute to the stationary phase in normal and reverse phase chromatography. As all stationary phases based on silica gel exhibit some exclusion properties, the ideal situation where the selective retention of two solutes is solely controlled by phase interactions is rarely met in practice. If the molecular size of the solutes differ, then the exclusion properties of the silica gel will always play some part in solute retention. 

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 ... 

Advances in column technology have improved the selectivity, stability, and reproducibility of LC analytical columns. For example, analytical columns are packed with a variety of stationary phases, providing enormous versatility in the separation process. This section describes (1) column dimensions, (2) particulate column packings, (3) monolithic column packings, and (4) the use of guard columns. 

Typical dimensions of packed columns and flow rates in pc-SFC are given in Table 12. This technique is becoming more and more popular compared to capillary SFC. A wide spectrum of stable stationary phases, showing different selectivities, is available commercially. Reproducible, quantitative sample introduction by means of the HPLC sample loop technique is no problem, nor is independent pressure or How control. 

The regular and rigid pore structures also make MOFs good shape and size selectivity for HPLC separation. Ahmad et al. reported the separation of benzene, naphthalene, anthracene, phenanthrene, pyrene, 1,3,5-triphenylbenzene, and 1,3,5-tris(4-bromophenyl)benzene on MOF-199- and MOF-5-packed columns based on the combination of molecular sieving and adsorption effects. Separation of ethylbenzene and styrene with very similar dimensions was also achieved on MOF-199 column. Unlike shape and size selectivity mechanisms, the separation of ethylbenzene and styrene was ascribed to the jt-complexation of styrene with Cu metal site in MOF-199. 

The hydrogen-bond donor and acceptor properties of a packing are another dimension of column selectivity. They are independent of the silanol ion-exchange activity described above, and affect the selectivity of a packing in a separate way. We have measured these properties by the relative retention between butylparaben, a hydrogen-bond donor, and dipropylphthalate. The same property, or at least a very similar property, has been called hydrogen bond basicity by Snyder.In our case, it has been defined as follows ... 

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