Stationary phases materials

A particular column can be used for different types of LC by changing the eluent components. For example, a column packed with RP-18 bonded silica gel can be used for SEC with THF, NPLC with n-hexane, and RPLC with aqueous acetonitrile. When separation cannot be achieved by improving the theoretical plate number of a column, it may be achieved by selection of an appropriate stationary phase material and/or eluent. 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

It was known from gas chromatographic theory that efficiency could be improved if the particle size of the stationary phase materials used in lc could be reduced. High performance liquid chromatography developed steadily during the late 1960s as these high efficiency materials were produced, and as improvements in instrumentation allowed the full potential of these materials to be realised. As hplc has developed, the particle size of the stationary phase used has... 

The frits in the cartridges are intended to retain stationary phase material in the separation channels (columns) while permitting the passage of the mobile phase during separations. Each frit is constructed from a permeable polypropylene membrane with an average pore size smaller than the... 

Study Table 12.3 and tell what stationary phase material would be useful for separating some low-molecular-weight alcohols. 

The particle size of the stationary phase material plays a very vital and crucial role in HPLC. In fact, high-elficiency-stationary-phase materials have been researched and developed exclusively for HPLC having progressively smaller particle size termed as microparticulate column packings. These silica particles are mostly uniform, porous, with spherical or irregular shape, and having diameter ranging from 3.5 to 10 pm. 

Name Mobile phase Stationary phase Materials separated... 

The physical and chemical aspects of liquid chromatography, in addition to mechanical aspects, are briefly described in this chapter. Theoretical approaches are explained in detail in later chapters. The effect of stationary phase materials on the chemical selectivity is described in Chapter 3, and the influence of the eluent components is covered in Chapter 4. The plate number theory is discussed in Chapter 5. Quantitative optimization is explained in Chapter 6. 

Comparison of these four chromatograms suggests that a fast separation can be performed either using a longer column with 10 pm stationary phase material with a high flow rate of the eluent, to give high resolution, or by a smaller... 

High-speed separations can be achieved with a short column packed with 3 pm stationary phase material, as shown in Figure 1.2. The sensitivity was also improved by the use of smaller-size stationary phase materials, due to less sample diffusion inside the column. The following conditions are required to obtain such a separation. 

The most important mechanism, O important property, A interaction depending on stationary phase material. 

Affinity liquid chromatography and chiral separations (enantiomer separations) require similar analyte properties. The solutes may have interactions through hydrogen-bonding, ligand formation, or Coulombic forces with the surface of stationary phase materials or the sites of additives however, the selectivity is controlled by the steric effects of the structures of the analyte molecules and the recognition molecules (chiral selectors). 

The physical and chemical properties of stationary phase materials are described in Chapter 3 (including methods for their synthesis) to clarify the differences in similar stationary phase materials supplied from different manufacturers. A detailed selection guide to solvents is given in Chapter 4. The unlimited selection of eluent components and their concentrations is a powerful force in developing separations in liquid chromatography. Although this area seems rather complicated, it is easy to understand the selection of a suitable eluent when you first identify the molecular properties of the analytes and solvents. 

The selectivity of different stationary phase materials can be applied using columns in sequence to provide high-speed isocratic separations instead of gradient elution. An example for amino acids analysis is shown later in Figure 4.15, where the same eluent was used for all of the separations and the fraction containing the sample components of interest was switched from one column to another. 

Preparation, Testing, and Selectivity of Stationary Phase Materials... 

The fundamental behaviour of stationary phase materials is related to their solubility-interaction properties. A hydrophobic phase acts as a partner to a hydrophobic interaction. An ionic phase acts as a partner for ion-ion interactions, and surface metal ions as a partner for ligand complex formation. A chiral phase partners chiral recognition, and specific three-dimensional phases partner affinity interactions. 

Stationary phase materials are synthesized from different raw materials. Those stationary phase materials synthesized from inorganic materials, such as silica and alumina, are physically strong but chemically unstable. Conversely, stationary phase materials synthesized from organic materials, such as polystyrene or poly(vinyl alcohol), are chemically stable but physically weaker. Improvements in the chemical stability of inorganic stationary phase materials and in the physical strength of organic stationary phase materials are required the marketed products do not have both and have to be used under restricted conditions in liquid chromatography. 

Table 3.1 Stationary phase material selection guide...

The selectivity of stationary phase materials can be understood if the method of their synthesis is understood. Differences in the same type of stationary phase material from different manufacturers or even from the same manufacturer depend on the synthetic methods and the quality control that has been employed. Details of the individual synthetic processes from different manufacturers have not been published, but are basically the same.1,2... 

Spherical porous silica gel is the easiest stationary phase material to handle however, although it is physically strong it is chemically unstable. Surface modification can expand its capability for different modes of chromatography, such as normal-phase, reversed-phase, size-exclusion, and ion-exchange liquid chromatography. These stable modifications are performed by chemical deriva-tization of the surface silanol groups. 

The chlorinated silica gel suspended in diethyl ether, dimethyl sulfoxide, or dioxane will also react with diamine or amino compounds. After being washed in an excess of amine and HC1, and then dried under vacuum, the final stationary phase materials are obtained ... 

Further modification of diamino bonded products with halogenated compounds can lead to a variety of stationary phase materials ... 

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