Bonded phase silica-based

It should be noted that most bonded-phase silica-based columns are less stable outside the pH range 2 to 8. A buffer suitable for HPLC should be transparent at the detection wavelength if UV-detection is to be used, should be stable, inert to the HPLC system and should not chemically react with the sample or with the mobile phase. The retention is adjusted by the addition of a moderate concentration of organic solvent to the mobile phase (up to 30-4098 acetonitrile, methanol or tetrahydrofuran. according to the solubility limits). 

Extraction discs (0.5 mm thick, 25 to 90 mm diameter) constitute a variation of column-based SPE. These discs allow rapid extraction of large volumes of sample, which is not possible using a small column. The discs are made of bonded-phase silica particles, a few micrometres in diameter, trapped in a porous Teflon or glass fibre matrix. The discs are operated in a similar way to a paper filter on a vacuum flask. After extraction, the analyte is recovered by percolating a solvent through the filter. The major application of this technique is the isolation of trace amounts of compound dispersed in an aqueous medium. 

The main advantage of the zirconium family of columns is their stability from pH 1 to 10 and at temperatures from ambient to 200°C. Their separating character also differs from silica-based columns due to the lack of ionizable surface molecules. Silica above pH 3.0 loses a proton to form anionic silicate moieties, giving the bonded-phase silica column some anionic as well as nonpolar organic column characteristics (Fig. 6.8a). 

Polymeric resins have several advantages over the commonly used silica-gel based reverse phase materials. The cost of polystyrene-based resins is considerably less than bonded phase silica gel adsorbents. Also, the potential... 

Ion-exchange chromatography (lEC). Stationary phases for the separation of mixtures of ionic solutes, such as inorganic cations and anions, amino acids and proteins, are based either on microparticulate ion-exchange resins, which are crosslinked co-polymers of st)U ene and divinyl benzene, or on bonded phase silicas. Both types have either sulfonic acid cation-exchange sites (-SO3 H ) or quaternary ammonium anion-exchange sites (-N RjOH") incorporated into their structures. 

The other restriction of SynChropak size exclusion columns is a general one for silica-based supports, that of pH. The most harmful pH is that above 7.5 due to silica dissolution. The bonded phase of SynChropak GPC has some polymeric properties therefore, it is not removed rapidly from the silica at pH 2-3. The bonded phase of SynChropak CATSEC is polymeric and stable at pH 2-7.5. 

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 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... 

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... 

A soluhon explored to definitely eliminate the problem of interachons with free silanol groups was the use of non-silica based stahonary phases. Many studies recently reviewed [9] were conducted on this class of stahonary phases to test their usefulness for UpophiUcity prediction. A few of these phases including polybutadiene-coated alumina [4] and octadecyl-bonded alumina show intereshng results. Conversely, several of them show poor correlahons or correlahons not better than those obtained on ODS stahonary phases between the retenhon parameter and log Pod-... 

---