Column packings micropores

It is a common procedure to assume certain conditions for the chromatographic system and operating conditions and, as a result, simplify equations (20) and (21). However, in many cases the assumptions can easily be over-optimistic, to say the least. It is necessary, therefore, to carefully consider the conditions that may allow such simplifying procedures and take steps to ensure that such conditions are carefully met when such expressions are used in practice. Now, the relative magnitudes of the resistance to mass transfer terms will vary with the type of columns (packed or capillary), the type of chromatography (GC or LC) and the type of particle, i.e., porous or microporous (diatomaceous support or silica gel). 

Figure 16.22 Breakthrough curve normalized to the inlet concentration of CH4 and C)2 carried by helium through a 1 x 30 cm column packed with a microporous carbon (Kureha MAC). Dimensionless time, t = tug/L. Symbols show experimental results, the lines the calculated profiles with the models indicated. The variation of the gas velocity along the column was accounted for. Reproduced with permission from L.. P. van den Broeke, R. Krishna, Chem. Eng. Sci., 50 (1995) 2507 (Fig 12).

Figure 19-32. Diagrams of the general types of column packings Lett rally porous, Middle pellicular, Right microporous.

The mass transfer term Cv is here subdivided into contributions from diffusion up to the surface of the particle (C ) and diffusion within the micropores of the particle (C,). According to Eq. (5-2a), the A term of Eq. ( 2) should approach zero at low solvent velocities, while at high solvent velocities the Cv term of Eq. (5-2) should approach C,v. The practical significance of Eq. (5-2a) as regards separation by liquid-solid chromatography is not yet clear. It is possible that the apparent variation of A and co with adsorbent particle size (4) arises from the form of Eq. (5-2a), rather than from variations in column packing with particle size. 

Pellicular particles technical term for a synthetic HPLC column packing material consisting of microglass beads, 10-50 pm in diameter with a 2-3 pm surface film of active stationary phase material. These packings have only about 10% of the capacity of microporous materials but produce columns with greater separation efficiencies due to the very rapid equilibrium processes that occur with such regular particles and thin stationary phase films. 

The size exclusion mechanism of the differentiation of electrolytes should be applicable to separations on any microporous column packing material, regardless of its chemical nature. Of course, the chemistry of the material and pore walls can contribute in one way or another to the retention of ions, but the exclusion phenomena in fine pores can barely be ehminated. Indeed, the general validity of basic ideas developed for neutral microporous... 

Actually, one of the first nanocomposites of this kind, although the purpose was not to improve the mechanical properties of polymers, was prepared by Kirkland, and Her and McQueston, [38] when they synthesized polymer-silica composites with average diameters ranging from 500 nm to 20 p,m by copolymerisation of either melamine or urea with formaldehyde in the presence of a silica sol. Calcination yielded microporous silica spheres, which were used as chromatographic column packing under the trade name Zorbex . 

Figure 13.2 Microporous structure of bonded-phase HPLC particle, (a) A particle of column packing is honeycombed by pores. Most of the surface area is inside these pores (this drawing is not to scale typical pore diameters are on the order of 1%-5% of the particle diameter), (b) Magnified cross-section of a pore, showing a layer of alkyl chains bonded to the silica surface. (Reprinted with permission of Supelco, Inc., Bellefonte, PA.www.sigmaaldrich.com/ supelco.)...

The major anion exchangers for ion chromatography are based on two substrate types macroporous and microporous (or gel type) materials. Microporous resins were formerly popular because of their superior ion-exchange kinetics. However, microporous substrates can be spongy. Columns packed with this material may eventually undergo bed compression leading to reduced column performance. 

A very common column configuration in elution chromatography is simply a tuhular column packed with porous particles, the packings, with or without a bonded liquid phase on the particle surfiices. Other column configurations include capillary columns or open tubular columns, in which a thin hquid film of adsorbents has been applied (or bonded) to the internal surface of the capillaries. A potential variation of this is the microporous hollow fiber membrane based column, wherein the stationary phase is heid in the pores of fiber wall and the eluent is passed through the bore of the fiber (Ding et al., 1989). 

Figure 11(A) shows a principle sketch of a SEC set-up. The eluent (solvent) is pumped at a constant flow rate through the system. A small amount of polymer solution (typically 200 pL) is injected manually or with an autosampler. The main part comprises a set of columns (typically 3-4 columns+pre-column) typically packed with microporous styrene-divinylbenzene, porous glass, or silica. In the routine analytical laboratory it is especially useful to have a pre-column in order to collect impurities that might be present in the samples. If many different samples are to be analyzed, it is necessary to check the reliability of the columns frequently to avoid artefacts due to residues from previous samples still held on the column. In order to avoid problems, samples should be purified before they...

---