Stationary phase porous layer bead

Finally, the useful life of an analytical column is increased by introducing a guard column. This is a short column which is placed between the injector and the HPLC column to protect the latter from damage or loss of efficiency caused by particulate matter or strongly adsorbed substances in samples or solvents. It may also be used to saturate the eluting solvent with soluble stationary phase [see Section 8.2(2)]. Guard columns may be packed with microparticulate stationary phases or with porous-layer beads the latter are cheaper and easier to pack than the microparticulates, but have lower capacities and therefore require changing more frequently. 

For use with 25 cm x 4.6 mm analytical columns, guard columns and scavenger columns are often 4.6 mm internal diameter and 3-10 cm in length. They can be packed with microparticulate stationary phases or with porous layer beads. Porous layer beads are cheaper than microparticulates and are easier to pack, but they have lower capacities and will require changing more often. It is usually difficult to know how long a pre-column will last before it requires changing. In routine work, precolumns are usually repacked or replaced to a fixed schedule. 

The solute molecules can enter and leave the particles only by diffusion. The particles are porous, like sponges, and most of the absorbent surface area is given by the surface of the inner pores. In bLC most of the stationary liquid phase is also inside the particles. Ofily porous layer beads which have been used in the past and may find sonie applications in the future represent an exception. The diffusion through the particles takes some time. The average time necessary for a molecule to diffuse across a distance dp is... 

The adsorbents in HPLC are typically small-diameter, porous materials. Two types of stationary phases are available. Porous layer beads (Figure 3.14A) have an inert solid core with a thin porous outer shell of silica, alumina, or ion-exchange resin. The average diameter of the beads ranges from 20 to 45 fim. They are especially useful for analytical applications, but, because of their short pores, their capacities are too low for preparative applications. 

The retention factor is directly proportional to the volume occupied by the stationary phase and more especially to its specific area (m g ) in the case of adsorbents. A column packed with porous-layer beads produces lower k values and hence shorter analysis times than a column containing completely porous particles if the other conditions remain constant. Silica with narrow pores produces larger k values than a wide-pore material. 

Various types of stationary phases are in use Porous particles, nonporous particles of small diameter, porous layer beads, perfusive particles, and monolithic materials. 

Microporous particles (3-10 (im) give columns that are as much as 20 times as efficient as porous layer-bead or pellicular (40 pm) packings. Whilst modem LC is based almost exclusively on microporous packing materials it is informative to relate the advances in particle design with the attempts to eliminate the deleterious effects on column performance since the latter as expressed by H is related to experimental variables, such as, the particle size (dp), the nominal stationary phase thickness (ds) and the mobile phase velocity (u). 

Packings for HPLC can be further described as either pellicular or porous. Pellicular particles are made from spherical glass beads, which are then coated with a thin layer of stationary phase. For example, a porous layer can be deposited onto the glass bead to produce a porous layer or a superficially porous particle. The porous layer can in turn be coated with liquid stationary phase or reacted to give a bonded stationary phase. Pellicular particles are generally less efficient than the porous layer of superficially porous particles. 

The only way to increase the rate of diffusion of solutes is to raise the temperature substantially, but this would lead to problems in thermally unstable analytes. The alternative is to reduce the distance through which the molecules diffuse. Efficient separation then requires the use of smaller particles for column packings. This apparently simple expedient has evolved into a new practice of LC that is competitive with GLC in speed and resolution of complex mixtures and applicable to many more materials than GLC. Packing materials comprised of particles as small as 5 pm are currently available. Smaller particles are extremely difficult to handle and give an almost impermeable column. To solve this problem, solid glass beads of 30-50 pm in diameter can be coated with a layer of porous material. These are called pellicular beads. The porous layer may serve as a solid stationary phase or be coated with a very thin layer of liquid stationary phase with an extremely large surface area. 

H2. Halasz, I., and Horvath, C., Micro beads coated with a porous thin layer as column packing in gas chromatography. Some properties of graphited carbon black as stationary phase. Anal. Chem. 36, 1178-1186 (1964). 

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