Silica-based packing materials

Column—A tube that contains packed solid material containing the stationary phase. Typical HPLC columns are stainless steel tubes packed with silica-based bonded phases. 

Initially packing materials were based on porous beads, usually silica or alumina, of 20-40 pm particle size they gave poor column efficiencies as... 

There is, however, one major problem with CO2 as a mobile phase and that is its low polarity. Thus only relatively non-polar analytes can be dissolved in CO2. Moreover, in columns packed with silica-based material there are always residual adsorptive sites. In reversed-phase HPLC the mobile phase deactivates these sites, but the CO2 is not polar enough to do that. As a consequence, the more polar analytes are adsorbed and these are then eluted as severely tailing peaks or are not eluted at all. It should be mentioned here that reports on more inert packings have been published (Li, Malik and Lee, 1994). There are some supercritical mobile phases other than CO2 that can be used, but those that are realistic to use are all non-polar. The only alternatives are the freons, of which chlorine-free freons are considered to be less harmful to the environment (Blackwell and Schallinger, 1994). 

Normal pore sizes vary from 6 to 30 nm. The sUica particles are rigid and noncompressible, which is one reason why many HPLC packing materials are based on a silica structure. The silica surface contains silanol groups with a pK from 6 to below 3. Thus, silica is acidic, causing extra strong retention of analytes with amino groups. 

Packed capiUary columns are prepared in 50-100 pm ID x 20-50 cm capillaries. They are packed with silica-based particles with the same or smaller particle diameter as in LC, but without metal frits. The frits cause disturbances by inhomogeneity of the field and formation of bubbles. Smaller particles than that in LC can be used because no pressure drop is present with potential-driven flow. The fused silica packed columns can be prepared by first packing the column against a frit. The column is subsequentiy sintered in the middle, and then flushed to remove the particles in the second part of the column. The sintering is performed hy local heating of the silica-paddng material. The next step is to sinter the inlet. Before use, the polyimide coating at the detection window is removed and the column is flushed with mobile phase by a mechanical pump. 

The reaction coil is 20 cm of coiled stainless steel capillary tubing. The guard column (1.5 cm length, inside diameter) which protects the more expensive Cjg reversed-phase separating column (length = 30 cm, inside diameter = 3.9 mm) is packed with silica-based Cig material. Typical electrochemical detector cells which can be used are (i) Model TL-5 thin layer detector cell obtained from Bioanalytical Systems, (ii) Model EA 1096 wall jet detector cell obtained from Metrohm. 

Mcntasty el al. [35] and others [13, 36] have measured methane uptakes on zeolites. These materials, such as the 4A, 5A and 13X zeolites, have methane uptakes which are lower than would be predicted using the above relationship. This suggests that either the zeolite cavity is more attractive to 77 K nitrogen than a carbon pore, or methane at 298 K, 3.4 MPa, is attracted more to a carbon pore than a zeolite. The latter proposition is supported by the modeling of Cracknel et al. [37, 38], who show that methane densities in silica cavities will be lower than for the equivalent size parallel slit shaped pore of their model carbon. Results reported by Ventura [39] for silica xerogels lead to a similar conclusion. Thus, porous silica adsorbents with equivalent nitrogen derived micropore volumes to carbons adsorb and deliver less methane. For delivery of 150 V./V a silica based adsorbent would requne a micropore volume in excess of 0.70 ml per ml of packed vessel volume. 

New templated polymer support materials have been developed for use as re versed-phase packing materials. Pore size and particle size have not usually been precisely controlled by conventional suspension polymerization. A templated polymerization is used to obtain controllable pore size and particle-size distribution. In this technique, hydrophilic monomers and divinylbenzene are formulated and filled into pores in templated silica material, at room temperature. After polymerization, the templated silica material is removed by base hydrolysis. The surface of the polymer may be modified in various ways to obtain the desired functionality. The particles are useful in chromatography, adsorption, and ion exchange and as polymeric supports of catalysts (39,40). 

The Jordi polyamine column is a polar column for simple sugar and polysaccharide applications. The amine groups are bonded to the DVB backbone and are stable in aqueous mobile phases. This material does not self-hydrolyze as do many silica-based amino packings (Fig. 13.14). 

Problems with adsorption onto the packing material are more common in aqueous GPC than in organic solvents. Adsorption onto the stationary phase can occur even for materials that are well soluble in water if there are specific interactions between the analyte and the surface. A common example of such an interaction is the analysis of pEG on a silica-based column. Because of residual silanols on the silica surface, hydrogen bonding can occur and pEG cannot be chromatographed reliably on silica-based columns. Eikewise, difficulties are often encountered with polystyrenesulfonate on methacrylate-based columns. 

The analysis demonstrates the elegant use of a very specific type of column packing. As a result, there is no sample preparation, so after the serum has been filtered or centrifuged, which is a precautionary measure to protect the apparatus, 10 p.1 of serum is injected directly on to the column. The separation obtained is shown in figure 13. The stationary phase, as described by Supelco, was a silica based material with a polymeric surface containing dispersive areas surrounded by a polar network. Small molecules can penetrate the polar network and interact with the dispersive areas and be retained, whereas the larger molecules, such as proteins, cannot reach the interactive surface and are thus rapidly eluted from the column. The chemical nature of the material is not clear, but it can be assumed that the dispersive surface where interaction with the small molecules can take place probably contains hydrocarbon chains like a reversed phase. 

The most common filter pack material is quartz (silica). Quartz is relatively inert, readily available, and workable therefore, it is preferred to replace formation materials removed from the borehole. The grain size of the filter material (i.e., sand or gravel) should be chosen based on the characteristics of the formation to be monitored and the slot size of the screen. Sand and gravel are available in various uniform sizes to accommodate different monitoring environments. 

Chemical composition of packings. Today, a wider variety of different support materials is available from which to choose. Silica is still widely used, though preparative grades often possess a relatively wide particle size distribution as compared to polymer-based supports. One serious limitation of silica-based supports is the low stability of silicas to alkaline pH conditions, which limits use of caustic solutions in sanitization and depyrogenation. Polymer-based supports, which include poly(styrene-divi-nyl benzene)- or methacrylate-based materials, are widely available and have gained increased acceptance and use. Nonfunctionalized poly(styrene-divinyl... 

The application of polymer monoliths in 2D separations, however, is very attractive in that polymer-based packing materials can provide a high performance, chemically stable stationary phase, and better recovery of biological molecules, namely proteins and peptides, even in comparison with C18 phases on silica particles with wide mesopores (Tanaka et al., 1990). Microchip fabrication for 2D HPLC has been disclosed in a recent patent, based on polymer monoliths (Corso et al., 2003). This separation system consists of stacked separation blocks, namely, the first block for ion exchange (strong cation exchange) and the second block for reversed-phase separation. This layered separation chip device also contains an electrospray interface microfabricated on chip (a polymer monolith/... 

Tanaka, N., Kimata, K., Mikawa, Y., Hosoya, K., Araki, T., Ohtsu, Y., Shiojima, Y., Tsuboi, R., Tsuchiya, H. (1990). Performance of wide-pore silica- and polymer-based packing materials in polypeptide separation the effect of pore size and alkyl chain length. J. Chromatogr. 535, 13-31. 

Recent advances in column stationary phases are remarkable. High performance silica-based reversed-phase 3 to 5 jxm packing materials have been developed for biological sample separations... 

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