Separator silica-based

Two classes of micron-sized stationary phases have been encountered in this section silica particles and cross-linked polymer resin beads. Both materials are porous, with pore sizes ranging from approximately 50 to 4000 A for silica particles and from 50 to 1,000,000 A for divinylbenzene cross-linked polystyrene resins. In size-exclusion chromatography, also called molecular-exclusion or gel-permeation chromatography, separation is based on the solute s ability to enter into the pores of the column packing. Smaller solutes spend proportionally more time within the pores and, consequently, take longer to elute from the column. 

Thus, in this case the simple form of equation (34) is quite adequate. Alternatively, employing a wide pore silica base in LC for separating small molecular weight... 

Resin-based TSK-GEL PW columns are durable to alkaline conditions, which cannot be used with silica-based columns. Such durability facilitates separations with basic eluents and allows the use of alkaline cleaning reagents. 

Unfortunately, exclusion chromatography has some inherent disadvantages that make its selection as the separation method of choice a little difficult. Although the separation is based on molecular size, which might be considered an ideal rationale, the total separation must be contained in the pore volume of the stationary phase. That is to say all the solutes must be eluted between the excluded volume and the dead volume, which is approximately half the column dead volume. In a 25 cm long, 4.6 mm i.d. column packed with silica gel, this means that all the solutes must be eluted in about 2 ml of mobile phase. It follows, that to achieve a reasonable separation of a multi-component mixture, the peaks must be very narrow and each occupy only a few microliters of mobile phase. Scott and Kucera (9) constructed a column 14 meters long and 1 mm i.d. packed with 5ja... 

An example of the efficacy of the resin phases used as an alternative to a conventional silica based reverse phase is shown in figure 12 where the separation of the three tocopherols are shown separated on the Cl 8 Polymer Column and The ODA-A 120A silica gel based columns. The columns were 15 cm long, 4.6 mm i.d., operated at a flow rate of 0.5 ml/min at 30°C with a mobile phase of 98% methanol/2% water. 

Comparison of the Separation of Some Tocopherols on a Polymer C18 Column and a Conventional Silica Based ODS... 

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 more useful types of chirally active bonded phases are those based on the cyclodextrins. There are a number of different types available, some of which have both dispersive or polar groups bonded close to the chirally active sites to permit mixed interactions to occur. This emphasizes the basic entropic differences between the two isomers being separated. A range of such products is available from ASTEC Inc. and a separation of the d and / isomers of scopolamine and phenylephrine are shown in figure 4. The separations were carried out on a cyclodextrin bonded phase (CYCLOBOND 1 Ac) that had been acetylated to provide semi-polar interacting groups in close proximity to the chiral centers of the cyclodextrin. The column was 25 cm long, 4.6 mm in diameter and packed with silica based spherical bonded phase particles 5pm in diameter. Most of the columns supplied by ASTEC Inc. have these dimensions and, consequently, provide a... 

An example of the separation of a mixture of explosives on a C8 column is shown in figure 7. The column was 3.3 cm long, 4.6 mm in diameter and packed with 3 pm C8 silica based reverse phase. This short column has a potential efficiency of 5,500 theoretical plates. 

Another example of the use of a C8 column for the separation of some benzodiazepines is shown in figure 8. The column used was 25 cm long, 4.6 mm in diameter packed with silica based, C8 reverse phase packing particle size 5 p. The mobile phase consisted of 26.5% v/v of methanol, 16.5%v/v acetonitrile and 57.05v/v of 0.1M ammonium acetate adjusted to a pH of 6.0 with glacial acetic acid and the flow-rate was 2 ml/min. The approximate column efficiency available at the optimum velocity would be about 15,000 theoretical plates. The retention time of the last peak is about 12 minutes giving a retention volume of 24 ml. 

An example of a separation primarily based on polar interactions using silica gel as the stationary phase is shown in figure 10. The macro-cyclic tricothecane derivatives are secondary metabolites of the soil fungi Myrothecium Verrucaia. They exhibit antibiotic, antifungal and cytostatic activity and, consequently, their analysis is of interest to the pharmaceutical industry. The column used was 25 cm long, 4.6 mm in diameter and packed with silica gel particles 5 p in diameter which should give approximately 25,000 theoretical plates if operated at the optimum velocity. The flow rate was 1.5 ml/min, and as the retention time of the last peak was about 40 minutes, the retention volume of the last peak would be about 60 ml. 

The preferred choice of a water-selective membrane up to now has been hydrophilic membranes because of their high water affinity. However, recently Kuhn et al. reported an all-silica DDR membrane for dehydration of ethanol and methanol with high fluxes (up to 20kg m h ) and high selectivities (H20/ethanol 1500 and H20/methanol 70 at 373 K) in pervaporation operation. The separation is based on molecular sieving with water fluxes comparable to well-performing hydrophilic membranes [51]. 

With the development of HPLC, a new dimension was added to the tools available for the study of natural products. HPLC is ideally suited to the analysis of non-volatile, sensitive compounds frequently found in biological systems. Unlike other available separation techniques such as TLC and electrophoresis, HPLC methods provide both qualitative and quantitative data and can be easily automated. The basis for the HPLC method for the PSP toxins was established in the late 1970 s when Buckley et al. (2) reported the post-column derivatization of the PSP toxins based on an alkaline oxidation reaction described by Bates and Rapoport (3). Based on this foundation, a series of investigations were conducted to develop a rapid, efficient HPLC method to detect the multiple toxins involved in PSP. Originally, a variety of silica-based, bonded stationary phases were utilized with a low-pressure post-column reaction system (PCRS) (4,5), Later, with improvements in toxin separation mechanisms and the utilization of a high efficiency PCRS, a... 

The most popnlar system is a reversed phase column (Cl8), on a silica base column. However, the use of C18 on a polymer-based column has been reported to provide better resolution, especially for the separation of complex anthocyanin mixtures containing acylated pigments. - Polymer-based columns also show better stability at low pH operating conditions. 

Optimized HPLC separation allows most betaxanthins to be separated on a Cl8 reversed phase stationary phase according to their respective polarities. - Considerable progress was achieved by the introduction of a highly polar silica-based column, which allowed major improvement of peak resolution, especially at early... 

Nonmodified silica gel is used most commonly for the separation of substances of medical interest. The separation is based on the interactions (hydrogen bonding, van der Waals forces, and ionic bonding) between the molecules of drugs, lipids, bile acids, etc., and the silica gel. Alumina has similar properties but is rarely used. Successful separation of endogenous substances, drugs, or their metabolites can also be achieved using physically or chemically modified silica gel. 

A cleanup procedure is usually carried out to remove co-extracted matrix components that may interfere in the chromatographic analysis or be detrimental to the analytical instrument. The cleanup procedure is dependent on the nature of the analyte, the type of sample to be analyzed, and the selectivity and sensitivity of the analytical instrument used in the analysis. Preliminary purification of the sample extracts prior to chromatographic separation involves liquid-liquid partitioning and/or solid-phase extraction (SPE) using charcoal/Celite, Elorisil, carbon black, silica, or aminopropyl-silica based adsorbents or gel permeation chromatography (GPC). 

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